Metal die casting
is an advanced technology with less or no cutting, having high production efficiency, few raw materials consumption, less production cost, good performance and high precision. Die casting is widely applied. Among that, the largest market of die castings is auto industry. For people paying more attention to sustainable development and environmental protection, light weight is the best pathway for realizing high efficiency, security, energy conservation, comfort and environmental protection. Use aluminum alloy to take place of steel for auto making which can reduce the weight of auto by about 30%. Die casting mold casts molten metal under high pressure. During casting, the mold is cooled and heated periodically. It is also eroded and corroded by scorching metal which is highly ejected. The materials used for mold should have higher hot fatigue resistance, thermal conductivity, good abrasion resistance, corrosion resistance and good high temperature mechanical property. Meeting the constantly improved usability cannot only depend on materials of mold. Using various surface treating technologies can meet the requirements for high efficiency, high precision and high service life of die casting mold.
When die casting mold operates, it contacts with high temperature molten metal. It is not only heated for a long time, but also has higher heating temperature than forging die. The temperature for die casting nonferrous metal is 300～800℃ and temperature for ferrous metal is higher than 1000℃. Moreover, die casting mold also endures high pressure 30～150MPa. The mold is heated and cooled repeatedly. It is also wear and corroded by high speed washing caused by molten metal flowing. Based on statics, when 3Cr2W8V is used as material for die casting mold, 65% of losing efficacy is caused by thermal fatigue, 15% caused by cracking, 6% caused by abrasion and 4% caused by erosion.
Factors influencing on thermal fatigue
Die casting has high speed and high pressure. The surface of mold is strongly impacted by load. The surface of mold contacts with high temperature flux. The highest temperature can be as high as 8700℃. Under such high temperature, thermal stress compression is caused on surface of mold. Before die casting, lubricant is injected into mold for quenching, then, tensile stress is caused on surface of mold. Alternating thermal stress causes thermal fatigue cracks on the surface when exceeding that yield strength on mold surface. The cracks spread radically and spread into center, forming chap, which cause injury and mold sticking of die castings. More seriously, chap can cause early cracking of mold.
Basic features of materials
For snap heat and cold, the materials for die casting mold have following materials:
1. Good thermal fatigue and thermal shock resistance. Cracks cannot be caused easily.
2. Good tenacity and malleability. Improve impact resistance of sharp corner and bulge.
3. Good thermal hardness, heat resistance, hardenability, abrasive resistance and high temperature oxygen resistance.
4. Less heat treatment distortion and small coefficient of thermal expansion.
Methods used to improve service life of die casting mold
Heat treatment on die casting mold
Heat treatment can modify metallographic structure of materials, ensuring necessary strength and hardness, dimensional stability under high temperature, thermal fatigue resistance and machinability of materials. After being heat treated, the components should have less deformation, no cracks and can reduce residual internal stress as much as possible. At present, die casting mold generally uses vacuum gas quenching without oxides on surface. The mold has less deformation, better maintaining quality of mold. The procedures are forging, spheroidizing annealing, rough machining, stabilizing treatment, finish machining and final heat treatment (quenching and tempering), polishing and nitrizing, accurate grinding or comminution, assembling. For H13 steel, high temperature quenching, double quenching and subzero treatment are adopted, improving performances of mold and extending service life of mold.
Surface strengthening on die casting mold
Surface strengthening is the most effective and economic method used to extend service life of mold. Strength and toughness of steel are high improved through heat treatment. Use different heat treatment technology and cooper with suitable center performance, which can give high hardness, which attach high hardness, corrosion resistance, adhesive resistance and low friction coefficient on surface of mold, improving service life of mold by several times and dozens of times. Surface strengthening on mold is classified into three categories
1. Not change chemical compositions on surface, such as laser hardening.
2. Change chemical compositions on surface, such as nitriding.
3. Form coating on surface, such as vapor deposition.
Optimize mold design and die casting technology
Reduce sharp corner on mold. Reasonably use raw materials. Specify processing and heat treatment technique. Nitriding of mold can control surface hardness of mold (HV>600). Depth of nitration case is 0.12~0.2mm. Correctly preheat mold. Optimize mold to improve internal cooling so that mold can acquire uneven effectiveness of heat balance. Mold is maintained at a lower temperature. Rationally spray coating. The coating is significant for delaying thermal fatigue cracks, improving service life and efficiency of mold.