MEDICAL HEAT TREAT TECHNICAL

HIP Innovation Maximizes AM Medical Potential

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The appeal of additive manufacturing (AM) for producing orthopedic implants lies in the “ability to design and manufacture complex and customized structures for surgical patients in a short amount of time.” To complement speed of production, learn how an innovative hot isostatic pressing (HIP) application is confronting the challenges of post-processing heat treatments when creating high quality AM medical parts.

Today’s Technical Tuesday article, written by Andrew Cassese, applications engineer; Anders Magnusson, manager of Business Development; and Chad Beamer, senior applications engineer, all from Quintus Technologies, was originally published in Heat Treat Today’s December 2023’s Medical and Energy Heat Treat magazine.

AM is playing a significant role in the medical industry. It gives manufacturers the ability to create customized and complex structures for surgical implants and medical devices. Additionally, medical device manufacturers have different material factors to consider – such as biocompatibility, corrosion resistance, strength, and fatigue – when selecting a material for a given application. Each of these factors plays a significant role. It’s no wonder that the most common metallic biomaterials in today’s industry are stainless steels, cobalt-chrome alloys, and titanium alloys (Trevisan et al., 2018).

In this article, learn about the application of Ti6Al4V in the medical industry, as well as ways to address some of the challenges when producing AM medical components.

The Future Demands Orthopedic Implants

Figure 1. Example of AM trabecular structure on a Ti6Al4V
acetabular cup (Source: Quintus Technologies)

The medical market for orthopedic implants is predicted to grow annually by approximately 4% where joint replacement, spine, and trauma sectors are reported to account for more than two-thirds of the market. The largest portion is joint replacement with over a third of global turnover, reaching in excess of 20 million U.S. dollars in 2022 (ORTHOWORLD® Inc., 2023). This confirms an earlier study by Allied Market Research where spine, knee, and hip implants made up over 66% of the entire market, with knee implants leading the way at 26% (Allied Market Research Study, 2022). This fact, combined with the expectation that the global population aged 60+ is predicted to double between 2020 and 2050, adds to the increasing demand on manufacturers to produce better quality and longer lasting orthopedic implants (Koju et al., 2022).

These factors have increased the predicted medical implant market for Ti6Al4V and other common orthopedic materials. Using AM processes such as electron beam melting (EBM) and laser powder beam fusion (L-PBF), manufacturers can produce thin-walled trabecular structures that are fabricated to promote bone ingrowth in a growing market that is in competition with traditional production methods.

Titanium-based alloys have been increasingly used in orthopedic applications due to their high corrosion resistance and a Young’s modulus similar to that of human cortical bone (Kelly et al., 2021). The high strength-to-weight ratio and bioinert-ness of Ti6Al4V has proven it to be an ideal candidate for orthopedic and dental implants. It is a titanium alloy with 6% aluminum and 4% vanadium that has low density, high weldability, and is heat treatable. Ti6Al4V demonstrates good osteointegration properties, which is defined as the structural and functional connection between living bone and the surface of a load carrying medical implant.

Many manufacturers are using L-PBF to create thin-walled complex structures on the surface of the implant. This makes use of the osteointegration properties as the implant integrates itself into the body over time without the need for bone cement (Kelly et al., 2021). Introducing a large metallic foreign body leads to challenges such as promotion of chronic inflammation, infection, and biofilm formation. Instead, porous AM Ti6Al4V implants have a biomimetic design attempt towards natural bone morphology (Koju et al., 2022).

AM Yields Production Solutions for Medical Alloys

The medical industry has been increasing the use of AM over traditional processing methods. AM facilitates weight reduction, material savings, and shortened lead-time due to reduced machining, but these are only a few of the benefits. Improved functionality and patient satisfaction are also key aspects through tailoring of designs to take advantage of AM over traditional forging and casting techniques. Additionally, the costs of machining a strong alloy like Ti6Al4V can be expensive, and any wasted material and time in turn lead to higher cost.

One of the main reasons for the interest in AM is the ability to design and manufacture complex and customized structures for surgical patients in a short amount of time. For example, if a patient needs an implant for surgery, an MRI scan can help reverse engineer a customized implant. Engineers prepare a design of a patient-specific implant according to the patient’s anatomy that is then printed, HIPed, and finished for surgery with a reduced lead time. This is especially important for trauma victims, where the speed of repair can mean the difference between losing a limb or returning to a fully functional life. Cancer victims and those requiring aesthetic surgery to the skull, nose, jaw, etc., can also benefit from this (Benady et al., 2023).

Some of the current challenges with AM titanium in the medical industry are related to the post-processing heat treatments that are required. These treatments can leave an oxide layer on thin-walled structures that is hard to remove by machining or chemical milling. Quintus Purus®, a unique clean-HIP solution, has proven to overcome this challenge and provide clients with a robust solution that both densifies and maintains a clean surface.

When HIP Meets AM

Figure 2. AM Ti6Al4V components HIPed without getter using conventional HIP (left) and Quintus Purus® (right) (Source: Zeda)

HIP is important in the AM world as a post-process that closes porosity and increases fatigue life. For medical implants, high and low cycle fatigue life properties are key as they affect the longevity of the repair. The mechanical strength and integrity are improved significantly by HIPing the implants, reducing the need for further surgery on the same patient. Modern HIP cycles have been developed to further increase this performance. When combined with Quintus Purus®, modern HIP cycles can minimize the thin, oxygen-affected layer that can result from thermal processing on surfaces of high oxygen-affinitive materials, such as titanium.

For Ti6Al4V, this layer is often referred to as alpha-case. The brittle nature of the alpha-case negatively impacts material properties resulting in medical manufacturers requesting their AM parts in the “alpha-case free” state. Alpha-case can be formed during heat treatment. As surfaces of the payload and process equipment are exposed to oxygen at elevated temperatures, they may be oxidized or reduced, depending on the oxide to oxygen partial pressure equilibrium. During heat treatment, evaporating compounds become part of the process atmosphere, and solids are deposited or formed on other surfaces, either as particles or as surface oxides.

For titanium alloys, surface oxides are formed at logarithmic or linear rates, depending on temperature and oxygen partial pressure. At the same time, oxygen can diffuse into the surface to form the brittle alpha-case, which is detrimental to the part’s fatigue performance. Changes of the surface color can often be seen as an indication that surface reactions have occurred during processing when using traditional thermal processes (Magnusson et al., 2023).

The HIP furnace atmosphere contaminants that cause this oxidation can originate from various sources including the process gas, equipment, furnace interior, and, most importantly, the parts to be processed. The payload itself often absorbs moisture from the surrounding atmosphere before being loaded into the furnace, which is subsequently released into the HIP atmosphere during processing. Industrial practice today attempts to solve the issue by wrapping parts in a material such as stainless steel foil or a “getter” that has a high affinity to oxygen protecting the Ti6Al4V component from exposure to large volumes of process gas, thus helping minimize the pickup of the contaminates.

This method adds material, time, and labor to wrap and unwrap parts before and after each HIP cycle. Also, wrapping in getter cannot guarantee cleanliness and may result in some uneven oxidation. This is where the tools of Quintus Purus® are of assistance; these tools allow the user to define a maximum water vapor content that can be accepted in the HIP system before the process starts. The tool utilizes the Quintus HIP hardware together with a newly developed software routine, ensuring that the target water vapor level is met in the shortest time possible. The result is a cleaner payload, without the need to directly wrap components with getter (Magnusson et al., 2023).

Table 2. Results from case study productivity analysis
(Source: Quintus Technologies)
Table 1. Input to case study (Source: Quintus Technologies)

Alpha-Case Avoided: Comparing Conventional HIP and Optimized HIP Technologies

Quintus Technologies performed a study with Zeda, Inc. to evaluate Quintus Purus® on L-PBF Ti6Al4V medical implant parts. The study was performed in the Application Center in Västerås, Sweden in a QIH 21 HIP. A conventional HIP cycle was performed as well as an optimized Quintus Purus® HIP cycle, both without the use of getter. No presence of alpha-case was found on the part processed with the Quintus Purus® cycle as shown in Figure 2 below (Magnusson et al., 2023).

