[China Aluminum Industry Network] In recent years, with the rapid development of China's automobile industry and the increasing prominence of the world's energy problems, automobiles have become more and more in the direction of lightweighting. Due to its low density and corrosion resistance, aluminum is widely used in the automotive industry. Cast aluminum alloys have higher mechanical properties and process performance than pure aluminum. Therefore, they are widely used in automotive engines, automotive chassis and automotive wheels and other components. However, the corrosion resistance and wear resistance of cast aluminum alloys are low, especially Al-Si alloys and Al-Cu alloys, which limit their wider application. Therefore, through surface modification to overcome the shortcomings of low wear resistance of cast aluminum alloy, in order to expand the application of cast aluminum alloy. At present, the surface modification methods used to improve the corrosion resistance and wear resistance of cast aluminum alloys include: micro-arc oxidation, laser surface treatment, electroless plating, electroplating, and anodic oxidation.
1. Main methods for surface modification of cast aluminum alloys 1.1 Micro-arc oxidation Micro-arc oxidation is a surface modification technique developed in the 1930s. It is the use of arc discharge to enhance and activate the micro-plasma oxidation reaction occurred in the anode, so that in the aluminum, titanium, magnesium and other metals and their alloys as the material on the surface of the workpiece to generate high-quality reinforced ceramic membrane. On the aluminum alloy, the oxide film formed by micro-arc oxidation is composed of externally loose γ-Al 2 O 3 and internal dense α-Al 2 O 3 . During the micro-arc oxidation process, the current density, electrolyte composition, and alloying elements in the aluminum alloy all have important effects on the quality and growth rate of the oxide film. In the oxidation process, both the cathode and the anode emit gas, which is accompanied by the growth process of the oxide film. Therefore, the current density of the electrode strongly influences the composition and structure of the oxide film and also affects the corrosion resistance of the aluminum alloy. Studies have shown that at relatively high anode current density, the oxide film mainly contains α-Al2O3, while at lower anode current density, the oxide film is almost all γ-Al2O3. JuhiBaxi's research shows that as the current density increases, the wear resistance of aluminum alloy decreases accordingly. The micro-arc oxidation electrolyte has an important influence on the quality of the oxide film. The traditional electrolyte is a silicate or aluminate. Polat believes that a low concentration of sodium silicate can obtain a ceramic film with higher hardness. Zheng believes that tungstates are more stable than silicates. The addition of rare earth elements to the electrolyte also has an important effect on the film formation rate of micro-arc oxidation. With the increase of Ce (III) concentration, the film formation rate increases first and then decreases. The addition of fluoride ions to the oxidizing electrolyte will reduce the porosity of the micro-arc oxidation film and increase the corrosion resistance of the oxide film. The addition of Na3AlF6 and sodium tartrate increased the hardness of the ceramic membrane.
1.2 Laser Processing 1.2.1 Surface Alloying In general, due to the characteristics of aluminum alloy itself, the wear resistance is not very high, that is, high-silicon aluminum alloys with high hardness do not have high bearing capacity. Therefore, the surface strengthening to improve its wear resistance has become a common method for expanding the application of aluminum alloys, especially cast aluminum. The laser surface alloying can form intermetallic compounds between the aluminum alloy matrix and the added metal or alloy. Increasing the hardness of the aluminum alloy, thereby increasing its wear resistance, can also improve its corrosion resistance to some extent.
The application of rapid laser alloying technology can add alloying elements on the surface of the aluminum alloy as required to produce the desired strengthening phase in the modified layer, thereby increasing the hardness of the material and improving the wear resistance. The laser alloy can increase the microhardness of the coating to about 900 HV. After the laser alloying, the corrosion resistance of the aluminum alloy is increased by more than one time. However, laser alloying is liable to over-remelt, especially in the case of low laser power, low scanning speed, and large thickness of the pre-coating layer, which can easily cause porosity defects in the coating. The current method of prevention is to use an increased amount of overlap or use two laser treatments, which increases the cost of the coating.
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