Laser heat treating overcomes issues of distortion that are frequent in conventional heat treating methods. Read this Technical Tuesday by Aravind Jonnalagadda (AJ), CTO and co-founder of Synergy Additive Manufacturing LLC, who examines how the automotive industry is achieving desirable dimensional tolerance while avoiding finishing operations like hard milling or grinding.
This informative piece was first released in Heat Treat Today’s August 2024 Automotive print edition.
Technology Overview
Laser heat treatment is a process in which a laser, with a typical spot size from 0.5” x 0.5” to 2” x 2”, illuminates the surface of a metal part to deliver very high energy flux with extreme precision both in time and geometry. This brings the metal’s surface up to the desired temperature very rapidly. Movement of the laser across the surface of the working piece produces hardened tracks.
The phase transformations induced by laser hardening of steels proceed according to the following stages:
- Formation of austenite from pearlite-cementite (hypereutectoid steels) or from pearlite-ferrite (hypoeutectoid steels) aggregate structure, during the heating stage.
- Martensite transformation from austenite, during the cooling stage.
During this process the short interaction time, in the range of 0.1–0.2 seconds, brings the surface temperature to 1337°F –2732°F (725°C –1500°C). Under these conditions, the original pearlite colonies transform into high-carbon metastable martensite due to self-quenching. This martensite phase increases the hardness.
Key Benefits of Laser Heat Treating
Consistent Hardness Depth: Laser heat treatment delivers consistent hardness and depth by precisely applying high energy to the metal. Millisecond-speed feedback control of temperature ensures specifications are met, as shown in the metallographic cross-section view of laser heat treated D6510 cast iron (Figure 3).
Minimal to Zero Distortion: The high energy density of laser heat treatment minimizes distortion, benefiting components like large automotive dies, gears, bearings, and shafts.
Precise Application of Beam Energy: The laser spot precisely heats the intended area, avoiding unnecessary heating of surrounding areas. This is particularly advantageous for surface wear applications, allowing for surface hardening while maintaining the rest of the material in a medium-hard or soft state, thus combining hardness and ductility.
No Hard Milling or Grinding Required: Laser heat treatment’s low-to-zero distortion reduces or eliminates the need for hard milling or grinding. Post-treatment material removal is minimal and can be managed with polishing. This reduction in finishing operations can save up to 20% in overall manufacturing costs.
Laser Heat Treatable Materials
Any steel with ≥ 0.2% carbon content is treatable by laser heat treatment. In real-world applications, the areas of dies that have been treated with laser heat treatment are generally as hard as, or harder than, the same areas of identical dies treated by conventional hardening treatment.
Common heat treatable automotive materials are indicated in Table 1. This is not a comprehensive list.
Cost Savings
In automotive tooling, the conventional practice is to mill the dies in soft state, intentionally leaving an extra 0.015” to 0.020” of material on the surfaces. This excess material acts as a buffer to accommodate distortions from subsequent heat treatments like flame or induction processes. After this initial phase, the dies undergo heat treatment and are then hard milled to achieve the specified tolerances before assembly.
An alternative method gaining traction, however, is laser heat treating (Figure 1). In this approach, the dies are machined to final tolerance from the beginning and then laser heat treated without causing distortions. This eliminates the need for a secondary hard milling operation. Automotive tool and die clients have reported cost savings exceeding 20% due to this streamlined process.
New Advancements
A Promising Application: Hardening Sharp Edges on Trim Dies
Within the automotive industry, trim dies hold a pivotal role in shaping sheet metal stampings (Figure 2). These dies are instrumental in cutting the metal sheets after forming operations. Typically, a trim die comprises numerous smaller steels assembled onto a die shoe. Ensuring the durability and hardness of these trim dies is imperative, as they must withstand considerable shear and fatigue loads.
Traditionally, heat treatment methods like flame or induction have been employed for treating trim inserts. However, these conventional techniques come with inherent drawbacks. Issues such as rolled edges and high heat input often lead to significant distortion in the dies. To compensate for this distortion, die makers commonly leave approximately 0.020” of stock material, which then requires hard milling to meet specifications. This process consumes substantial time and resources.
To address these challenges effectively, many die makers have recently turned towards laser heat treating for their trim inserts.
Multi-Point Temperature Control System (MPTC)
Another innovation has been the use of more advanced temperature control units. The need to overcome temperature control challenges led to the development of the Multi-Point Temperature Control System (MPTC). This system enables Synergy to regulate laser power and temperature distribution over the entire cutting edge, ensuring consistent and controlled heat treatment without melting the cutting edge.
