Over the past decade, the automotive industry has experienced rapid growth. The quality of cars has continuously improved, yet their prices have decreased.
As a result, reducing production costs has become a pressing issue for every car manufacturer. In the realm of metal stamping, one critical measure to cut costs is to ensure stable production.
Factors affecting stability include issues like wrinkling, cracking, and marking of stamped parts. The primary solution to these defects is to treat the surface of the dies.
Surface treatment of the dies can reduce their surface roughness, increase their hardness, and enhance their wear resistance. This not only boosts production stability but also reduces costs.
Causes of Production Instability
1. Marking:
During the forming process, when the metal sheet slides relative to the die, adhesive wear occurs due to the forced contact between the sheet and the die, leaving streaks on the sheet as shown in Figure 1.
Marking usually results from three main reasons:
- The hardness of the drawing die’s fillet reduces and becomes close to that of the sheet, causing them to stick together.
- The die or sheet is dirty, with hard foreign objects between them, scratching both during the sliding process.
- Fillet of the die is damaged or has cracks or pores left after welding, scratching the sheet as it flows over the fillet.
2. Cracking:
Generally, cracks can be categorized into strength cracks and plasticity cracks, as illustrated in Figure 2.
Strength cracks, also known as α cracks, occur when the strength in the force-bearing area of the sheet cannot meet the deformation force required in the deformation zone. An example is the crack generated at the punch fillet when forming a cylindrical part.
Plasticity cracks, or β cracks, arise when the sheet’s deformation capability in the deformation zone is less than the needed deformation level.
For instance, the cracks in the outer panels of car doors, where the draw bead locks the material in place, making it almost immobile.
Under these conditions, cracks are due to plastic deformation. Based on the load conditions, there are two types of cracks. One results from tensile stresses in both the X and Y directions, typically causing lateral cracks.
The other type occurs when one direction experiences tensile stress and the other compression stress, causing relative movement between adjacent metal lattices and resulting in longitudinal cracks.
3. Wrinkling:
During the drawing or forming process, parts experience complex and unpredictable forces.
Since sheet thickness is much less compared to its length and width, it’s highly susceptible to instability under tensile and compressive stresses, leading to wrinkles as shown in Figure 3.
Wrinkles can arise from compressive stress, uneven tensile stress, or shear stress. Compressive stress wrinkles appear perpendicular to the direction of the die opening, mostly at the corner of the die opening.
This is due to the compressive stress on the metal lattice in the direction parallel to the die fillet, leading to instability when the compressive stress surpasses the sheet’s limit.
Uneven tensile stress wrinkles commonly occur in the smoother regions of outer panels, showing up as shallow pits or waves. Shear stress wrinkles occur when adjacent metal lattices flow at different rates, causing misalignment and shear-induced wrinkling.
Solutions for Production Instability
To ensure production stability, the dies need to be tested and found satisfactory.
If, during production, the die surface wears out and the original processing equilibrium breaks, leading to instability, the die’s surface needs treatment to achieve a new stable state.
Measures for surface treatment include:
1. Polishing of Die Corner
During the stamping production process, dies should be cleaned and maintained periodically. This includes removing zinc and oil residues from the surface and inspecting the roughness of the die corners.
If a deterioration in roughness is detected, the die corners require polishing. At a minimum, 600-800 grit sandpaper should be used.
A rectangular grinding block is inserted underneath, and grinding paste is applied to the corner. Polishing should follow the material flow direction until the desired roughness is achieved.
2. Weld Repair of Die Corner
Serious local issues like burring, wrinkling, or cracking might arise during production.
Once equipment and process issues are ruled out, these problems generally arise from wear on the drawing corner, causing the hardness to drop below 40HRC or the appearance of cracks and damage in the drawing corner. The remedy is weld repairing the problematic areas.
(1) Before welding, the damaged area must be cleared. Use an angle grinder to remove a 20mm deep and 10mm wide chamfer, ensuring the edge of the chamfer exceeds the corner tangent point by at least 5mm.
