Because die-casting mold is exposed to high temperature, high pressure, and high speed working environment for a long time, periodic stress and strain are generated on the surface of mold. After a certain period of use, cracks, ablation, cracks and other damages will occur, resulting in the mold being unable to continue to be used. Only after repairs can it continue to be put into production. Therefore, during use of die-casting molds, mold repair is very important, which can not only ensure production of qualified products, but also extend service life of mold. This paper introduces failure conditions such as ablation, cracking, and aluminum adhesion of fixed core of a certain die-casting mold, and analyzes causes. Mold repair process is divided into marking, welding, CNC milling and EDM processes. Quality of repaired molds meets technical requirements. At the same time, measures to improve service life of die-casting molds were discussed, including reasonable analysis of mold structure, correct selection of mold materials, optimization of heat treatment processes, improvement of use environment, and standardized maintenance.
Graphical results

Figure 1 shows fixed core of a die-casting mold for an aluminum alloy cylinder block. Material is H13 steel. Failure occurred during production. Failure form and causes are analyzed as follows.

 

Figure 1 Fixed mold core
(1) Ablation
There are 2 ablation spots about 1mm deep adjacent to lateral runner and sprue sleeve. There are 12 ablation spots on convex edge and concave root of mold core. Ablation reduces size of mold, and burned part must be replaced to obtain a die-casting part with acceptable dimensions. Main reason for ablation phenomenon is that high-temperature aluminum liquid at about 670℃ washes mold repeatedly and at high speed under high pressure. Due to heat concentration at the corners of mold, temperature is relatively high, and steel material is burned off after a long time.
(2) Cracks
Small network cracks appeared on molding surface facing inner gate. This was because strong impact of molten metal produced a large compressive stress. At the same time, spray paint is chilled after high-temperature mold is opened. Thermal stress is generated under alternating action of cold and heat, small cracks gradually occur under action of composite internal stress. As time goes by, cracks become larger and larger. When cracks reach a certain level, mold may break.
(3) Stick aluminum
There is aluminum stuck at the root of inner gate. High-temperature and high-pressure aluminum liquid enters steel matrix along cracks. Because affinity between iron and aluminum is greater at high temperatures, they are bonded together.
(4) Design changes
During production, products at the end of lateral runner sometimes appear to be under-poured. Reason is that size of two inner gates near sprue is too large, resulting in a large flow of molten metal here and a small flow of inner gate at the end. Molten metal solidifies before flowing at the end and filling cavity. Therefore, it is necessary to reduce size of the first section of inner gate.

(1) Mark
Use a white marker to circle area with larger ablation size and write "B" next to it to indicate need for repair. An "H" is written next to smaller ablation, indicating that it is a hole and does not need to be repaired for time being. Design changes are pointed out with arrows, written "Design Change", and need to be repaired. Word "remove flesh" is written where aluminum is stuck, indicating that material needs to be processed to remove it.
(2) Welding
Argon arc welding has little thermal impact on base metal and is less prone to defects such as cracks, undercuts, and pores, and welding quality is easy to guarantee. 9188GS welding machine is used in production, and welding rod is AST0506. Its composition is similar to H13 steel, it has good tempering resistance and thermal fatigue resistance, as shown in Figure 2. Before welding, cracks, oxides and oil stains on welding surface need to be thoroughly removed and dried.

 

Figure 2 Argon arc welding
(3) CNC milling
Repair welding area needs to be machined to remove excess material. Fixed mold core model file is opened in UG NX software, rough machining and finishing CNC programming are performed respectively. Post-processing generates program files and transfers program to machine tool. Lift fixed mold core onto workbench of machine tool, calibrate it with a table, press and secure it. Calculate tool accurately according to requirements of program sheet, set processing coordinate system G54, call up roughing program, set processing parameters, and start processing. Then call up finishing program, set processing parameters, and start processing, see Figure 3.

 

Figure 3 CNC milling
(4) Electrical discharge machining
If there are narrow grooves in the area that needs to be processed, EDM processing should be used to design and process graphite electrodes, install and fix electrodes on EDM machine tool, fix fixed mold core on machine tool, set reasonable discharge parameters, and perform Electric discharge machining, see Figure 4.

 

Figure 4 EDM

(1) Mold structure design
Wall thickness of die-casting parts should meet normal wall thickness and minimum wall thickness requirements of alloy. On the premise of ensuring strength and stiffness, try to design thin-walled parts with uniform wall thickness to prevent shrinkage cavities and shrinkage porosity. According to connection method of the two walls of casting, size of casting fillet should be reasonably selected to facilitate filling and exhausting of molten metal and ensure strength of mold. Draft angle should be appropriately selected according to alloy type and surface characteristics so that casting can be demoulded smoothly.
(2) Mold material selection
Choose hot work die steel with good thermal fatigue resistance and thermal stability. It is recommended to use 8407 or refined H13. Service life of aluminum die-casting mold can reach 70,000 to 100,000 times. E38K is suitable for temperature range below 700 ℃, and service life of aluminum die-casting mold reaches 200,000 to 400,000 times. 2367 is suitable for temperature range below 700 ℃, and service life of aluminum die-casting mold reaches 400,000 to 600,000 times. In production, mold materials should be correctly selected based on the type of processing materials, product characteristics, and production batches to improve economic benefits.
(3) Heat treatment of mold
Heat treatment changes workpiece's microstructure, thereby changing its strength, hardness, toughness, wear resistance and other performance properties. Correctness of heat treatment is directly related to service life of mold. Therefore, quenching temperature and time, cooling rate and tempering temperature must be reasonably controlled. Design hardness of mold core (HRC) is set at 44~46, and heat treatment process is 480, 700, 850 ℃ graded heating + 1050 ℃ vacuum oil quenching +600 ℃ secondary tempering. Mold core is tempered again when it is transferred to mass production to eliminate effects of wire cutting, repair welding, and discharge on mold core structure.
(4) Mold usage environment
According to alloy type, casting wall thickness and structural complexity, preheating temperature and working temperature of die-casting mold should be reasonably selected. Preheating temperature of aluminum alloy die-casting mold is 150~180℃, and working temperature is 180~240℃. Appropriate selection of higher preheating and working temperatures can significantly increase life of mold. Ensure that mold is properly cooled. Temperature of cooling water should be maintained at 40-50℃. If machine is temporarily shut down, mold should be closed as much as possible and amount of cooling water should be reduced to avoid thermal shock to mold when machine is restarted. On the premise of ensuring good forming, it is better to use a lower pouring temperature.
(5) Mold repair and maintenance
Standard maintenance can keep mold in good condition. After new mold is tested, stress relief and tempering should be performed. When new mold is used for 1/6 to 1/8 of its design life, that is, 10,000 mold times for aluminum die-casting molds, 5,000 mold times for magnesium and zinc die-casting molds, and 800 mold times for copper die-casting molds, mold cavity should be tempered at 450 to 480℃, cavity should be polished and nitrided to eliminate internal stress and slight cracks on cavity surface. In the future, perform same maintenance every 12,000 to 15,000 mold times. After mold has been used for 50,000 mold times, maintenance can be performed every 25,000 to 30,000 mold times (aluminum die-casting mold). Using above method can significantly slow down speed and time of cracks caused by thermal stress.