July 29, 2008
Proper selection and use of tool coatings in HSLA and AHSS forming applications will extend tool life and yield the best part results.
Physical vapor deposition (PVD), chemical vapor deposition (CVD), and thermal diffusion (TD) coatings have been used successfully for years to enhance the productivity of forming tools. Almost all the available products provide enhanced performance when used in forming low-carbon steel.
However, the increasing use of high-strength, low-alloy (HSLA)—and more recently advanced high-strength steel (AHSS)—materials has resulted in a narrowing of the available coating options that can produce acceptable levels of productivity in these new, demanding applications (see AHSS Materials sidebar). These materials have placed a premium on matching the coating's features and capabilities with the application's mechanical demands to produce the desired tool performance.
The key is selecting the right coating for the application and material, which is based on the tool function, tool material, tool tolerances, tool design, and effectiveness of the lubricant.
For thin-film PVD, CVD, and TD coatings, AHSS forming applications are some of the most demanding to date.
The CVD and TD processes result in tools that are extremely hard, wear-resistant, and tough. Both are conducted at high temperatures (around 1,000 degrees C). The high coating temperatures necessitate rehardening and tempering of the tool steel material after coating to restore the desired mechanical properties. Additionally, the high temperatures and reheat treatment can cause material distortion, so careful consideration of dimensional tolerances is needed before choosing a CVD or a TD coating.
The processes differ in the manner in which the coating is created. CVD coating is done in a heat-treating furnace under a controlled atmosphere. A mixture of gases interacts with the substrate surface, resulting in the decomposition of some of the constituents of the gas mixture and the formation of a solid coating on the tool surface. Adhesion is maintained through a chemical bond between the coating and the substrate surface.
The TD process takes place in a furnace containing a molten salt. At the desired coating temperature, carbide-forming elements—namely, vanadium (V), niobium (Nb), and chromium (Cr)—diffuse into the surface of the tool steel and react with the carbon in the steel to form a coating both within and on top of the surface of the steel. The diffusion into the surface produces an excellent bond with the tool.
The PVD process can be described as the creation of vapors in a vacuum from solid material sources and their subsequent condensation onto a substrate. This line-of-sight process is intended to be conducted at lower temperatures (180 to 500 degrees C), and a physical bond is created between the coating and substrate rather than a chemical bond.
The PVD process is intended to be used below the tempering temperature of the tool steel to prevent softening or distortion. Not all coatings are routinely deposited at the same temperature, and not all coating suppliers employ the same process. Therefore, it is always important to compare the tool's heat-treatment history with the intended coating and process to ensure that the risks of softening and distortion are minimized.
Figure 1 provides a summary of the product types available. The coating designs and properties may vary somewhat from supplier to supplier.
The high tensile strength levels, work-hardening characteristics, and springback behavior of AHSS materials present unique challenges for coatings to deliver the expected performance enhancements. The combination of these characteristics results in significantly higher forming pressures for a coating to withstand compared to the same thickness of low-carbon steel.
For a coating to perform in AHSS forming applications, it must possess certain properties.
Toughness. The coating must be able to withstand the forming pressure without cracking. Toughness, which is difficult to quantify with thin-film coatings, should not be confused with hardness. Most coatings have hardnesses greater than 80 RC, yet not all coatings exhibit great toughness. Toughness is more related to coating type, coating design (single versus multilayer), and deposition method.
Smoothness. A smooth coating will enhance the metal flow within a die in a forming application. Predictable, consistent metal flow results in fewer tears in the material and less metal pickup on the die surface. Conversely, a rougher coating will inhibit flow, resulting in lower productivity.
Support. All coatings, regardless of type, require mechanical support from the die material to perform. The coating does not absorb any of the applied force in a forming application; it all gets transferred to the die material. If the die material deflects under the forming pressure, the coating can crack or begin to wear at an accelerated rate, which results in lower productivity.
