As a common issue in the production process, strain on stamping parts is prevalent in major automobile factories.
On one hand, it decreases stability and efficiency of the production process and raises the rate of defective parts. On the other hand, it leads to increased wear on the die, shortening its lifespan and reducing the accuracy of the stamped parts, resulting in more frequent die repairs and causing production downtime.
The essence of galling occurs when the workpiece and the die surface become partially adhered (occasionally).
There are several methods to address the issue of galling.
The fundamental idea is to alter the friction characteristics between the die and the machined parts, such that they are replaced with materials that are less prone to adhesion.
After the mold enters the production site commissioning stage, the following methods are generally used to improve the galling problem:
1. Change the mold material and enhance its hardness by increasing its chromium content.
2. Treat the surface of the mold to improve its durability and wear resistance, such as through hard chromium plating, physical vapor deposition (PVD), and thermal diffusion (TD).
3. Apply a nano coating to the mold cavity using advanced technologies such as RNT, to further improve its wear resistance and reduce friction.
4. Add a separating layer between the mold and the processed parts to prevent direct contact, such as applying a lubricant or special lubricant, or using a layer of PVC or other materials.
5. Consider using self-lubricating coated steel plates for the mold to reduce friction and wear.
When it comes to mold materials, SKD11 and Cr12MoV, which are wear-resistant and anti-bite, are widely used. These materials can attain a hardness of around HRC58-63 after heat treatment and are suitable for molds with simple shapes and small parts.
However, the material becomes difficult to process after heat treatment, and is prone to cracking due to its brittleness. In addition, these materials are expensive and have limited size options, as well as large deformations after heat treatment. Furthermore, extensive research and matching work are required after heat treatment.
The shapes of automobile inner panels are becoming increasingly complex and high-strength steel plates are being used more frequently. This results in higher requirements for the overall performance of the mold.
Therefore, the inlay structure is usually adopted for such products.
The current methods of surface treatment for inlays include TD (thermal diffusion carbide coating process), hard chromium plating, nitriding, and PVD.
TD treatment, also known as the Toyota diffusion process or TD Process for short, was originally patented by the Toyota Central Research Institute in Japan in the 1970s. In China, it is referred to as the molten salt infiltration process.
The process involves placing the workpiece in a molten borax mixture and creating a metal carbide coating on the surface through high-temperature diffusion. Regardless of its name, the principle remains the same.
The main features of TD cladding treatment are:
The TD coating boasts high hardness (HV up to around 3000) and excellent properties, including high wear resistance, tensile damage resistance, and corrosion resistance.
With an estimated service life of 100,000 cycles, TD coating is a durable solution.
However, the application of TD coating has strict requirements for mold materials. The mold may deform or crack during high-temperature treatment due to changes in thermal stress, phase transformation stress, and specific volume.
Moreover, repairing a mold that has undergone TD coating treatment is challenging and often results in cracking at the weld. The processing quality and shape of the mold must meet stringent standards for TD coating to be effective.
Additionally, reprocessing after TD coating treatment is difficult and may not accommodate design changes or mold adjustments and repairs.
If a mold has undergone other surface treatments, it is imperative to completely remove them before TD coating to ensure proper surface quality.
Please note that after 3-4 cycles of TD coating treatment, the service life of the technology may decrease.
Fig. 2 TD processing
PVD (Physical Vapor Deposition) is a method of depositing a surface coating through the Physical Vapor Deposition process. The resulting coating is referred to as a PVD coating.
PVD coatings have excellent tensile damage resistance and a hardness that can range from HV2000-3000 or even higher, making it highly resistant to wear. The process also offers the benefits of a low treatment temperature, minimal deformation of the treated workpiece, and the ability to be repeated multiple times without affecting its service life.
However, one drawback is that the bond between the coating and the substrate is weak. When used on deep drawing dies or dies with high forming pressure, it can easily cause the coating to peel off, reducing its tensile damage resistance and wear resistance capabilities.
Fig. 3 PVD coating
The outer plate mold is typically large in size.
To avoid strain at the seams, the overall structure is preferred over the mosaic block structure. The most common materials used are ductile iron and other cast iron alloys.
The feeding part of the mold after flame quenching has a hardness of approximately HRC50-55 degrees.
Hard chromium plating is the most commonly used surface treatment for the outer plate mold of the overall structure, but it has limited surface hardening effects, resulting in a surface hardness of approximately 1000HV.
However, the hard chromium plated coating is prone to falling off when subjected to high forming pressure, as it is mechanically bonded to the base metal of the die. This can result in the loss of tensile damage resistance.
Additionally, once the surface hardening layer wears off, galling can occur again, reducing the service life of the surface hardening layer to approximately 5-10 years.
Fig. 4 chromium plating
RNT is a newly developed technology in recent years. Its working principle involves applying an RNT coating solution to the mold cavity, which diffuses nano molecules onto the mold surface to form a nano metal carbide coating. The process of expansion occurs from the inside to the outside, resulting in an increase in thickness and hardness with the increase in the working time of the mold.
The coating thickness ranges from 0.1 to 1 μm, with a hardness of HV1100-1600. The coating will not peel off even under heavy loads, due to the plastic deformation of the substrate, and its thickness and hardness will continue to increase with each working time and coating.
A single application of RNT coating can typically ensure 100-500 pieces without galling. However, the application of this technology to parts with significant roughening, parts that are heated during production, and ultra-high strength plates is not yet advanced, and its use cost is high.
Fig. 5 roughening before using RNT
Fig. 6 roughening after RNT
The use of appropriate lubricants in the production process can effectively improve friction conditions and prevent galling. The main purpose of lubricants is to separate the contact surfaces with a lubricating oil film. Typically, wire heads are lubricated either manually or with automated equipment.
The use of lubricants can also effectively reduce the risk of hidden damage and cracking. However, it can also contribute to a dirty and slippery work environment.
To mitigate the negative impact of lubricants on the work environment, iron and steel enterprises such as Baosteel, WISCO, and Masteel have developed self-lubricating steel plates in recent years. These steel plates have superior self-lubricating properties, as well as excellent resistance to corrosion, fingerprints, formability during processing, and paint performance.
The self-lubricating coating is achieved by rolling an organic coating onto the steel plates, eliminating the need for additional lubricating oil during the stamping process. However, the cost of using self-lubricating steel plates is slightly higher and their usage has not yet become widespread.
When selecting the most appropriate method to solve the problem of workpiece strain, it is important to consider the effectiveness of the solution, the batch size of the product, the feasibility of implementation, and the economy, in addition to the forming load and the type of material being formed.