Quintus Purus® can be further enhanced with the use of a Quintus custom-made getter cassette supplied as part of the installation, which consumes or competes for the remainder of contaminant gaseous compounds still present in the system after all other measures such as best practice handling, adjustment of gas quality, etc., have been implemented.

Titanium is considered the getter of choice for Quintus Purus® and is included as an optional compact getter cassette placed at the optimum position in the hot zone of the HIP furnace. Although the custom-made getter cassette occupies a small space, its use can significantly increase loading efficiency. The traditional way of individually wrapping components with stainless steel or titanium foil will consume more furnace volume, through reduced packing efficiency, leading to less components per cycle when compared to the Quintus Purus® titanium getter cassette strategy. Using an average spinal implant size of 2 in3 (32 cm3), one can calculate the packing density in a standard HIP vessel assuming two shifts per day and a 90% machine uptime. For example, a Quintus Technologies QIH 60 URC with a hot zone diameter of 16 in (410 mm) and a height of 40 in (1,000 mm) can pack up to 1,280 implants per cycle, with clearances for proper spacing and load plates.

Figure 3. Quintus Technologies QIH 60 URC outfitted with
Quintus Purus® technology (Source: Quintus Technologies)

The typical Ti6Al4V HIP parameters include a soak time of two hours at 1688°F with 14.5 ksi argon pressure (920°C with 100 MPa). Accounting for heat up and cool down time, this HIP cycle can take less than eight hours, allowing two cycles per day on a two-shift work schedule. A typical case of wrapping each component in getter material adds time, cost, resources, and uses up to an estimated 50% of the load capacity. With the increased efficiency enabled by Quintus Purus®, clients have the opportunity to HIP 552,960 spinal implants per year (Tables 2 and Figure 3).

In conclusion, the growing Ti6Al4V market in the medical industry demands innovative developments to keep up with ever-increasing production volumes, whilst quality demands in lean production are becoming more significant. Solutions like the Quintus Purus® will allow manufacturers to have control over the quality of their titanium parts during a HIP cycle. It can be applied to produce alpha-case free components ensuring the optimal performance of orthopedic implants with increased service life.

References
Ahlfors, Magnus, Chad Beamer. “Hot Isostatic Pressing for Orthopedic Implants.” (2020): https://quintustechnologies.com/knowledge-center/hiporthopedic-implants/.
Allied Market Research Study performed for Quintus Technologies, 2022.
Benady, Amit, Sam J. Meyer, Eran Golden, Solomon Dadia, Galit Katarivas Levy.
“Patient-specific Ti-6Al-4V lattice implants for critical-sized load-bearing bone defects reconstruction.” Materials & Design 226 (Feb. 2023): https://www.sciencedirect.com/science/article/pii/S0264127523000205?via%3Dihub.
Kelly, Cambre N., Tian Wang, James Crowley, Dan Wills, Matthew H. Pelletier, Edward R. Westrick, Samuel B. Adams, Ken Gall, William R. Walsh, “High-strength, porous additively manufactured implants with optimized mechanical osseointegration.” Biomaterials (Dec.2021): 279, https://www.sciencedirect.com/science/article/abs/pii/.

About the Authors

Andrew Cassese is an applications engineer at Quintus Technologies. He has a bachelor’s degree in welding engineering from The Ohio State University.

Contact Andrew at andrew.cassese@quintusteam.com

Anders Magnusson is the business development manager at Quintus Technologies with an MSc in engineering materials from Chalmers University of Technology.

Contact Anders at anders.magnusson@quintusteam.com

Chad Beamer Applications Engineer Quintus Technologies

Chad Beamer is a senior applications engineer at Quintus Technologies, and one of Heat Treat Today’s 40 Under 40 Class of 2023 award winners. He has an MS from The Ohio State University in Materials Science and has worked as a material application engineer with GE Aviation for years and as a technical services manager with Bodycote. As an applications engineer, he manages the HIP Application Center located in Columbus, Ohio, educates on the advancements of HIP technologies, and is involved in collaborative development efforts both within academia and industry.

Contact Chad at chad.beamer@quintusteam.com

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Potential for L-PBI Titanium Alloy in Aero and Medical Industries

/ AEROSPACE HEAT TREAT, AEROSPACE HEAT TREAT TECHNICAL CONTENT, FEATURED NEWS, MEDICAL HEAT TREAT, MEDICAL HEAT TREAT TECHNICAL, TITANIUM PROCESSING TECHNICAL CONTENT, VACUUM FURNACES TECHNICAL CONTENT

Source: TAV Vacuum Furnaces 

Those familiar with vacuum heat treatments are surely acquainted with the vacuum heat treatment of titanium and how such furnaces create the ideal environment for titanium's heat treatment. However, not all titanium and its alloys are created equal. Enter the beta titanium alloy.

In this best of the web article from TAV Vacuum Furnaces, discover the potential applications for beta titanium alloys, as well as the effects that various vacuum heat treatments can have on the mechanical properties of the alloy. Additive manufacturing (AM) technologies, specifically laser powder bed fusion, are gaining increased interest in the treatment of beta titanium alloys, due to their efficiency and their cost-cutting potential. Learn more about the chemistry and applications of this unique material below.

An excerpt:

Beta titanium alloys have an unique combination of desirable properties: their high specific strengths, creep resistance, oxidation and corrosion resistance, excellent temperature resistance up to 600°C and hardenability, make them very attractive for aerospace applications. On the other hand, the excellent biocompatibility and low elastic modulus, closer to that of human bone compared to other alloys, make Ti beta alloys an excellent material for biomedical applications.

Read more: "Vacuum Heat Treatment of L-PBF Beta Titanium Alloys-TAV Vacuum Furnaces at ECHT 2023”

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A Quick Guide to Alloys and Their Medical Applications

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OC

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Do you know what are the most popular alloys in the medical market? What are their applications?  This medical alloys reference graphic gives a quick overview of alloys and their specialized uses in the medical industry.

Ask average people walking along the street what metal/alloy comes to mind when they think of medical uses - things like hip and shoulder joints, the orthodontia their kids might wear, the forceps used to remove stitches - they might come up with the answer "titanium." While this certainly is correct, there are lots of other metals and metal alloys that are used in the medical industry. They probably wouldn't answer "nitinol," a titanium alloy. Nitinol is actually used in the aforementioned braces! Nitinol can be found in other things too: stents, staples, septal defect devices, etc. Take a look at the graphic to see what all these alloys, in fact, can do; you might be surprised!

Such important implements, devices, and components that are used in and on the human body need to be durable and reliable. These medical pieces can improve the quality of life (to put it mildly) or actually save a life (to put it dramatically). Some of these alloys are actually used in and around the heart and blood vessel system! Only the best of the best will do to make up these medical items; lives are literally preserved and saved with them.

What alloys have you found in medical applications? Maybe you have experience with a loved one or yourself incorporating one of these medical pieces in your life? Are you a heat treater involved in the making of these products? Let Heat Treat Today know in the Reader Feedback.  

Download the full graphic by clicking the image below.

Source: Heat Treat Today

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Heat Treating: The Best Medicine

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OCHeat treating solutions are important for more than keeping an airplane flying in the sky or a bridge suspended above the water. These two examples are high profile, but what about the heat treating solutions that do not zoom through the air or mark the skyline above rivers? In the medical industry, heat treating solutions are often unseen unless something goes wrong.

When it comes to medical implant and device heat treating, what options are available to manufacturers that will benefit patients? What should we know about the heat treating processes that make metal parts functional as knees, hips, and elbows? Find out in this expert analysis from Quintus Technologies and ECM USA, Inc.

This Technical Tuesday article was first published in Heat Treat Today's December 2022 Medical and Energy print edition.

