During the stretching process, two relatively sliding metal surfaces, the raw material and the mold cavity, move relatively under high pressure and high speed. Due to the plastic deformation and flow of the material, a bonding phenomenon similar to the initial friction welding occurs. However, this kind of adhesion is only local, and when multiple consecutive adhesion occurs at a certain point or piece, it can produce a binding tumor. The common adhesive bumps that occur on the straight wall of the concave mold can scratch the surface of the drawing part, causing deep and continuous scratches, worsening the surface quality of the drawing part, and producing defective products from the surface. Timely detection and removal of adhesive nodules is of great significance for improving the quality of tensile surfaces. Only by eliminating the adhesive phenomenon can the formation of adhesive tumors be avoided. It is particularly important to take different measures to reduce adhesion for stretched parts of different materials and shapes.
Factors contributing to tensile bond tumors: 1. The properties of mold materials: A small surface roughness of a mold has a high ability to resist adhesion. However, when the surface finish of the mold is too high, due to the difficulty in storing lubricating oil, and the increase in the number of surface contact points, the molecular adsorption force increases, which increases the friction coefficient and is not beneficial to resist adhesion. It is also important to select appropriate processing patterns for the parts of the mold surface that are prone to adhesion. Experiments have shown that the pattern processed by the mold is better along the direction of material movement than perpendicular to the direction of material movement, and the coefficient of friction can be reduced by 20%. 2. The properties of the material to be processed. Brittle materials have a stronger resistance to adhesion than plastic materials. Materials with a greater degree of mutual solubility (affinity) have a greater tendency to adhere, generate more heat from sliding friction, easily melt and weld locally, and have a high bonding strength. From the perspective of the metallographic structure of materials, multiphase metals have a lower tendency to adhere than single metals. Group A elements have a greater tendency to form friction and adhesion with iron. Materials with a greater tendency to adhere have a higher melting point than materials with a lower melting point have a greater ability to resist adhesion 3. During the deep drawing process, due to frictional expansion and plastic deformation, the kinetic energy and deformation work are converted into thermal energy, which increases the temperature of the friction surface. When the temperature reaches a certain value, the viscosity of the lubricant will decrease, the thickness of the lubricating oil film will decrease, and the strength will also decrease. In severe cases, the local lubricating oil film may rupture, forming direct metal contact, resulting in dry friction. At the same time, local high temperatures will also reduce the OS of submicroscopic protrusions on the surface of deep drawing materials, so the bonding trend will increase and promote the formation of bonding nodules. 4. Generally speaking, as the sliding speed increases, there is not enough time for heat dissipation, and adhesion is easy to occur. In addition, it is not difficult to see from the above formula that the size of the adhesive bead is directly proportional to the normal pressure and sliding distance, and inversely proportional to the yield stress of the soft material. During the deep drawing process, the position where the adhesive tumor first forms should be the position with the maximum normal pressure. 5. Surface oxide film, lubricating film or other surface facial mask. 6. Measures to reduce adhesion due to environmental factors Reduce the temperature rise of the contact surface Polishing the surface of the drawing punch and concave mold Reduce and balance the degree of tensile deformation Drawing materials and semi-finished products Use highly effective lubricants and reasonable lubrication.
