Metal die casting is an advanced less and no cutting technology, with high production efficiency, saving raw materials, reducing production costs, good casting performance, high precision, widely used. The biggest market of die-casting castings is the automobile industry. As people pay more attention to sustainable development and environmental protection, automobile lightweight is the best way to achieve high efficiency, safety, energy saving, comfort and environmental protection. Using aluminum alloy instead of traditional steel to make a car can reduce the weight of the vehicle by about 30%. Because the die casting mold is under high pressure (30 ~ 150MPa) to 400 ~ 166000 C molten metal die casting molding. During the molding process, the mold is periodically heated and cooled, and is scoured and corroded by hot metal sprayed at high speed. The die materials are required to have high thermal fatigue resistance, thermal conductivity, good wear resistance, corrosion resistance and high temperature mechanical properties. It is still difficult to meet the needs of constantly improving performance only by the application of mold materials. It is necessary to apply various surface treatment technologies to the surface treatment of die casting mold to meet the requirements of high efficiency, high accuracy and high life of die casting mold.
Die casting die failure form
- Die casting die failure mode Die casting die working with high temperature liquid metal contact, not only the heating time is long, and the heating temperature is higher than the forging die, die casting non-ferrous metal temperature 300 ~ 800℃, die casting black metal temperature of more than 1000℃. It also bears a high pressure of 30 ~ 150MPa, which is subjected to repeated heating and cooling as well as high-speed erosion of liquid metal flow and wear and corrosion, and is repeatedly heated and cooled, and the processing environment is harsh. According to the failure mode statistics, when 3Cr2W8V is used as die casting mold material, 65% is thermal fatigue, 15% is cracking, 6% is abrasion and 4% is erosion failure.
- Fatigue crack Thermal fatigue crack is the most common failure form of die casting die, accounting for a large proportion of the failure. In the process of die casting, the thermal cycle of die casting die at 300 ~ 8000C and the alternating cycle of tensile stress and compressive stress caused by the release agent repeatedly undergo the thermal stress caused by sudden cooling and sudden heat, resulting in the gradual emergence of micro cracks on the surface or internal thermal stress concentration of the cavity. Most of the morphology is mesh, called cracking, and some are radial. Thermal stress causes the thermal fatigue crack to expand into macroscopic crack. Thus lead to die casting die failure. Thermal fatigue crack is produced by the joint action of thermal cyclic stress, tensile stress and plastic strain. Plastic strain promotes crack formation and tensile stress promotes crack propagation and extension. From microscopic analysis, thermal fatigue cracks are initiated in the concentration area of carbides and inclusions at grain boundaries. High quality die steel with clean steel and uniform microstructure should be selected, which has higher thermal fatigue resistance
- Integral brittle cracking is the catastrophic fracture of die casting die caused by accidental mechanical or thermal overload. The stress value reached when the material is fractured is generally much lower than the theoretical strength of the material. Due to the existence of micro cracks, stress concentration will be caused after the stress is stressed, so that the stress at the crack tip is much higher than the average stress. There are many reasons for the brittle cracking of die casting die, and the plasticity and toughness of the material is the most important mechanical property of the box. The inclusions in die steel are reduced, the toughness is improved obviously, and the overall brittleness is less in production.
- Dissolution or erosion molten metal liquid at high pressure, high speed into the cavity. Intense impact and erosion on the surface of die casting die forming parts, resulting in mechanical erosion of the cavity surface, high temperature makes die casting die hardness decreased, leading to cavity softening, plastic deformation and early wear. , in the process of filling liquid and produce turbulence caused by cavitation erosion effect of particles or liquid scour, impurities in high temperature liquid metal and slag on the cavity surface complex chemical change, chemical corrosion, molten metal escape bubble cavitation in the cavity, a result of the combined action of the mechanical and chemical wear and tear are accelerating corrosion and crack on the surface of the generated. Improving the high temperature strength and chemical stability of the mold material is beneficial to enhancing the corrosion resistance of the material.