Introduction

Dan McCurdy, former president at Bodycote, Automotive and General Industrial Heat Treatment for North America and Asia, knows full well just how much time, energy, and pain the right medical heat treating practice and alloy composition can save a patient. Dan’s wife suffered from complications due to a nickel allergy in a traditionally thermally-processed ASTM F75 knee implant. She dealt with constant inflammation, swelling, and pain. Physical therapy and a second procedure did nothing to ease the discomfort. The best medicine for Dan’s wife? A specially heat treated medical implant (more of Dan's story can be found at the end of this article).

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To understand the stories behind final medical products, Heat Treat Today asked Quintus Technologies and ECM USA, Inc. to share two different approaches on medical implant and device heat treatment. These two companies at the forefront of the medical heat treating industry shared about hot isostatic pressing (HIP) with additive manufacturing, and vacuum heat treating. Read their answers to our questions and learn how, when it comes to implantable medical devices, heat treating can be the best medicine.

 

How do you ensure your equipment maintains the precise specifications required in the medical industry? What specifically is necessary to maintain compliance when it comes to medical implants?

Quintus Technologies

Chad Beamer
Applications Engineer
Quintus Technologies

Quintus Technologies has observed a trend in bringing Nadcap to the medical industry. Historically the medical industry has focused on the standards and regulations for the quality management system of their approved supplier, but a consistent transition to technical aspects of critical processes (including HIPing) is becoming the norm. Quintus Technologies’ background is one of delivering HIP equipment in line with Nadcap and AMS2750 specifications. The medical industry requires best-in-class temperature uniformity and accuracy; systems designed with production driven flexibility (such as thermocouple quick-connectors for T/C sensor installation
to minimize downtime); HIP furnaces equipped with uniform rapid cooling (URC®) for optimized cycle productivity; active involvement in standards committees; and working directly with the industry.

Requirements are increasing in terms of productivity and the introduction of more complex surface requirements. It is crucial to work closely with the industry to reduce oxidation of orthopedic implants during the HIP and heat treatment processes.

Steering of the HIP cycle is key, along with in-HIP heat treatments to achieve the desired microstructure for the application, which is a standard offering for High Pressure Heat Treatment™ (HPHT™) equipment.

ECM USA, Inc.

Dennis Beauchesne
General Manager
ECM USA, Inc.

Some of the features that are most important are leak rate at deep vacuum along with a chamber and furnace design that does not contribute to any contamination. In our systems, these features, along with others, are of the utmost importance when supplying equipment for the medical implant market.

What are the top 3–5 key requirements or compliance/quality issues needed to heat treat medical implants?

Quintus Technologies

There are several industry standards that have been released to establish key requirements for the HIP process that are often leveraged for medical applications demanding performance and reliability. For example, Nadcap has released AC 7102/6 which details the audit criteria for HIP. This document was developed with significant input from the industry and the government to define operational requirements for quality assurance. It offers a checklist for the HIP processing of metal products and includes requirements for:

  • managing the equipment per pyrometry standard AMS2750
  • qualifying technical instructions and personnel training
  • handling product during the loading and unloading operations
  • complying with gas purity requirements of the pressure medium
  • controlling temperature, including uniformity and accuracy evaluations and management

These aspects are critical to ensure product quality meeting medical customer requirements and expectations. Recent additions beyond conventional requirements highlighted above include high speed cooling in the HIP process (>200 K/min) for some materials which is important for metallurgical results.

ECM USA, Inc.

Key requirements include thermal performance (both uniformity and ramp control); real-time vacuum and gas management; traceability and production lot follow up through human machine interface (HMI); quality procedures for all sensor calibrations; and remote access for control and troubleshooting.

Can you share an example of how your equipment could be used to heat treat a medical implant/device from start to finish?

Quintus Technologies

Many medical implants — whether fabricated using conventional processing techniques such as casting, or more novel approaches such as additive manufacturing — require HIP to eliminate process related material defects. Defects include shrinkage porosity for castings and lack-of-fusion and keyhole defects for fusion based additive manufacturing techniques. These defects can have a negative impact on product quality, impacting performance and reliability. Once HIP has been applied to a material, post processing is often not complete, with additional thermal treatments required to achieve the optimum microstructure leading to the desired material properties and performance. Such thermal treatments are material and process dependent, but could include a stress relief, solution anneal, rapid cooling or quenching, and aging and are often applied in separate heat treat equipment.

Hot Isostatic Press QIH 60 offering our most advanced Uniform Rapid Cooling (URC®) furnace technology with industry leading temperature control and accuracy

Quintus Technologies has introduced HIP systems providing capabilities beyond conventional densification. Decades’ worth of work in equipment design, system functionality, and control now offers an opportunity to perform HIP and heat treatment in a combined cycle, referred to as HPHT. Combined HIP and heat treatment for castings and AM implants can mitigate the risk of thermally induced porosity, as well as grain growth, which can offer advantages for mechanical and chemical properties in implants. This methodology provides a more sustainable processing route with improved productivity and energy efficiency. A joint HIP and heat treatment offers significant advantages with lead time, and this improvement in lead time couples well with the demands placed on the personalized medical implants. It also offers opportunities to further optimize microstructures for improvement in material properties coupled with ease of manufacturability. HPHT and modern HIP equipment may allow for a higher performing material system, which produces an implant with improved reliability and life.

Within the medical industry, fine grain AM microstructure, repeatability, and low porosity are key concerns. There are many reported benefits by applying the combined HPHT route such as reduced number of process steps, reduced cycle time and lead time, and improved process and quality control. Other advantages include spending less time at elevated temperatures helping to preserve the fine grain AM microstructure by minimizing grain growth. Tight control and steering of the cooling rates during the different steps of the HPHT cycle ensures repeatability of the properties. Manufacturability can be improved through HPHT as this approach reduces the cooling or quench severity during cooling segments which can often lead to part distortion or cracking. Improved functionality and
control go hand-in-hand with the high quality and reliability demanded in the medical industry.

ECM USA, Inc.

We have several customers making titanium alloy prothesis for various applications: shoulders, hips. Our furnaces are used for post printing processes, such as stress relieving and solution annealing.

Given concerns of metal poisoning, do you know of any changes in alloy composition of medical devices over the last decade?

Quintus Technologies

There are some metals that are becoming more common for implants, including tantalum, magnesium, CP Titanium, etc., and there have been major steps in improving ceramic materials to compete with metals for many applications.

ECM USA, Inc.

As a vacuum furnace equipment supplier, we are not deeply involved in the entire process of material selection. In the early stages of 3D printing joint replacements, from 2013 to 2014, we saw cobalt being part of some alloys. Lately it seems, indeed, that there is a trend in removing that element from the finished parts.

A Happy Ending

Dan McCurdy
Former president, Bodycote, Automotive and General Industrial Heat Treatment for North America and Asia

(The rest of Dan's story from the beginning of the article....) The effects of metal poisoning and metal allergies post-surgery can be
devastating. In the narrative below, Dan McCurdy shares the story of his wife’s struggle with an allergic reaction to a knee implant, and the heat treating solution that proved to be the best medicine for her.

My wife, an avid runner up and down the hills of Cincinnati, was diagnosed with osteoarthritis in both knees at the age of 53. Her orthopedist suggested a knee replacement for the most degraded one. The replacement was a well-known brand, made from investment-cast ASTM F75 (nominally a Co-Cr-Mo alloy) with full FDA-approval. After a successful surgery and diligent physical therapy, her recovery plateaued, and she experienced chronic inflammation, swelling, and pain.

A blood test, designed to detect allergies to materials used in orthopedic implants, showed a reaction to nickel that was nearly off the charts. We were surprised, as she had previously tested negative for nickel allergies through skin patch testing. The ASTM F75 specification allows for up to 0.5% bulk nickel as a tramp element in implantable devices; however, depending on foundry practices, the concentration of tramp alloys at any point on the surface of a casting can vary significantly. Titanium implants may be the solution to this, but FDA-approved titanium alloys can still contain up to 0.1% Ni.