(2) In selecting welding rod materials, the weld generally consists of two layers: an inner filler layer and an outer working layer. The filler layer’s welding rod should have mechanical properties consistent with the die body, especially in hardness and impact toughness, even if the material differs.
The working layer’s welding rod should fuse well with the die body, ensuring a robust bond that avoids cracks and undercutting. After welding, the hardness should reach post-quench levels of the die body, above 50RC.
Our drawing die body material is ductile iron, with the grade being GGG70L. The filler layer welding rod chosen is TC-3F, and NH-100R is used for the working layer, yielding satisfactory results.
(3) Welding speed should be consistent during the process, opting for intermittent welding.
During welding, it’s also necessary to hammer the area to eliminate gas pockets and internal stress, preventing cracks from significant temperature differences or internal stresses.
(4) After welding, CNC machining should be performed, leaving a 0.1mm allowance. The remainder is precision ground manually, ensuring the repaired area smoothly transitions with the original surface, without undulations.
Following the repair, a precision polish is necessary to guarantee the die’s flatness and roughness, maintaining maximum consistency with its original state.
3. Chromium Plating
Electroplating involves coating a metal surface with a thin layer of another metal or alloy, aiming to prevent metal oxidation, enhance surface hardness, and improve surface roughness.
Generally, the metal being plated acts as the cathode, and the plating metal as the anode. Through electrolysis, metal cations in the solution deposit on the base metal surface, forming a plated layer.
In the stamping industry, electroplating typically refers to hard chrome plating, which primarily improves the die surface’s hardness, wear resistance, and reduces surface roughness.
The electrolyte used is a chromic acid solution. A relatively thick layer of chrome is plated onto the base surface, with its thickness generally ranging from 10-20μm.
4. PPD Treatment
PPD refers to Pulse Plasma Nitriding, a lifetime wear-resistant treatment technology for automotive panel stamping dies, designed as a replacement for electroplating.
In a low vacuum environment (less than 2000Pa), with the furnace body acting as the anode and the metal product to be treated as the cathode, an electric current produces a voltage of several hundred volts between the two.
This causes nitrogen and other gas molecules to separate into nitrogen ions, hydrogen ions, and electrons. Nitrogen ions begin to react with the base ions on the mold surface, forming a hardened nitrided area.
After nitriding, an extremely hard layer of nitride alloy (Fe4N.Fe2N) is formed on the mold surface.
The nitriding layer typically achieves a hardness of 50~60HRC, offering high surface hardness and wear resistance. It’s suitable for most cast iron and cast steel materials used in automotive dies.
PPD treatment has many advantages:
(1) The surface can be treated with 800-grit or higher oil stone.
(2) Damaged surfaces can be welded.
(3) Welded surfaces can undergo high-temperature heat treatment without changing shape.
(4) Molds treated with PPD generally guarantee up to 500,000 stampings.
(5) After PPD treatment, molds can undergo subsequent treatments without additional process treatments.
(6) Environmentally friendly.
For PPD treatment of the mold body, the following requirements should be met:
- Clean the parts to be treated and remove inlays, screws, guide plates, and other attachments.
- Ensure welding quality with no voids, welding pores, or cracks.
- Any previous coating treatments need to be removed before PPD.
- For mold corner areas, the surface roughness should be Ra0.2-0.4um, and for flat areas, Ra0.4-0.6um.
Conclusion
By applying the five-element analysis method, the root causes of production instability and their corrective measures were identified.
This included weld repairs, grinding, and polishing of the drawing die corners, and hard chrome plating or PPD treatment of the mold surface. This ensured the mold corner’s hardness and roughness met technical requirements.
In subsequent production, issues like burring were essentially eliminated, with no signs of cracking or wrinkling. The performance was consistent.
The surface treatment processes and procedures of these three projects offer valuable insights for similar future projects.