Factors affecting thermal fatigue
- Mold temperature affects the speed of die casting is very high, the pressure is very large, the mold surface is subjected to a strong impact load, the mold surface contact high temperature melt, its temperature up to 8700C, in such high temperature acute heat, the mold surface produces compressive thermal stress. Before each die casting, lubricant is sprayed into the die for rapid cooling, and tensile stress is generated on the surface of the die. When the alternating thermal stress exceeds the yield strength of the die surface, thermal fatigue micro-cracks are generated on the surface, which spread rapidly and spread to the center to form cracking. Will cause casting strain and mold adhesion, serious cause mold early cracking
- Basic Properties of materials Die casting in the die casting environment of rapid heat and cold, die casting die materials have the following requirements: (1) thermal fatigue resistance and thermal impact resistance, not easy to crack. (2) Good toughness and ductility, improve the impact resistance of the sharp corners and convex parts of the mold. (3) good thermal hardness, thermal strength, hardenability, wear resistance and high temperature resistance to oxygen. (4) Heat treatment deformation is small, thermal expansion coefficient is small and so on. Hot working die steels 3Cr2W8V and H13 (4Cr5MoSiVl), which are mainly chromium, tungsten and molybdenum, are commonly used in die casting dies. At present, the use of H13 steel die mold used extensively, and was mainly used for chromium alloying elements in steel, it has good toughness and thermal fatigue resistance and oxidation resistance, through the appropriate surface treatment, its service life can reach very high levels, has now become a mature, die-casting die steel being widely applied abroad by more than 90% of the casting cavity die H13 steel manufacturing. Mold material control is very important. H13 steel for die casting mould must be clean steel, uniform structure, slight segregation, good isotropy quality steel. A series of advanced technologies have been adopted in the production process of high quality H13 steel abroad, such as vacuum degassing, electroslag remelting and other refining technologies to improve the cleanliness, and then through multi-directional binding or repeated block forging and ultra-fine processing technology, H13 steel has excellent internal quality. The use of advanced smelting process to provide more high purity die casting die steel is the future direction
The heat treatment process of die casting die can change the metallographic structure of the material through heat treatment to ensure the necessary strength and hardness, dimensional stability at high temperature, thermal fatigue resistance and cutting properties of the material. The parts after heat treatment require less deformation, no cracks and minimize the existence of residual internal stress. The die-casting die generally adopts the vacuum gas quenching, surface oxide, mold deformation is small, better ensure the quality of the mould, the spheroidizing annealing process for forging _ _ a _ stabilizing treatment rough machining finish _ the final heat treatment (quenching and tempering) _ QianXiu polishing _ _ + nitriding (or carbonitriding) _ fine grinding or lapping _ assembly. H13 steel is treated with high temperature quenching, double quenching, controlled cooling speed quenching and cryogenic treatment, so as to improve the performance of the die and improve the life of the die.
Surface treatment of die casting die is the most effective and economical method to prolong the life of die casting die. The strength and toughness of steel are improved by adjusting the general heat treatment process. Using different surface strengthening processing technology, with suitable core performance, can give the die surface with high hardness, wear resistance, corrosion resistance, bite resistance and low friction coefficient and many other excellent properties, so that the life of the die increased several times or even tens of times. There are three main types of die surface strengthening: ① do not change the surface chemical composition, laser phase transformation hardening, etc.; ② change the surface chemical composition, nitriding, etc.; (3) Surface formation of overlay, gas deposition technology treatment, etc.
Do not change the surface chemical composition of enhanced laser enhanced treatment: laser as a heat source to strengthen the material surface, phase change hardening, surface melting, surface coating, etc. It is characterized by a power density of at least 103VC/em2 on the surface of the supplied material. The method of surface treatment of metal with high power and high density laser beam is called laser surface heat treatment. It can be divided into laser phase change hardening, laser surface alloying and other surface modification, resulting in the surface composition, microstructure and properties of other surface heating hardening can not reach the change.
In order to improve the surface performance, the die surface of laser cladding technology is covered with a thin layer of cladding material with certain properties. The hardness of H13 steel after conventional treatment is 44HRC, and the surface hardness can reach 772HV(equivalent to 62HRC) after laser quenching, and the depth of hardened layer is 0.63ram. The hardness, tempering stability, wear resistance and corrosion resistance of the quenched layer are significantly improved because the microstructure is dominated by ultra-fine and high-density dislocated martensite and the dispersed carbides are precipitated during the self-tempering process after laser heating. Laser cladding technology has great potential application value because of its high machining accuracy, small thermal deformation and small post-processing amount.
Edm surface strengthening: Edm surface strengthening is the use of the electrode and workpiece, the effects of the spark discharge in the gas between the conductive material for electrode melt into the workpiece surface, the formation of alloying surface strengthening layer, commonly used electrode materials have a TiC, WC, ZrC and cemented carbide, etc., because of the electrode material deposition occurred regularly, smaller grow up, improve workpiece surface physical and chemical properties. Such as cemented carbide electrode to strengthen the workpieces, surface hardness up to 1100~1400HV, strengthen the layer and matrix combination of firm usebieveryday.
Change the surface chemical composition to strengthen (1) carburizing. Carburizing is a surface chemical heat treatment process in which the steel is placed in the carburizing medium, heated to the single-phase austenite area, and held for a certain time to make the carbon atoms penetrate into the steel surface. Carburizing at AT 3 above (850℃ - 950℃). The purpose of this method is to increase the carbon concentration of the surface of the die after heat treatment, so that the hardness, wear resistance and contact fatigue strength of the surface are improved more than those of the core, while the core maintains a certain strength and high toughness. There are solid carburizing and liquid carburizing.