The solution for my wife, as it turned out, was a different material, originally developed for the nuclear industry, along with an innovative heat treatment process. Created with an alloy of zirconium and niobium (with a maximum nickel content of 0.0035%), her new knee was heat treated at a high temperature in an oxidizing environment, which converts the soft zirconium surface into hard
ceramic zirconia, increasing hardness and wear resistance. With this specially heat treated implant in place, my wife is back to nearly 10K steps a day.

 

References

[1] Magnus Ahlfors and Chad Beamer. “Hot Isostatic Pressing for Orthopedic Implants.” quintustechnologies.com/knowledge-center/hot-isostatic-pressing-for-orthopedic-implants. Quintus Technologies. 2020.

[2] Chad Beamer and Derek Denlinger. “Hot Isostatic Pressing: A Seasoned Player with New Technologies in Heat Treatment — Expert Analysis.” www.heattreattoday.com/processes/hot-isostatic-pressing/hot-isostatic-pressing-technical-content/hot-isostatic-pressing-a-seasoned-player-with-new-technologies-in-heat-treatment-expert-analysis/. Heat Treat Today. 2020.

For more information

Contact Chad Beamer at chad.beamer@quintusstream.com

Contact Dennis Beauchesne at DennisBeauchesne@ECM-USA.COM

Find heat treating products and services when you search on Heat Treat Buyers Guide.com

 

Heat Treating: The Best Medicine Read More »

Corrosion Behavior of DMLS Titanium Alloy for Orthopedic Applications

/ FEATURED NEWS, HIPING TECHNICAL CONTENT, MEDICAL HEAT TREAT, MEDICAL HEAT TREAT TECHNICAL, SINTERING POWDER/METAL TECHNICAL CONTENT, TITANIUM PROCESSING TECHNICAL CONTENT

OCIn this article, explore the importance of alternative advanced manufacturing processes and the effects of post-process heat treating of DMLS titanium alloy parts. In a recent study, a team at Worcester Polytechnic Institute (WPI) evaluated the effects of these processes. Read along to see what they found.

This Technical Tuesday article was first published in Heat Treat Today's December 2022 Medical and Energy print edition.

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Jianyu Liang
Professor of
Mechanical and Materials Engineering
at Worchester Polytechnic Institute
Source: WPI

According to Markets and Markets reports, the metal implants and medical alloys market 1 will reach $17.64 billion by 2024, at a CAGR of 9.4%, with titanium metal implants and medical alloys accounting for the largest share of the market. Since it was first reported in the 1940s that titanium had excellent compatibility with human bones, titanium has been used in a wide range of biomedical applications, including arthroplasty and bone replacement, prostheses, craniofacial, maxillofacial, and dental implants, as well as surgical instruments and healthcare goods. 2,3

Although Ti-6Al-4V alloy was originally developed for aerospace applications, its many attractive properties — such as high strength-to-weight ratio, satisfactory biocompatibility, and good corrosion resistance — resulted in it being one of the most widely used biomedical alloys. 4

However, Ti-6Al-4V alloy is very difficult to machine. Traditional Ti-6Al-4V manufacturing processes include casting, wrought (forging/milling from ingots), and powder metallurgy (P/M), with wrought products accounting for 70% of the titanium and titanium alloy market. 5

In recent decades, additive manufacturing (AM) processes have been rigorously

Richard Sisson
Key Heat Treat
Researcher and Lecturer at Worchester
Polytechnic Institute
Source: WPI

developed as an alternative advanced manufacturing process for Ti-6Al-4V, especially in personalized biomedical applications. Alternate processes, including powder-bed fusion (PBF), directed energy deposition (DED), and sheet lamination (SL) have been applied in AM processing of titanium and its alloys. 6 Direct metal laser sintering (DMLS), a PBF technology, was the first commercial rapid prototyping method to produce metal parts in a single process and is one of the most widely used AM technologies to manufacture Ti-6Al-4V parts. 7 However, even with the protective oxide film (mainly TiO2), titanium alloys still suffer from pitting and crevice corrosion. Localized breakdown of the protective film leads to the formation of pits. These pits can grow and propagate into macroscopic cracks, which lead to catastrophic failure in orthopedic applications. 8,9

It was reported that post-heat treatment of Ti-6Al-4V parts fabricated by AM techniques could improve its mechanical properties, especially increasing ductility and fatigue strength.

Yangzi Xu
Yield & Module
Process Engineer at Intel Corporation
Source: WPI

However, the changes in corrosion behavior with various post-heat treatments of Ti-6Al- 4V parts fabricated by AM techniques have not been fully understood. In a recent study, a team at Worcester Polytechnic Institute (WPI) evaluated the effects of various post-process heat treatments (including solution treatment and aging, annealing, stress relief, and hot isostatic pressing (HIP)), on the corrosion behavior of Ti-6Al-4V parts manufactured by DMLS. The researchers then proposed a desirable posttreatment procedure that can obtain a good combination of mechanical properties and corrosion behavior of as-printed parts in a simulated body environment. 10,11,12

Ti-6Al-4V dumbbell-shaped tensile testing bars were fabricated by DMLS, according to ASTM standards. The microstructure, phase fraction, porosity, and residual stress of as-printed parts were examined and compared to those of the commercial Grade 5 alloy. It was found that the as-printed samples, mainly composed of acicular α’ martensite phase with a small amount of nano-scaled β precipitates, dispersed in the α’ matrix due to rapid cooling during laser processing, whereas the Grade 5 alloy has an α + β two phase with an equiaxed microstructure. The β phase fractions in the as-printed and Grade 5 alloy were 1.6% and 20%, respectively, based on the results of x-ray diffraction refinement. Furthermore, porosity and defects due to lack of fusion or entrapped gas were observed in the DMLS samples. The rapid cooling rate also resulted in residual tensile stress in the as-printed parts.

The microstructure and phase changes due to different heat-treatment processes were examined and compared to those of the commercial Grade 5 alloy. The corrosion behavior of the heat-treated DMLS parts was studied in simulated body fluid by well-established electrochemical methods.

Microstructure: coarsening of the α lath thickness, more spherical β precipitates.
Phase identification: narrowed α characteristic peaks (reduced compressive residual stress)
Source: WPI

Transformation from α’ to α phase, coarsening of the α lath microstructure, and the development of β phase were observed in samples after heat treatments. The greatest fraction of β phase was obtained in the high temperature annealed sample. Enhanced corrosion resistance was found in all heat-treated samples. The reasons for improved corrosion resistance after heat treatments include: 1) a passive layer that was developed on the sample surface after heat-treatments; 2) increased β phase fraction and size after heat treatments that led to the reduction of the corrosion susceptible sites. Furthermore, only a single passive layer has been observed in the as-printed sample, whereas double passive layers have been observed in samples after heat treatments at temperature higher than 550°C. However, this second layer, which was largely composed of Al2O3 and V2O5, had very low corrosion resistance compared to that of the primary passive layer that was primarily TiO2.

Microstructure: coarsening of the α lath, and grain boundary can be observed
Phase identification: narrowing of α characteristic peaks (reduced microstrain, increased grain size) and evolution of β phase
Source: WPI

It was also found that the surface roughness had an exponential effect on the corrosion current density and calculated corrosion rate. A rough surface led to a higher corrosion rate, but a rough surface is known to enhance osteointegration. Therefore, surface roughness needs to be adjusted, based on specific applications.

 

Microstructure: no significant change in the α lath thickness
Phase identification: narrowing of α characteristic peaks (reduced microstrain), evolution of β phase
Source: WPI

The effect of porosity was analyzed by using a crevice corrosion test. After a one-month immersion in Ringer’s solution at body temperature, pits were found on the Ti-6Al-4V sample surface near the pores in the as-printed samples, which was due to the formation of localized O2 concentration cells near the pore. Porosity in the as-printed parts was confirmed to impair crevice corrosion resistance. To reduce porosity, HIP was applied at three different temperatures. Based on polarization tests and electrochemical impedance spectroscopy tests, different degrees of reduction in porosity and corrosion-current density were observed in samples after HIP; this reduction was most significant after high-temperature HIP at 799°C (1470°F).

