In 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.
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
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.
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.
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.
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.
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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