Laser processing technology is a high-tech that has developed rapidly in recent decades. The research and application of laser surface modification technology on ferroalloys are reviewed. Explore
The development trends and broad prospects of laser surface treatment technology in magnesium alloys are discussed.
Magnesium alloy has low density, high specific strength, high specific rigidity, good thermal conductivity, good electromagnetic shielding characteristics and damping capacity. Magnesium alloys have replaced many materials such as zinc, aluminum, cast iron and steel, and are widely used in aerospace, automotive, computer, communications and other industries, especially the automotive industry, the demand for magnesium alloy parts has increased sharply, making the application of magnesium alloys show Strong development momentum. However, the poor wear resistance and corrosion resistance of magnesium alloys limit the potential of magnesium alloys. Therefore, researchers have attached great importance to the surface treatment technology of magnesium alloys.
Laser surface treatment technology is a new technology developed in recent decades. Compared with the current surface modification treatment technology of magnesium alloy materials, the laser method can perform rapid local heating and select a specific chemical reaction by selecting the wavelength. Therefore, the surface modification of the material can play a huge role. Laser surface treatment methods include laser surface phase transformation and impact hardening, laser surface fusion, laser surface alloying, and laser surface cladding. Among them, the laser surface modification technologies used in magnesium alloys are mainly laser fusion, laser alloying, and laser cladding.
First, laser surface fusion of magnesium alloy
Laser fusion uses a high-power-density laser to interact with the metal in a very short period of time, instantly heating a localized area of the metal surface to a relatively high temperature and melting it, and then absorbing heat and transferring heat by means of a cold metal substrate The thermal process causes the molten metal surface to solidify rapidly, thereby changing the surface structure and properties of the part. Because this process is completed under rapid heating and rapid cooling, the resulting hardened layer has a finer structure and a higher hardness than that of conventional quenching. This technology improves the surface hardness, wear resistance, corrosion resistance, strength and high temperature performance of metal materials and parts.
Under vacuum conditions, Gao Yali et al. Performed laser fusion treatment on AZ91HP magnesium alloy. It was found that the laser fused layer of magnesium alloy was mainly composed of α3 / Mg phase and β-Mg17Al12 phase. As the laser scanning speed increases, the hardness and wear resistance of the fused layer also increase. However, at different scanning speeds, the corrosion resistance of the laser fused layer is lower than that of the base magnesium alloy, and the corrosion resistance of the fused layer decreases with the decrease of the scanning speed.
KA Kousomichalis and Zeng Aiping used a KrF laser to illuminate the polished AZ3lB4H sample surface under vacuum. It was found that the surface micro-morphology of the sample was corrugated, and the surface of the sample showed tensile stress after laser treatment compared with the sample without treatment. The micro-hardness of the laser-treated layer at 30 μm is lower than that of the substrate, and the corrosion resistance of the laser-treated sample is greatly improved.
Nd: YAG laser, D Dube and other laser fusion treatment of AZ91D and AM06B magnesium alloy, its microstructure is dendritic, the grains are refined, although the corrosion performance of laser heat treated AM60B magnesium alloy is higher than AZ91D, but did not get a significant improvement. Corrosion resistance can be significantly reduced even under certain process parameters. Yaojun et al. Performed laser fusion on the surface of AZ91D magnesium alloy, and the microstructure of the fused layer was more uniform, which improved the wear resistance of the laser fused layer accordingly. Using a 2kW continuous wave CO2 laser, G.Abas, etc. performed laser melting on the surface of A3Z1, AZ61 and WE43 magnesium alloys. The fused samples were immersed in sodium chloride at a pH of 10.5 at 20 ° C (mass fraction 5 %) In solution for 10d, the results show that the corrosion resistance of the magnesium alloy after laser melting has been improved, the microstructure of the alloy has been refined, and the alloying elements have been increased. The concentration in the phase solid solution makes the β phase more uniformly distributed, forming a corrosion-resistant layer.