In summary, it was found that high temperature heat-treatment enhanced the corrosion resistance of DMLS Ti-6Al-4V parts. HIP was effective in reducing porosity and improving corrosion resistance. HIP below the annealing temperature (799°C, 1470°F) was recommended as a post-treatment for DMLSprintedTi-6Al-4V, to achieve a good corrosion resistance.

References

[1] “Metal Implants and Medical Alloys Market – Global Forecast to 2024,” 2019. https://www.marketsandmarkets.com/Market- Reports/metal-implant-medical-alloy-market-256117768.html.

[2] R. Bothe, et al., “Reaction of bone to multiple metallic implants.” Surgery, Gynecology and Obstetrics, 1940, 71:598–602.

[3] M. Sarraf, E. Rezvani Ghomi, S. Alipour, et al., “A state-of-the-art review of the fabrication and characteristics of titanium and its alloys for biomedical applications,” Bio-des. Manuf., 2022, 5, 371–395. https://doi.org/10.1007/s42242-021-00170-3.

[4] L.-C. Zhang and L.-Y. Chen, “A Review on Biomedical Titanium Alloys: Recent Progress and Prospect,” Adv. Eng. Mater., 2019, 21: 1801215. https://doi.org/10.1002/adem.201801215.

[5] L. E. Murr, S. A. Quinones, et al., “Microstructure and mechanical behavior of Ti–6Al–4V produced by rapid-layer manufacturing, for biomedical applications,” Journal of the mechanical behavior of biomedical materials, 2009, 2(1), 20-32. https://doi. org/10.1016/j.jmbbm.2008.05.004.

[6] A. Hung Dang Nguyen, A. K. Pramanik, Y. Basak, C. Dong, S. Prakash, S. Debnath, I. S. Shankar, Saurav Dixit Jawahir, and Budhi Dharam, “A critical review on additive manufacturing of Ti-6Al- 4V alloy: microstructure and mechanical properties,” Journal of Materials Research and Technology, 2022, 18: 4641-4661. https://doi.org/10.1016/j.jmrt.2022.04.055.

[7] “Direct Metal Laser Sintering (DMLS) Technology,” Additive News. https://additivenews.com/direct-metal-laser-sintering-dmlstechnology/.

[8] O. Cissé, O. Savadogo, M. Wu, and L’H Yahia, “Effect of surface treatment of NiTi alloy on its corrosion behavior in Hanks’ solution.” Journal of Biomedical Materials Research, 2002, 61/ 3 :
339-345. https://doi.org/10.1002/jbm.10114

[9] Sara A. Atwood, Eli W. Patten, Kevin J. Bozic, Lisa A. Pruitt, and Michael D. Ries,”Corrosion-induced fracture of a double-modular hip prosthesis,” The Journal of Bone & Joint Surgery, 2010, 92/ 6: 1522-1525.

[10] Y. Xu, Y. Lu, K.L. Sundberg, et al., “Eff ect of Annealing Treatments on the Microstructure, Mechanical Properties and Corrosion Behavior of Direct Metal Laser Sintered Ti-6Al-4V,” J. of Material Eng and Perform, 2017, 26: 2572–2582. https://doi.org/10.1007/ s11665-017-2710-y

[11] Ibid.

[12] Z. Yang, Y. Xu, R. D. Sisson, & J. Liang, “Factors Influencing the Corrosion Behavior of Direct Metal Laser Sintered Ti-6Al-4V for Biomedical Applications,” Journal of Materials Engineering and Performance, 2020, 29/6: 3831-3839.

About the Authors

Professor Richard Sisson is a key heat treat researcher and lecturer at Worchester Polytechnic Institute. His main research interest is the application of diffusion and thermodynamics to the solution of materials problems. Currently, he is working on modeling the surface treatment of steels and the postprocessing of AM ceramics and metals. His research endeavors have resulted in over 300 publications and over 300 technical presentations.

Dr. Yangzi Xu is currently working at Intel Corporation as a Yield & Module Process Engineer. She received her PhD at Worcester Polytechnic Institute (WPI) and focuses her research on understanding the mechanical and electrochemical properties of AM Ti alloys with different types of heat treatments, and their corrosion performance in biofluid for potential orthopedic applications. Her background includes research in polymer and food science and engineering.

Professor Jianyu Liang is a Professor of Mechanical and Materials Engineering at Worchester Polytechnic Institute, with affiliated appointments in the departments of Civil and Environmental Engineering, Chemical Engineering, and Fire Protection Engineering. Her research work on nanomaterials, AM, agile manufacturing, machine learning for materials science and manufacturing engineering, and sustainability has been funded by NSF, NASA, DoD, ED, and industry. Her work has resulted in over 300 research papers and technical presentations. As an educator, Liang strives to equip students with the confidence, enthusiasm, knowledge, and skills to allow them to enjoy learning throughout their lives.

For more information

Department of Mechanical and Materials Engineering Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609 Or email jianyul@wpi.edu and sisson@wpi.edu

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Corrosion Behavior of DMLS Titanium Alloy for Orthopedic Applications Read More »

Hot Take on HIPing

/ FEATURED NEWS, HIPING TECHNICAL CONTENT, MEDICAL HEAT TREAT, MEDICAL HEAT TREAT TECHNICAL

OCHot isostatic pressing. . . What is it? How is HIPing benefiting the medical industry? What is its place in additive manufacturing? In today's Technical Tuesday, Heat Treat Today is doing a deep dive into HIPing and its benefits. Check out these resources for some hot takes on HIPing.

Can You HIP It? Investigating Hot Isostatic Pressing

What exactly is HIPing? It's taking over the additive manufacturing world. In this article, written in 2020 by Derek Denlinger, corporate lead metallurgist at Paulo, find out the answer and also discover the applications, materials, and advantages of HIPing.
"HIP was initially developed as a diffusion bonding technique. In diffusion bonding, high heat and pressure work together to weld similar or dissimilar metal surfaces without filler materials."

Free ebook — High Pressure Heat Treatment: HIP

Product efficiency, reduced environmental impact, and improved process reliability are becoming more and more important everyday. HIPing's future has never been brighter. It's about to see a renaissance. To explore HIPing in depth, read this free ebook from Heat Treat Today and Quintus Technologies. 

"Modern HIP machinery is an extremely good fit with the traditional heat treatment market, offering the opportunity to further adjust material properties through tailored HIP cycles."

Hot Isostatic Pressing for Orthopaedic Implants

Check out what Chad Beamer and Magnus Ahlfors at Quintus Technologies had to say about HIPing. Shrinkage, gas porosity, and lack of fusion between layers are all things that do not belong in medical implants. Implants manufactured with metal injection molding and casting often still contain defects, but HIPing eliminates those defects and produces a 100% dense material. HIPing is widely used across the medical industry to reduce the occurrence of these issues.

"The elimination of defects results in improved fatigue properties, ductility, and fracture toughness. For this reason, HIP is widely used for orthopaedic implants like hip, knee, spine, ankle, wrist as well as dental implants to ensure quality and performance and prevent early failure of the implant inside the patient."

Heat Treat Radio: Hot Isostatic Pressing – Join the Revolution

High temperatures, high pressures. That's HIPing. Cliff Orcutt of American Isostatic Presses, Inc. describes HIPing as "pressurize sintering." Because of the high pressure, HIPing is faster and leads to less part deformation. In this episode of Heat Treat Radio, learn the many applications of HIPing (including ceramics) and learn if outsourcing is right for you. 

"In HIP, since you’re starting with powders that are solid, you can blend things like graphite powder and steel. You couldn’t blend them very well in a molten state, but in here, you can. And, you can squeeze it to solid, you can get interlocking and bonding and diffusion bonding materials that you couldn’t otherwise.  So, you can make things you couldn’t make any other way."