Proper support can be attained by selecting a die material with an appropriate compressive strength level or by using case-hardening techniques, such as nitriding, with lower-grade die materials to boost the elastic compliance of the system when subjected to forming pressure.
Lubrication. One of the myths regarding traditional coatings like chromium nitride (CrN) or TD is that they act as lubricants. Actually, all traditional coatings require some type of lubrication to maximize performance. The amount of lubricant required can be reduced by producing an exceptional surface finish on the tool, both before and after coating.
For AHSS materials, it is even more critical that lubrication be used because of the higher amounts of energy required to form these alloys. The higher energy presents itself in the form of heat, so an external source of cooling (lubricant) is required, even if newer-generation coatings that have very low coefficients of friction are used.
Most coating suppliers would identify a single product from within their respective portfolio that they say works best in AHSS applications. However, from a customer perspective, it is better to think of multiple options for these types of applications, because each will have different operating conditions under which a coating selection is to be made.
Field tests of products in a variety of AHSS applications have identified a coating recommendation matrix (see Figure 2). The rankings are based on performance in these AHSS applications.
Selecting a coating for a particular application requires a comparison of the application's operating conditions with the customer's goals. Following are the operating conditions that must be evaluated, and the impact each has on the decision-making process:
1. Tool failure mode —The failure mode defines whether or not a coating—any coating—can help. Sometimes the solution lies with the tool material, the heat treatment of the tool steel, or the die setup conditions.
2. Tool material, heat treatment, and dimensional tolerances —An evaluation of these three issues will determine if high-temperature processes like CVD and TD can be used. If a PVD coating is required, an assessment can be made to determine if adequate support for the coating from the substrate will be present.
3. Tool function —Some coatings clearly perform better in certain tool functions than in others.
4. Workpiece material type and thickness —The material type and thickness will provide a relative measure of the forming pressure present. The higher the strength level and thicker the material, the higher the forming pressure. A higher forming pressure necessitates more consideration for the better or best solutions.
5. Die clearances —Die clearances also provide an assessment of the forming pressure. As die clearances are reduced, the forming pressure and friction level increase, which again necessitates more consideration of the better or best solutions.
6. Production volume —High production volumes suggest that more serious consideration should be given to the better or best solutions, while the good solutions are appropriate for low-volume applications.
7. Effectiveness of lubrication —Lubrication effectiveness is a critical component of the coating selection process. A highly effective lubricant enables all products with the matrix to be considered. On the contrary, less effective lubrication eliminates all but the best options from being considered and puts a premium on the surface finish on the tool before and after coating.
You cannot get to forming utopia by coatings alone. But choosing the proper coating for your AHSS application can extend tool life and improve tool performance.
AHSS materials are relatively new grades of steel alloys with composite microstructures containing different combinations of ferrite, austenite, and martensite that produce unique combinations of strength and formability.
While several grades of AHSS are available, the two most common categories are dual-phase (DP) and transformation-induced plasticity (TRIP) steels. DP steels have microstructures containing soft ferrite with varying concentrations of martensite, while the microstructure of TRIP steels contains ferrite, martensite, and retained austenite.
The soft phases of DP and TRIP steels give these materials excellent ductility and formability. When they are being formed, the martensite in DP steels and the martensite and retained austenite in TRIP steels have very high work-hardening rates. This leads to much higher ultimate tensile strength levels than those of conventional steels or even traditional HSLA materials.
This combination of formability and increased strength has led to higher usage of DP and TRIP steels in the automotive industry, where the material has improved crash test performance and the strength-to-weight ratio for better fuel economy. AHSS materials originally were used for structural components, but more and more they are being considered for other components, even body panels.
Even more so than HSLA materials, AHSS alloys present serious forming challenges. First, their high tensile strength requires significantly more force and energy in the forming operation than what are needed to form conventional steel and even HSLA grades. This force is compounded by the need for high blank holder forces to control the tendency of AHSS materials to wrinkle, and for overforming to accommodate their springback properties.
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