Laser surface alloying of magnesium alloys
Laser surface alloying is by melting the pre-coated film and part of the substrate on the surface of the substrate, or injecting some powder while the surface is melting. The film or surface is rapidly solidified after liquid mixing in the molten pool, so as to form 1 thin layer of alloy with desired properties to improve matrix properties. In recent years, research on laser surface alloying of aluminum and aluminum alloys has been active, but not much research on laser surface alloying of magnesium and magnesium alloys.
AWang A A et al. Used Mg and Al alloying treatment on the magnesium substrate with laser, and found the existence of magnesium aluminum alloy through the selected electron diffraction spot of TEM. The corrosion resistance of the sample is better than that of cast magnesium and magnesium alloy. R Galun et al. Used elements such as aluminum, copper, nickel, and silicon to alloy the surface of magnesium alloys with a 5 kW CO2 laser. The melting depth is 700-1200 μm, and the surface hardness reaches 250 HV. The mass fraction of the alloying elements on the surface alloy layer is 15% -55%. When the copper alloy is added, the corrosion resistance is greatly improved, and when the aluminum alloy is added, the corrosion resistance is significantly enhanced.
3. Laser surface cladding of magnesium alloy
Holmium laser cladding technology is a rapid solidification process that uses a high-energy density laser beam to rapidly melt alloys with different compositions and properties on the surface of the substrate and form an alloy layer on the surface of the substrate that has completely different compositions and properties from the substrate. The generated surface alloy layer isolates the substrate from the corrosive medium, and the corrosion performance of the material is determined by the alloy layer. Since the magnesium alloy laser surface treatment does not require harsh environmental conditions such as vacuum, the size of the workpiece is less restricted. In recent years, with the continuous improvement of laser cladding technology, its application on the surface corrosion resistance of magnesium alloys has been increasingly affected. Attention of domestic and foreign researchers.
Wang Anan uses a 10kW CO2 laser to laser-clad a magnesium-aluminum alloy in a vacuum-filled reaction chamber. A eutectic layer was formed on the interface, and the composition phase of the modified alloy layer was α-Al and β-Mg2Al3. The corrosion potential was positively shifted by about 0.7v, indicating that the corrosion resistance of laser-treated magnesium alloy was better than that of pure magnesium.
Yue T M performed Nd: YAG laser surface cladding of Al-Si eutectic alloy on Mg-SiC composites under conventional conditions, and laser cladding of stainless steel on Mg-ZK60 / SiC composites using a two-step process. The corrosion electromotive force of the Al-Si alloy layer deposited by the former has not changed significantly, but the corrosion current density is reduced by two orders of magnitude compared with the base material. The metallurgical bonding layer of the latter sample is excellent. The corrosion potential is 820-1090mV, which is much higher than that of ordinary ordinary materials and spray materials. The corrosion current 1 is reduced by 2 to 4 orders of magnitude, and both of them increase the magnesium-based metal composite accordingly. Material corrosion resistance. In addition, Yue TM has also developed a laser cladding of an amorphous alloy Zr65Al7.5NI10Cu17.5 on a magnesium-based metal using a blown powder method. The cladding thickness is about 1.5 mm. . The cladding sample has an excellent metallurgical bond with the surface, without porosity and cracks. The wear resistance and corrosion resistance are better than those of the cladding sample.
Fourth, the future
The method of laser surface fusion of hafnium-magnesium alloy is simple, and the hardness and corrosion resistance are improved, but the degree of improvement is limited. Laser surface alloying can design alloy layers with different hardness. Although the interface is metallurgical, it is difficult to form a uniform alloying layer. Laser cladding can introduce elements with high hardness, high corrosion resistance and high melting point on the surface of magnesium alloys, can form a strengthening layer with excellent surface properties, and is metallurgically combined with the substrate, which is an effective method to improve the surface properties of magnesium alloys.
Sheet metal With the development of sheet metal laser, sheet metal robots, and automatic control technology, laser surface modification technology will move towards high-power automation and intelligence. Laser surface treatment technology has almost no negative effects on environmental protection. As a high-speed and efficient surface modification technology, a lot of results have been achieved in many aspects, and it has become more and more obvious in improving the surface properties of magnesium alloys and extending their service life. The economic benefits obtained are also more and more significant, and there will be great application prospects in anti-corrosion projects in the future.
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