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Hot Take on HIPing Read More »

Fringe Friday: Amazing Medical Machined Metal

/ FEATURED NEWS, MEDICAL HEAT TREAT, MEDICAL HEAT TREAT TECHNICAL

Source: Global Medical via LinkedIn

Sometimes our editors find items that are not exactly "heat treat" but do deal with interesting developments in one of our key markets: aerospace, automotive, medical, energy, or general manufacturing. To celebrate getting to the “fringe” of the weekend, Heat Treat Today presents today’s Heat Treat Fringe Friday best of the web video that announces and describes how a thoughtfully designed and machined medical implant allows orthopedic surgeons to increase their precision in treating a variety of distal femur fractures.

Read more at: "Introducing the ANTHEM. . ."

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Fringe Friday: Amazing Medical Machined Metal Read More »

Exploring Thermal Sensors in Hydrogen Atmosphere for Turbines and Other Applications, Part 2 of 2

/ FEATURED NEWS, MEDICAL HEAT TREAT, MEDICAL HEAT TREAT TECHNICAL, THERMOCOUPLES TECHNICAL CONTENT

OCSome thermal sensing systems are not able to measure the lower end of the spectrum, while other systems are not able to measure the higher end. In Part 1, we learned how Nanmac and Rhenium Alloys, Inc. worked together to discover a thermal sensing system in hydrogen atmospheres that answered these issues.

In Part 2, explore thermal sensors in hydrogen atmospheres for temperatures above 2642°F to discover if ceramics can reach 4000°F. Can these ultra-high temperature systems be built commercially?

Today's Technical Tuesday was written by Herbert Dwyer, chief technical officer of Nanmac and president of Herb Dwyer & Associates, LLC.; Todd Leonhardt, metallurgist and director of Research & Development at Rhenium Alloys, Inc.; and Joe Johnson, senior metallurgical technician at Rhenium Alloys, Inc. This article was originally published in Heat Treat Today’s March 2022 Aerospace Heat Treating print edition.

Joe Johnson
Senior Metallurgical
Technician
Rhenium Alloys, Inc.
Photo Credit: Rhenium Alloys

Herbert Dwyer
CTO, Nanmac
President, Herb Dwyer & Associates

Todd Leonhardt
Metallurgist and Director of Research & Development
Rhenium Alloys, Inc.

Introduction

Temperature sensors for use in stationary or aviation turbines and/or test stands must also work in high wind shear, thermal shock, mixed gas environments and vibration which add more challenges. Key sections of the turbine, that contribute to the increased efficiency of the turbine operation, require higher temperatures than the traditional 2642°F (1450°C) to be measured. No thermocouple exists that can make these measurements reliably today. Theoretically, the temperature has been calculated to be near 4262°F (2350°C).

While there is significant history of using optical pyrometers above 2642°F (1450°C), the optical pyrometer is not practical on the actual turbine or in the test stand. The PIWG (Propulsion Instrumentation Working Group) consortium developed a matrix that added a requirement for directly measuring the hot section of the turbine to 4262°F (2350°C).

Figure 1. W5Re/W26Re wire with an alumina insulator and molybdenum sheath
Photo Credit: Herb Dwyer and Rhenium Alloys

To get the most useful result, we combined the insulator (a form of ceramic), the sheath (molybdenum), and the Type C wire, tested them and then performed a full lab analysis after each test period. The various step temperatures are shown in Figure 1 and started at 3362°F (1850°C) and the exposure time varied from one to six hours and compared this thermocouple assembly to a calibrated pyrometer in the same hydrogen-based atmosphere furnace. Our previous testing showed that a better understanding of the interactions between these materials was critical to longer life and performance at these UHT ranges. Our lab analysis also looked at both the mechanical and the chemical properties of these interactions as well.

This turns out to be a significant challenge because of the interaction of the material systems that cause the resulting eutectic temperatures to be much lower than their individual temperature ratings. This includes: the ceramic insulators, refractory metal sheath, and W-Re wires. A key question now is, are we approaching the material systems maximum capabilities? Further testing up to 4172°F (2400°C) is planned in 2022 to determine that answer. In addition to the material requirements, the real questions include: how accurate are these direct reading thermocouples and can they be calibrated at these UHT (Ultra-High Temperatures); what is their overall life and what are the drift factors?

Generally, the ASTM E230 Table for the Type C wire shows an accuracy of +/- 1% up to the maximum of 4199°F (2315°C). Earlier tests by the National Institute of Standards and Technology (NIST) showed that the typical accuracy of the Type C assembly (in this case the wire and its insulators only, not the sheathed versions) above 3182°F (1750°C) starts to degrade from those shown in the E230 Table. The accuracy may be closer to +/- 1.5% which at these temperatures may become a critical determiner for the life and maintenance costs. While we used a Type C wire supplied by a highly recognized manufacturer for our test program, our emphasis was to address the insulator since it was the weaker link in the overall assembly.

Nanmac set a goal to achieve the ASTM E230 Table accuracy of +/- 1% or better up to the maximum of 4199°F (2315°C) with the insulator life being analyzed. In addition, they have set their sights on developing a direct comparison measuring system by using a NIST traceable optical pyrometer calibrated and a NIST traceable thermocouple for Ultra-High Temperature measurements. NIST has previously used this type of system.

Experimental Setup

The temperature measuring experiments used a 33 KVA Spectra-Mat furnace which has three tungsten rod elements for heating as shown in Figure 2. The outer bell and pedestal are water cooled to prevent overheating during operation. The heating elements are surrounded on the outside diameter by a multilayer of 0.009” thick molybdenum sheet as shown in Figure 3. The NIST traceable thermocouple is fed through the pedestal and attached via molybdenum clamp above the pedestal shown in Figure 4. The thermocouple is centered in the hot zone to provide uniform temperature. The calibrated NIST traceable optical pyrometer is set at a specific distance from the quartz window imbedded into a water-cooled bell. The optical pyrometer is aimed 1.0” below the tip of the thermocouple and the emissivity is set for molybdenum, since the thermocouple sheath is molybdenum as shown in Figure 2. The optical pyrometer output was adjusted for the reflection angle, spot location, spot size, and to ensure that it was perpendicular to the assembled thermocouple, because these were identified as the critical variables for this calibration process.

By running the optical pyrometer/thermocouple experiment several times, it was demonstrated that this combination of a thermocouple and pyrometer can give reliable and repeatable comparative data as shown in Figures 5 through 7.

The furnace is increased in power in timed set points (steps) which show up on the graph as temperature versus time. Experiments were run at 2912°F (1600°C) and 3632°F (2000°C). As shown in the graphs, the yellow line (thermocouple) tracks these step point changes as the power is increased. The blue line (optical pyrometer) starts at 1472°F (800°C) since the optical requires color to measure temperature. After 1472°F (800°C) both thermocouple and optical pyrometer track temperature well. At the 2912°F (1600°C) the optical pyrometer was reading slightly lower temperatures than the thermocouple in all experiments at the higher temperatures.

It is believed that this is caused by the thermocouple reading the combination of convective, conductive, and radiated thermal energy while the pyrometer is responding to the radiated energy as potentially attenuated by the hydrogen gas atmosphere.

While the Type C matched legs (W5Re/W26Re) wire, according to the ASTM E230 Table, covers a range from 32°F to 4199°F (0°C to 2315°C) the initial test furnace was limited to a range of 1472°F to 3632°F (800°C to 2000°C) due to the type of insulator being used in this experiment.

The next set of experiments will have an operational temperature of 3992°F (2200°C) with the same ramp rate holding the same variables. In the near future, Rhenium will use a front loading Centorr furnace which is rated up to 4532°F (2500°C), but for now, tests up to 3992°F (2200°C) are planned in 2022. The experiments helped to identify key elements of the assemblies and suggested additional long duration tests that will address each element in more depth. These experiments are ongoing as of this Part II article and further reports of the results will be published over the next year.

Our tests used different types of insulators rated at these temperatures and after the tests, we performed a cross sectioned lab analysis to determine the interaction of these insulators with the wire and sheath.

