Study Of Laser Producing Al-Si-based Nano-Si₃N₄Strenthening Layer On Magnesium Alloy Surface

As the lightest metal structural material on earth, magnesium and its alloys have not only a series of advantages such as higher strength, lighter quality and better recycle performance, but also abundant reserves. Simultaneously, they play a significant role in the fields of aviation and aerospace industry, automotive and national defense application and so on, which have been well known as" the most potential green engineering materials in the21st century ". Unfortunately, the property superiority of magnesium alloys has a serious limit in its applicability due to its poor corrosion resistance and wear-resistance performance. Thus, it has a practical importance to improve its superficial comprehensive performance by taking the appropriate surface modification technology of magnesium alloy.This paper has investigated the feasibility of forming composite coating based on the surface of AZ31B magnesium alloy which is made of Al-Si based nano Si3N4powder. Respectively, the experiment consists of three methods: laser cladding, laser cladding with liquid nitrogen which is under the condition of the extreme cooling and laser remelting after plasma spraying. Meanwhile, the discussion about the microstructure and properties of the modified layers got by the three different ways mentioned above have been made.Laser cladding study showed that the cladding layer is mainly composed of the upper dendrites, the central black phase which presenting a linear arrangement and the bottom thick branches crystal microstructure. When nano-Si3N4content is1%and5%, the phase of cladding layer are Al, AlN, Al9Si, Mg2Si and Al, AlN, Al9Si, Al3.21Sio.47, Mg2Si respectively; the hardness are235HV0.05and260HV0.05respectively, which is4-5times of the matrix’s; the corrosion potential are-1204mV and-1212mV, which raised382mV and374mV respectively compared the AZ31B magnesium alloy substrate, the corrosion current density are0.0705mA·cm-2and0.1056mA·cm-2which reduced one order of magnitude compared with the substrate.It is showed that there are a lot of irregular black particles dispersed distribution under extreme cooling condition with liquid nitrogen in laser cladding experiment. Compared with the air cooling method, the grains refined obviously and the organization are more compact. The phases of cladding layer consist of Al, AlN, Al9Si, Al3.21Si0.47, Mg2Si and the highest hardness of cladding layer reach to286HV0.05which is about5～6times of matrix’s. The corrosion potential and corrosion current got-1185mV and0.0695mA·cm-2, which has been greatly improved compared with substrate of-1581mV and 0.5214mA-cm"2.The results of laser remelting show that the interface bewteen composite coating and magnesium matrix achieve a good metallurgical bonding after remelting. The phases of plasma sprayed layer and laser remelted layer are consist of Al, Si, Al9Si, Al3.2iSi0.4, Al0.403Zn0.597, Al and Al, AlN, Al9Si, Al3.21Sio.47, Mg2Si respectively. Under the role of laser remelting. The intermetallic compound of A1N, Mg2Si and the dispersed aluminum silicon solid solution strengthening phase precipitated effectively improved the hardness of remelted coating. The maximum value of microhardness is514HV0.05,10times of matrx’s. The excellent corrosion resistance of remelted layer is due to the integrated action of precipitation strengthening phases, the self-corrosion current reduced one order of magnitude compared with as-sprayed layer and substrate.