Research On The Structure And Performance Of Laser Modified Surface Of AZ91D Magnesium Alloy

In order to achieve the purpose to enhance the surface hardness and anti-corrosion ability of Magnesium alloy, this thesis uses CO2 laser with a maximum laser power of 5kw to conduct the laser melting and cladding process on the surface of Magnesium alloy. In the process of experience, the author respectively analyzes three surface modified crafts, that are the laser melting process, laser cladding nano-Al2O3/WC/Co composite coating and the Al powder added in the coating, and compares the original Magnesium alloy and the surface modified layer on the microstructure, phase composition, microhardness, electrochemical corrosion and other performances.First, the laser melting is conducted on AZ91D magnesium alloy. The melted layer likes a waned moon, and the grains of melted layer are smaller than those of the matrix, only consisted ofα-Mg and Mg17Al12. There is no crack, pore and other deficiencies in the melted layer, which could combine with the matrix well. Under the comprehensive function of fine-grained intensification and precipitation intensification, both the microhardness and anti-corrosion ability of the melted layer are obviously improved.Second, in consideration of that the A12O3, as a hard ceramic particle, has possessed a series of excellent features such as high hardness, high stability and low density, and Al2O3 is the reinforcement in laser cladding layer; WC grain has the characteristics of high hardness and good anti-corrosion ability, which is decomposed into W and C through laser action. The W carbides, precipitated uniformly random, play an excellent role in dispersion strengthening and solid solution strengthening; Co can intensify the anti-corrosion ability, In this thesis, the nano-Al2O3/WC/Co compound layer is added on the surface of AZ91D magnesium alloy through laser, but only a little Co could be added since it will surround the surface of WC grains and obstacle the decomposition of WC grains. The laser cladding layer forms tiny dendrite and columnar particles near the matrix.The grain size gradually becomes smaller and smaller from the bottom to the surface of the cladding layer without obvious cracks or pores or other deficiencies. The cladding layer is consisted of Mg, Mg17Al12, Al2O3, W2C, WC and Co, generating the new material W2C. Compared with the original magnesium alloy AZ91D, both the microhardness and anti-corrosion ability of the laser cladding layer have been greatly improved, due to the fine-grained, solid solution and dispersion strengthening. The micro hardness has improved by more than ten times, the corrosion potential increases about 255mV, and the current density decreased by about 5150μA/cm2 than the matrix.This thesis also compares of two matrix materials die-casting magnesium alloy AZ91D with casting magnesium alloy AZ91D after the laser melting on the mictrostructure, mictrohardness and anti-corrosion ability, also gets the suitable laser power and scanning speed of the two materials from two influential factors of laser power and scanning speed.Finally, in consideration of the excellent metallurgy combined performance of Magnesium and Aluminum, Aluminum powder is added into the above-mentioned layer, that is to say laser cladding of nano-Al2O3/WC/Co-Al compound layer is added on the surface of AZ91D magnesium alloy, thus form melted layer without crack or pore based on excellent combination with the matrix. The phase of the cladding layer is consisted of Mg, Mg17Al12, Al2O3, W2C, WC, Co, and Al3Mg2, generating the new material W2C and magnesium aluminum intermetallic compound Al3Mg2. Compared to the original magnesium alloy, the microhardness and anti-corrosion ability of the laser melted layer have been dramatically improved, due to the intensification of grains and the enhancement of the hardness and anti-corrosion ability of Al and Al3Mg2.