Comparison of the Assembled Thermocouple and the NIST Traceable Pyrometer Output Curves

As the curves indicate, it is possible to directly compare the output of the Type C based thermocouple to the NIST traceable pyrometer, and, after some experimentation, we were able to develop a repeatable process which showed that the pyrometer tracked the assembled thermocouple.

Using this approach, we more realistically determined the actual accuracy of the assembled thermocouple under UHT and hydrogen atmospheres. Our life testing has achieved 100+ hours and these tests continue.

Typical Applications of These Types of Assembled Thermocouples

  1. Ultra-high temperature sintering and alloying of unique material(s) is used in turbine blades; hypersonic vehicles; space craft; nuclear reactors.
  2. Ultra-high temperature furnaces where critical temperature measurements at these elevated temperatures are important (tantalum materials for capacitors etc.).
  3. Very high temperature section of the stationary or aviation turbine where true temperature measurements, at UHT up to 4262°F (2350°C) can help to plan maintenance, contribute to life cycle calculations, and enable the optimization of the turbine’s combustion efficiency.
  4. Replacement of the Type S (platinum-rhodium), Type R (also a platinum-rhodium), and a Type B (platinum-rhodium). These are short lived at temperatures above 3002°F (1650°C) (and, in the case of the Type B, has a limited lower temperature measuring range of 1112°F (600°C) due to its low millivolt output).
  5. The Type C output at 1112°F (600°C) is 10.609 MV or almost 6x greater than that of the Type B (1.792 MV) allowing more accurate temperature measurement. The Type C at 572°F (300°C) has an output of 4.865 MV or almost 3x that of the Type B at 1112°F (600°C) enabling a wider temperature measuring range at a much lower total cost and a more robust temperature measurement.

About the Authors:

Herbert Dwyer is the CTO of Nanmac, and president of Herb Dwyer & Associates, LLC. Herb specializes in international business development, electromechanical manufacturing, heat treating furnace optimization, and thermal measurements up to 4172 °F. Herb has over 50 years of experience in the field of thermal and pressure sensors for the aerospace industry.

Contact Herb at herbdwyer1@gmail.com

Todd Leonhardt, a metallurgist and director of R&D at Rhenium Alloys, Inc., possesses an in-depth knowledge of high temperature refractory metal and is an expert in rhenium. As a 38-year veteran of industrial and government research in the areas of material characterization and processing refractory metals, Todd has shared his knowledge in over 25 publications including NASA technical memorandum, peer review journal articles, and conference proceedings.

Contact Todd at Todd.Leonhardt@rhenium.com

Joe Johnson is the senior metallurgical technician at Rhenium Alloys, Inc. and has been working with refractory metals, specifically rhenium and its alloys, for over 15 years. While his background is in material processing, most of his tenure has involved process metallurgy. In addition to co-authoring several technical publications, Joe enjoys performing failure analysis and designing custom tools and equipment.

Contact Joe at Joe.Johnson@rhenium.com or 440.309.2098

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Exploring Thermal Sensors in Hydrogen Atmosphere for Turbines and Other Applications, Part 2 of 2 Read More »

Terrifically Titanium Heat Treating Results

/ AEROSPACE HEAT TREAT, FEATURED NEWS, MEDICAL HEAT TREAT, MEDICAL HEAT TREAT TECHNICAL, TITANIUM PROCESSING TECHNICAL CONTENT

Source: Total Materia

Heat treaters in the medical and aerospace sectors will eagerly tell you about titanium alloys. The hot alloy can be fantastic for intense applications once you reduce residual stresses that are developed during fabrication and increase their strength. This article is specifically about how duplex heat treatment of Ti alloys helps in relieving stress, optimizing ductility and machinability properties, and increasing strength.

An excerpt:

“Most commonly known for their excellent strength, corrosion resistance and low density, titanium alloys are a key material for important applications in the aerospace and medical industries. Duplex heat treatments of Ti alloys helps in stress relieving, optimizing ductility and machinability properties and increases strength further.”

Read more at “Duplex Heat Treatment of Titanium Alloys: Part One“

Terrifically Titanium Heat Treating Results Read More »

Exploring Thermal Sensors in Hydrogen Atmosphere for Turbines, Part 1 of 2

/ FEATURED NEWS, MEDICAL HEAT TREAT, MEDICAL HEAT TREAT TECHNICAL, THERMOCOUPLES TECHNICAL CONTENT
OC

Direct thermal measurement of temperatures within a turbine is limited due to many factors. Some thermal sensing systems are not able to measure the lower end of the spectrum, while other systems are not able to measure the higher end. In this article, learn how Nanmac and Rhenium Alloys, Inc. worked together to discover a thermal sensing system in hydrogen atmospheres that solved these issues and more.

Today's Technical Tuesday was written by Herbert Dwyer, chief technical officer of Nanmac and president of Herb Dwyer & Associates, LLC., and Todd Leonhardt, metallurgist and director of Research & Development at Rhenium Alloys, Inc. This article was originally published in Heat Treat Today’s December 2021 Medical & Energy print edition.

Introduction

Todd Leonhardt
Metallurgist and Director of Research & Development
Rhenium Alloys, Inc.
Herbert Dwyer
CTO, Nanmac
President, Herb Dwyer & Associates

Direct thermal measurement of temperatures within the turbine (both fixed and aviation) and on the test stand, has been limited to 2642°F (1450°C). This uses a precious metal thermocouple composed of platinum (Pt) and rhodium (Rh) which are very expensive and have limited life above 3182°F (1750°C).

The conditions within the turbine also limit the choice of direct measurement systems due to the combustion by-products, wind speeds, pressures, shear forces, vibration, and thermal shock.

The recent focus on “green energy” gases that are more friendly to the atmosphere and offer excellent energy density per volume of gas points to a gas that has been around for many years — hydrogen. However, the use of hydrogen as a combustion gas within the turbine can be challenging as well. Molecular hydrogen is generally smaller than molecular oxygen. The by-product of the combustion of hydrogen and oxygen forms water. Water vapor is more climate friendly than carbon monoxide, carbon dioxide, or other forms of carbon found in turbines using standard jet fuels, natural gas, or combinations. Another challenge is the wide variety of temperatures to be measured at various points within the turbine, from inlet air to combustion to outlet air, the range can be from -22°F (-30°C) or lower to a predicted high of 4172°F (2300°C). No one type of thermal measuring system exists today that covers this total range. With this as a baseline, Nanmac and Rhenium Alloys, Inc. worked together to explore various combinations of material systems that could operate at the lower temperatures, plus reaching the upper temperatures of 4172°F (2300°C).

The system that could cover this range was the Type C thermocouple with a 5% tungsten and 26% rhenium wire composition. One key part of the system is the insulator which separates the two legs of the thermocouple, the second key part is the refractory sheath like tantalum or molybdenum. The range of temperature covered by this combination was from about 662°F (350°C) to 4172°F (2300°C). The actual testing temperature was performed from 1292°F (700°C) to 3992°F (2200°C).

Application and Testing Objectives:

  • Fixed and aviation turbines (includes direct mount and test stands)
  • Test temperatures above the 2642°F (1450°C) range
  • 100% hydrogen atmospheres
  • Thermal shock issues
  • Life issues at elevated temperatures and stress
  • High shear stress caused by air flow
  • Low and high frequency vibrations
  • Mounting options to accommodate space issues
  • Atmospheric corrosion caused by particulates in combustion gases
  • Cold junction transition location
  • Mounting depth

Objectives, Equipment, Assembly, and Test Times

Of concern, as atmospheres approach the ultra-high temperatures (UHT) region above 2642°F (1450°C), there are materials interactions between the components of the thermocouple (sheath, insulators, and thermocouple wires) and the furnace environment (representing the combustion section of the turbine) at elevated temperatures. Individually, the materials have high melting points, but combining these materials within the thermocouple system can cause low melting point eutectic to form a reaction between materials to occur. These material interactions can cause the thermocouple to fail prematurely in service at unexpectedly lower temperatures than predicted.