Case Study: Press Brake Tooling Hardening
High precision press brake tools are essential for the metalworking industry, providing the necessary precision and durability for bending and shaping sheet metal. These tools are crafted from a variety of materials, including 4140, S7, A2, and D2 steels, each known for their unique properties and performance characteristics. However, hardening these tools presents significant challenges due to their lack of mass, which often leads to serious distortion, especially in longer pieces.
Traditional hardening methods can cause substantial distortion in press brake tooling. This is particularly problematic for long tools, where uneven heating and cooling can lead to warping. The need for precise dimensions and smooth operation in press brake tooling makes any level of distortion unacceptable, as it can affect the accuracy and quality of the final product.
Laser hardening of press brake tooling at Synergy has demonstrated remarkable results. For tools less than 10 inches in length, the recorded distortion is less than 0.001 inches. Even for longer tools, measuring up to 10 feet, the overall distortion was maintained at less than 0.050 inches.
Case Study: Hem Die Laser Heat Treatment
Hemming is a critical operation in the production process and has a significant impact on the overall quality and performance of a vehicle. Hemming involves bending the edge of a sheet metal over itself, and it is performed on various components such as hoods, doors, tailgates, and fenders. Hemming dies, also known as anvils, play a crucial role in this process and are compact compared to conventional stamping dies, but this presents a new set of challenges for die makers.
Conventional heat treating methods, such as induction and flame hardening, can cause substantial distortion in hemming dies and result in inconsistent hardness across the profile. Additionally, the dies require a great deal of post-machining to bring them back to the desired tolerance. This not only results in substantial cost but also adds time to the production process, leading to increased time to market (TTM).
Laser heat treating offers a solution to these challenges and helps to maintain the quality of hemming dies. With Synergy’s laser heat treating process, the die is laser heat treated after it is machined to its final dimensions, resulting in minimal to no distortion and consistent hardness. This eliminates the need for additional hard milling processes and helps to reduce the TTM. Extensive testing by Synergy’s clients has shown that laser heat treated anvils exhibit consistent hardness within ±1 HRC and do not require additional hard milling operations.
Case Study: Punch Pins Laser Hardening
Uniform laser heat treating of punch pins with distortion of less than 0.0005” can be achieved with laser heat treating on pins and other cylindrical components. A demonstration of this application on a 4140 alloy steel part is depicted in the Figure 6. Laser hardening resulted in a surface hardness of 60 HRC with a case depth of 0.010”.
Conclusion
The automotive industry increasingly requires precise, repeatable methods to not only meet standards but also remove steps for manufacturers creating these components. As the three case studies demonstrate, laser heat treating is a key tool that heat treaters should use to improve energy efficiency, avoid distortion, and increase overall quality.
References
Asnafi, Nader, Tuve Johansson, Marc Miralles, and Andreas Ullman. “Laser Surface-Hardening of Dies for Cutting, Blanking or Trimming of Uncoated DP600.” Recent Advances in Manufacture & Use of Tools & Dies and Stamping of Steel Sheets, Olofström, Sweden (October 5-6, 2004).
Beyer, E., F. Dausinger, A. Ostendorf, A. Otto. “State of the Art of Laser Hardening and Cladding.” Proceedings of the third International WLT-conference on Lasers in Manufacturing, (2005): 281–305.
Pashby, I.R., S. Barnes, and B. G. Bryden. “Surface hardening of steel using a high power diode laser.” Journal of Materials Processing Technology, University of Nottingham, Nottingham, UK b Warwick Manufacturing Group, University of Warwick, Warwick, UK,139, (2003): 585–588.
Jonnalagadda, Aravind and Brian Timmer. Great Designs in Steel Presentations: Laser Heat Treating of Automotive Dies for Improved Quality and Productivity. Michigan, 2021. https://www.steel.org/wp-content/uploads/2021/06/GDIS-2021_Track-3_08_-Jonnalagadda.Timmer_Laser-Heat-Treatment-of-Auto-Dies.pdf.
Selvan, J. Senthil, K. Subramanian, and A. K. Nath. “Effect of laser surface hardening on En18 (AISI 5135) steel.” Journal of Materials Processing Technology 91, 1–3 (June 1999): 29–36.
About the Author:
CTO and Co-Founder
Synergy Additive Manufacturing LLC
Source: LinkedIn
Aravind Jonnalagadda (AJ) has over 20 years of expertise in laser material processing. Synergy provides high power laser-based solutions for complex manufacturing challenges related to wear, corrosion, and tool life specializing in laser systems and job shop services for laser heat treating, metal based additive manufacturing, and laser welding.
For more information: Contact AJ at aravind@synergyadditive.com or synergyadditive.com/laser-heat-treating/.
