First, the heat treatment process of zinc alloy castings 1. Stabilization treatment (low-temperature aging) The solid-phase de-elution and eutectoid transformation of cast zinc alloys are cooled faster and alloy elements ((Cu, M townships are inhibited, so as to obtain At the room temperature, the metastable structure will gradually and slowly change to a stable state and cause changes in the size and mechanical properties of castings, and stabilizing treatment at a low temperature can accelerate the transformation of the structure and stabilize the size and properties of the material. The higher the temperature, the shorter the time required to achieve stabilization, which is 3-6 hours at 100°C, 5-10 hours at 85°C, and 10-2.0 hours at 70°C. Air-cooled.
The transformation of the metastable Cu-rich phase during the ageing of a cast zinc alloy with a higher aluminum content will significantly increase the size of the casting.
2. Homogeneous heat treatment The homogenization treatment of the zinc alloy is followed by furnace cooling at 320-400°C for 3-8 hours.
Homogenization treatment can reduce or eliminate dendrite segregation and obtain a fine platelet eutectoid structure. The plasticity and toughness of the alloy increase and the strength slightly decreases.
3. Intensified heat treatment The strengthening of the zinc alloy comes directly from solid solution strengthening and mesophase dispersion strengthening caused by the alloy elements in the matrix. The casting is heated to a temperature as high as possible, close to the melting point of the eutectic, held at this temperature for a sufficient period of time, and then rapidly cooled to maximize the dissolution of the strengthening component. This high-temperature state of the tissue is fixedly preserved. The process to room temperature is called solution treatment. The purpose of solution treatment is to eliminate dendrite segregation during the solidification of the casting and to dissolve all coarse or coarse raw phases into the matrix in whole or in part, and then to precipitate a fine and evenly distributed secondary phase during the cooling process. Solution treatment can increase the strength and plasticity of the casting and improve the corrosion resistance of the alloy.
The factors that affect the solution treatment effect include solution treatment temperature, holding time, and cooling rate. If the temperature is too high, the faster the strengthening element dissolves, the better the strengthening effect will be. In order to obtain the best effect of strengthening the solid solution without overheating the alloy, a stepwise heating method can be adopted. That is, at the low melting point eutectic temperature, the low-melting eutectic does not exist after the dispersive dissolution of the constituent elements. Raise to a higher temperature for holding and quenching.
Second, the surface treatment of zinc alloy castings Although many zinc alloy castings can be used in the as-cast state, but in some cases also need to be surface treatment, on the one hand to protect the casting from corrosion, on the other hand can also play a decorative The effect is to make the appearance more beautiful. Zinc alloy castings mainly have the following surface treatment processes.
1 Plating. Freshly polished zinc alloy castings look like they are plated with chromium, but will soon lose their luster. The common electroplating process for zinc alloy castings is nickel plating on the bottom plating of copper followed by chromium. In addition, zinc alloy castings can also be directly chrome plated. Direct chrome plating can increase the hardness and wear resistance of the casting, reduce the friction factor and improve the corrosion resistance.
2 paint. Zinc alloys can be coated with a variety of different paints. Before painting, the workpiece usually needs to be treated with phosphate or chromate solution. For some of the less expensive parts, acrylic paints with weak adhesion and acidic corrosion components can be used. For parts requiring high corrosion resistance, it is best to use epoxy paint or various amine-based paints, baking after painting.
3 metal spraying. The metal spraying method applies a very thin metal film on the surface of the treated part under high vacuum. Metal spraying can simulate the appearance of copper, silver, brass, gold, etc. This process is mostly used for die castings. If the primer is coated with a primer, the surface defects can be masked. Then, the workpiece to be processed needs no polishing. After low-temperature baking, aluminum vapor is deposited on the part under vacuum to form a thin film. After the second spray varnish is baked to obtain a silver-white paint layer that can be dyed in any color.
4 anodizing. The anodizing treatment of zinc alloy castings is performed in an anodizing treatment liquid at a voltage not exceeding 200 V. The treated casting surface is porous and can be sealed in a heated water glass dilute solution or with an organic coating.
Anodizing oxidation treatment can effectively improve the zinc alloy corrosion resistance.
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