During Nanmac’s material compatibility testing, interactions between the sheath, insulators, and Type C thermocouple wire occurred. The weak link in the thermocouple system is the high temperature insulators of hafnia, alumina, and boron nitride. As the temperature approached the 3812°F (2100°C) test temperature, the insulators decomposed. Some of the observed failures appeared to be due to the hydrogen gas penetrating the end closure welds or even through the sheath walls; some of the insulators failed at these temperatures at much lower levels than expected. Some failure modes were caused by the insulator melting and attacking the thermocouple wires leading to fractures of the junction welds and the individual thermocouple wires.

Figure 1. The hafnia insulator failed above 3542°F (1950°C)
Photo Credit: Herb Dwyer & Associates, LLC
Figure 2. 99 alumina failure above 3542°F (1950°C) during the transition to 3812°F (2100°C) 6 inches back from tip
Photo Credit: Herb Dwyer & Associates, LLC

Nanmac and Rhenium Alloys, Inc. used hydrogen cover gas because of access to a furnace which used that atmosphere. The use of hydrogen as a future combustion gas gave insight into how these material systems would operate in that harsh environment. The furnace also gave insight into the thermal shock issues from quenching the furnace with hydrogen gas for rapid cooling which allowed for a quick turnaround in testing. Additionally, the processes helped evaluate the possible impact of 100% hydrogen atmosphere on the insulator’s materials, wire junction welds, and sheath end closure weld. The furnace used for compatibility testing used a calibrated control system with a reference thermocouple and a calibrated optical pyrometer.

The test assemblies for compatibility testing were smaller lengths of typical thermocouple systems composed of the 0.050” walled molybdenum sheath. The insulators tested included: hafnia (HfO2); 99% alumina (Al2O3), and boron nitride (BN insulator and Type C 24 AWG [0.020]) thermocouple wire assembly, which was back filled with argon gas to prevent oxidation of the components. Test durations were one hour, two hours, and six hours, at elevated temperature and a complete post-mortem evaluation was performed on all test articles to evaluate compatibility of the thermocouple components at UHT.

Discussion:

The ASTM E20 Committee is exploring the possible increase in calibration temperatures from the existing 2642°F (1450°C) to 3182°F (1750°C) or higher. While it appears the individual materials can achieve these and even higher temperatures, tests indicate that this is not the case for all the combined thermocouple components.

Discussion of some of the material issues includes:

1. As atmospheres reach 3182°F (1750°C) and up to 4172°F (2300°C), materials are limited to refractory materials like:

  • Molybdenum, tantalum, platinum, and other alloys of these materials.
  • Ceramics like 99% alumina, zirconia, hafnia, boron nitride, silicon nitride (SiC), and others were tested.
  • Wire materials are limited to some alloys of tungsten (W) and rhenium (Re).

2. Combining these materials also lowers the system’s overall temperature. For example, the boron nitride, on its own, can reach temperatures up to 5252°F (2900°C), but when combined with the Type C wire and molybdenum sheath, it can only operate reliably to 3632°F (2000°C). Figures 1 and 2 are examples of failures of ceramic insulator exposed to high temperature service conditions.

NOTE: During lab analysis, at the various temperatures, it was observed that decomposition and significant degrading of the insulators had occurred. The exposure to the UHT not only attacked the ceramic insulators but also attacked the Type C wire and its sheath. Part two of this article will show some of the pictures of this attack and discuss some of the approaches to address this material issue at these UHTs, above the threshold of 3182°F (1750°C).

3. Cost now becomes a significant driver. Some of these materials, including alloys of platinum and rhodium can cost upwards of $2,000 or more per inch, are very hard to machine and form, and can contain hidden cracks and voids that under these extreme temperatures lead to reduced mechanical life.

4. The operating atmospheres have a significant impact on the alloys used, and high carbon loads from unburned fuel can also impact these refractory materials, for example:

  • Oxygen attacks molybdenum, tungsten, and even tantalum, although to a lesser extent than the attack on molybdenum.
  • Hydrogen’s small molecules can attack the insulator by penetrating the welds; the insulators exposed to this reducing gas oxidize, melt, and shrink causing potential grounding, secondary junction(s), and further mechanical failure.
  • Nitrogen becomes a significant factor when used above 1832°F (1000°C).

5. The operating environment is not friendly to the following:

  • The need to directly insert into the combustion gas flow chambers exposes the tip to very concentrated thermal and mechanical forces.
  • Space restrictions limit the wall thickness and lengths (to resist H2 penetration and handle the extreme heat, thicker walls and an OD of ¼ inch or higher are required).
  • A turbulent air flow at speeds up to 400 mph.
  • Air pressures to 500 psi or higher.

6. The shorter the thermocouple (TC) length, additional thermal transfer issues are exacerbated. For example, four inch or shorter lengths can adversely impact any brazed joints from the TC to extension wire [(the temperature in this area where the tip may be at 3632°F (2000°C)], can be 1832°F (1000°C) or higher. Braze joints fail at much lower temperatures.

  • The transition at this point may also have insulation issues since it may be difficult to control the addition of an acceptable insulator in the transition area. The potential of secondary junctions is quite high (any significant mechanical movement (expansion and contraction) can cause high stress and weak insulators.
  • A technique using swaging has been somewhat successful but requires materials that can be swaged, limiting options to very expensive alternatives today. Some mineral insulated (MI) cables may be acceptable but need more testing.
  • The cold reference junction may be unacceptably close to the high temperature (1832°F or 1000°C), thus requiring a relocation further away from this point, requiring possibly a unique analog to digital converter (A/D) like those used in cars. This has not yet been fully developed at this point.
  • Nanmac is working on a method to carry the TC wire further into the test stand or turbine. This will address the transition issue, enable the use of existing A/Ds, and offer potential integration into the turbine engine itself.
  • By using this method (see d, above) on the test stand, it is possible to economically investigate this method, maintain safe operations, and make it useable once the system issues are resolved at the test stand.

The Type C was chosen for this temperature requirement (4172°F or 2300°C)

  1. Other than the Type D or A (both of which are in very limited supply and are basically of the same alloy construction), the Type C is well known and characterized, can operate up to 4172°F (2300°C), and has some significant history of use in this temperature realm [the tungsten (W)/Rhenium (Re) alloys are used regularly in high temperature metallurgical furnaces and even within the turbines].
  2. Type C is recognized by ASTM and NIST, its accuracy is 1%, and by comparison calibration Type C has been shown to be capable to about 0.5%, not the 0.25% of the Type S or R, but at a cost of 75% or less and its life, at elevated temperatures, is good and predictable.
  3. Type C has existing MI cable matching extension wire.
  4. Type C has existing A/D systems; thus, it is easier to integrate.
  5. Type C has existing connectors, color coded wire, and terminal connections.

Can other ceramics reach 4000°F (2204° C)? Can these ultra-high temperature systems be built commercially? To find out the answers to these questions, don’t miss the second part of this article in March’s (2022) Aerospace Heat Treating magazine and learn about the results, conclusions, and next steps.

About the Authors:

Herbert Dwyer is the CTO of Nanmac, and president of Herb Dwyer & Associates, LLC. Herb specializes in international business development, electromechanical manufacturing, heat treating furnace optimization, and thermal measurements up to 4172 °F. Herb has over 50 years of experience in the field of thermal and pressure sensors for the aerospace industry.

Contact Herb at herbdwyer1@gmail.com

Todd Leonhardt, a metallurgist and director of R&D at Rhenium Alloys, Inc., possesses an in-depth knowledge of high temperature refractory metal and is an expert in rhenium. As a 38-year veteran of industrial and government research in the areas of material characterization and processing refractory metals, Todd has shared his knowledge in over 25 publications including NASA technical memorandum, peer review journal articles, and conference proceedings.

Contact Todd at Todd.Leonhardt@rhenium.com

Exploring Thermal Sensors in Hydrogen Atmosphere for Turbines, Part 1 of 2 Read More »