Heat treatment plays a crucial role in fortifying castings through the modification of their physical attributes. It’s a later-stage process of die casting aimed at increasing the tensile strength and hardness of parts. Furthermore, it guarantees that castings meet the user’s exact specifications. The desired structural changes in castings can be achieved using heat treatment.
The heat treatment process involves heating castings to a steady temperature for a specific period and then cooling them down. It results in a change in the part’s internal structure, which leads to enhancing its physical properties. Heat treatment aids die casting by increasing the parts’ wear resistance, machinability, hardness, tensile strength, ductility, and dimensional stability
Heat Treatment in Die Casting
The role of heat treatment in die casting is to change the physical and chemical properties of alloys. A controlled heating and cooling of alloys is performed to enhance their hardness, strength, toughness, ductility, elasticity, etc. Heating the alloys results in changing their crystalline structure.
Types of Heat Treatment for Die Casting:
There are nine main varieties of heat treatment processes exist for die castings. What heat treatment your castings require depends on the type of casting, the alloy structure, and what type of physical properties you seek.
T1 (Untreated Artificial Aging)
T1 is a heat treatment process to improve the machinability of parts and increase surface smoothness. It is for castings that already have some quenching effect during wet sand mold or metal mold casting.
T2 (Solution Treatment)
T2 as a solution treatment removes casting or residual stress, removes work hardening produced by machining, and improves alloy plasticity. The selection of insulation time and temperature for this heat treatment depends on the application of your die-casting parts.
T3 (Quenching)
It improves the strength of alloys by natural aging at low temperature and for long insulation time.
T4 (Quenching and Natural Aging)
Parts with high corrosion resistance (below 100°C) require T4 heat treatment. It’s also natural aging at low temperatures for long insulation time.
T5 (Quenching and Incomplete Artificial Aging)
T5 heat treatment is for parts that require high strength and high plasticity. The temperature of T5 artificial aging is lower or the insulation time is shorter.
T6 (Quenching and Full Artificial Aging)
Parts that require maximum strength are T6 heat-treated. Maximum strength is achieved with T6, but with some reduction in ductility in parts. Compared to T5, its temperature is higher or the insulation time is longer.
T7 (Quenching and Stabilizing Aging)
Excellent for high-temperature working parts. T7 aging is carried out at temperatures higher than T6 and close to the working temperature of the part. Parts heat-treated this way achieve high strength and a relatively high structure as well as dimensionally stable.
T8 (Quenching and Softening Aging)
T8 heat treatment increases the part’s ductility and dimensional stability. However, at the cost of reducing strength of parts. The aging temperature of T8 is higher than T7.
T9 (Cold Treatment or Cold-Hot Cycling Treatment)
T9 is an ideal heat treatment process to stabilize the dimensions of the parts. Depending on the application of the part, multiple treatments can be carried out in T9, and the temperature is determined based on the working conditions of the part and the alloy used.
Heat Treatment Process of Die Casting
The thermal treatment process for die casting is structured around four sequential steps. Each step plays an important part in stabilizing the size & crystalline structure of the parts, enhancing their mechanical properties, improving their machining performance, and removing any internal stress.
Step 1: Casting Solidification
Injecting molten metal into the mold cavity at high speed and rapidly cooling the metal down is the initial step of die casting. This complete process is done with die casting machines. Fast cavity filling and fast cooling removes any chance of filling gaps in the mold or disturbed structural stability of the parts.
Step 2: Solution Annealing
Annealing helps with stabilizing the shape and size of die casting parts. It’s an important step which also removes the casting stress of the parts caused by machining. Annealing of die casting parts is done at 450-490 ℃ temperature for a brief time.
Step 3: Quenching
Die casting parts undergo solution treatment, also known as cold treatment, to maximize the dissolution of reinforcing elements. This process involves annealing or heating the parts to around 500°C for several hours, followed by rapid quenching in water at 60-100°C.
Step 4: Artificial Aging
Aging encompasses a low-temperature tempering technique that includes both natural and artificial aging. Natural aging refers to the aging of parts at ambient room temperature, while artificial aging involves subjecting them to temperatures ranging from 120 to 280℃ for extended durations. Artificial aging includes:
- Incomplete artificial aging: It is performed at ~120℃ temperature for 3-4 hours. Incomplete artificial aging of die casting parts increases their tensile strength, toughness, and plasticity at the cost of lower corrosion resistance.
- Complete artificial aging: It is performed at ~150-180℃ temperature for >5 hours. It sufficiently increases the tensile strength of parts at the cost of lower elongation.
- Over-aging: It is performed at ~280℃ temperature for >3 hours. It strengthens the parts by increasing their stress resistance and corrosion resistance.
Advantages of Heat Treatment Process for Die Casting
Heat treatment holds tremendous significance in die casting due to its remarkable effectiveness in enhancing the performance and lifespan of parts.
The following are some of the primary advantages of heat treatment process of die casings:
- Increases parts’ wear resistance, hardness, tensile strength, ductility, and dimensional stability
- Increases machinability of parts and surface smoothness.
- Removes casting or residual stress and work hardening produced by machines.
- Improves alloy plasticity and stabilizes the size and shape of parts.
- Strengthens the parts by increasing their corrosion and stress resistance.
However, thermal treatment of castings can cause bubbling and porosity in parts, but it’s extremely rare and the chances of its happening depend on various factors such as:
- The alloy type
- The temperature and duration of the heat treatment process
- The design of the part itself
It’s important to carefully control the heating/cooling rates and the atmosphere surrounding the part during its heat treatment process.
Conclusion
The heat treatment of casting is an essential step in enhancing their physical properties and overall performance. By carefully controlling and stabilizing the heating and cooling of parts can effectively change their properties exactly to the user requirements.
Through meticulous attention to detail and innovative approaches, Sunrise Metal leverages the heat treatment process to deliver high-quality die casting parts that meet the rigorous demands of modern industries.