Surface treatment process of aluminum die casting

Aluminum and aluminum alloy surface treatment process

Due to the poor adhesion of the coating film on the surface of aluminum and aluminum alloy substrates, the surface must be pretreated to form a conversion film on the surface. This not only enhances the bonding force of the coating film, but also greatly improves the corrosion resistance. The surface treatment of aluminum and aluminum alloys is generally: alkaline washing → hot water washing → cold water washing → deoxidation film → cold water washing → cold water washing → chemical conversion → cold water washing → cold water washing → deionized water washing → draining → drying.

The surface treatment process of aluminum and aluminum alloy is different from the performance treatment of steel parts as follows.

(1) Degreasing: The purpose of alkaline washing is mainly to remove oil stains attached to the aluminum alloy surface. The commonly used alkaline lotion formula is composed of Na2CO3, Na3PO4, Na2SiO3 and additives. It is mainly used for dipping and spraying, but also electrolytic cleaning and ultrasonic cleaning.

(2) Removal of oxide film: The oxide film naturally formed on the surface of aluminum is a non-uniform non-continuous film, which must be thoroughly removed before coating. For rolled aluminum alloy parts, a mixture of chromic acid, sulfuric acid, and hydrofluoric acid can be used To remove the surface oxide film, and the die-casting parts are mainly nitric acid.

(3) Chemical conversion: The purpose of conversion treatment is to form a dense and uniform continuous film on the aluminum alloy surface. The function of this process is equivalent to the phosphating process in the surface treatment process of steel parts, but its solution is different. The conversion coating of aluminum and aluminum alloy is based on chromic acid/hydrochloric acid, phosphoric acid/hydrochloric acid and hydrofluoric acid. The quality of the conversion coating is usually It is 2.5g/m2. After the conversion film is formed, it must be washed with water and then dried with hot air to enhance the hardness of the film.

Painting process

The coating process can generally be divided into: spray paint, powder spray and electrophoresis. After the surface treatment of the workpiece, according to the requirements of different workpieces for appearance and corrosion resistance, select the appropriate spraying process and equipment, and pay attention to the control of leveling, drying, cooling and other processes, otherwise it will have a negative impact on the product quality.

Under normal circumstances, the spray paint process can achieve better appearance quality. The spray paint coating has good gloss, color and weather resistance, and is usually used in applications with high appearance requirements such as automobile exterior coatings and motorcycle fuel tanks. For occasions with high anti-corrosion requirements, such as motorcycle frames and refrigerators in the kitchen, powder spraying technology is generally used. The electrophoresis process is generally used in high requirements such as salt spray test and impact resistance, and acts as a primer. Of course, like car wipers, high-end door locks, etc., only one electrophoretic paint can meet the requirements. Sometimes three processes can be applied to a product, which depends on the comprehensive factors of various aspects.

The three processes have their own advantages and disadvantages. In terms of equipment investment, the one-time investment in electrophoresis equipment is large, and the key equipment mainly depends on imports; the one-time investment in powder spraying equipment is the least, but due to the high powder baking temperature, the equipment (energy consumption) operation cost high. At present, powder coatings with good leveling properties and low curing temperature have been developed abroad and paints with anti-corrosion properties comparable to powder coatings.

A good precision die-casting company must have complete die-casting surface treatment system equipment, so when choosing a die-casting company, you must pay attention to the surface treatment capabilities of aluminum alloy die-castings.