Research On AZ31B Wrought Magnesium Alloy And Its Weldment Treated By Laser Shock Processing

Magnesium alloys, as the lightest metal structural material, are regarded as the21st century green engineering material because of their excellent properties such as low density, superior strength-to-weight ratio, better thermal and electrical conductivity, good vibration and shock absorption ability, perfect electromagnetic interference shielding property and easy recycling. There are great application prospect for magnesium alloy in aerospace, transportation and other fields. However, a number of undesirable properties such as poor strength, wear resistance, stress corrosion cracking (SCC) resistance and fatigue resistance hinder the play of its advantages. With the various laws and regulations of energy conservation and emissions reduction, lightweight of the component becomes an important way to solve the problem. Therefore, to intensify the basic research of magnesium alloy could give full play to the advantages of magnesium alloy performance, broaden the application range of magnesium alloy, which not only meets the needs of scientific development, but also can achieve considerable economy and society benefits. In the paper research on macro properties, microstructure and strengthening mechanism is carried out for AZ31B wrought magnesium alloy and its pulsed tungsten inert-gas (TIG) welding treated by Laser Shock Processing (LSP), which could enrich material plastic deformation theory under super high stain rate, provide new technology method for magnesium alloy surface nanocrystallization and stress corrosion cracking (SCC) resistance, and provide theoretical basis for fatigue damage tolerance design of magnesium alloy. In LSP induced grain refinement and magnesium alloy surface modification processing, there are important application value and theoretical significance.Based on theoretical calculation and effect of different laser processing parameters on surface micro-dents, surface roughness and microstructure of AZ31B magnesium alloy, optimized laser processing parameters could be achieved. When AZ31B magnesium alloy plates are treated by using a YAG laser with a wavelength of1.054μm, a pulse width of15ns, a pulse energy of10J and a spot diameter of3mm.The water with a thickness of3mm was used as the transparent confining layer and the aluminum foil with a thickness of0.1mm was used as the opaque absorbing layer. Nano-sized grains are generated in the surface layer. The microstructure of the plastic deformation layer along the depth direction in different cross section is characterized by means of X-ray diffraction (XRD) and transmission electron microscope (TEM). The formation reasons of micro-twins and a possible LSP induced AZ31B magnesium alloy surface nanocrystallization mechanism are discussed. Residual stress, micro-hardness, potentiodynamic polarization curves, tensile property, impact toughness, wear resistance, stress corrosion cracking resistance and fatigue performance are examined.According to the optimized welding technology parameters:welding current range from45to50A, tungsten electrode diameter of2mm, welding wire diameter of2mm, shield gas rate range from8to10liter per minute, welding speed based on accepting ideal welded joint. TIG welding was used to weld AZ31B Mg alloy plates with the thickness of2.2mm by using single-side welding double-side molding process, the ideal welded joints can be obtained. The macrostructure and microstructure of TIG welded joints are observed. The residual stress and mechanical properties of the welded joint are examined. The changes of microstructure for welded joint and reason for residual tensile stress formation are discussed.According to optimized laser processing parameters, TIG welded AZ31B magnesium alloy sheets surface are processed using a YAG laser with a wavelength of1.054μm, a pulse width of15ns, a pulse energy of4J and a spot diameter of3mm. The water with a thickness of3mm was used as the transparent confining layer and the aluminum foil with a thickness of0.1mm was used as the opaque absorbing layer. A nano-structured surface layer can be produced on heat effect zone of welded AZ31B Mg alloys. Residual stress, potentiodynamic polarization curves, mechanical property and stress corrosion cracking performance are examined.The innovative results are obtained through the above study as followings.Firstly, in the aspect of mechanism research, LSP technology is used to produce nanocrystalline in AZ31B magnesium surface layer according to optimized laser processing parameters. Microstructures of LSP induced plastic deformation layer along the depth in different cross section is studied systematically. For the first time the mechanism of surface nanocrystallization for AZ31B wrought magnesium with LSP is proposed as followings:In original grains, dislocation slipping leads to the formation of dislocation tanglings and stress concentration leads to deformation twinning; in the subgrains and refined grains, dislocation tanglings and dislocation cells are formed continuously; dislocation tanglings are transformed into subgrains boundary with low angle, which divides coarse grains into subgrains; subboundaries are gradually developed into grains boundary with high angle, finally, nano-scale grains with equiaxed shape and random orientations are formed.Secondly, in the aspect of processing technology, the paper has provided a new technology method for AZ31B wrought magnesium alloy and its TIG weldment surface nanocrystallization and the improvement of performance of stress corrosion cracking resistance, etc. Stress corrosion cracking contrast experiment results in acid deionized water for AZ31B wrought magnesium alloy and its TIG weldment before and after LSP show that the time of stress corrosion fracture for AZ31B wrought magnesium alloy and its TIG weldment without LSP is261hours and192hours, respectively, however, no cracks are observed on samples with LSP after ten months. So it is concluded that LSP can significantly improve stress corrosion cracking resistance of AZ31B magnesium alloy and its TIG weldment. Comparing with the as-received sample, the tensile strength and the yield strength are improved by16.9%and16.3%for samples treated by LSP, respectively, however, the elongation is decreased by4%, the mean impact energy of samples with LSP is increased by70.3%, the wear rate of the treated specimen is decreased by384.5%and the value of steady-state friction coefficient decreased by60%, surface micro-hardness is improved by91.8%, the depth of plastic layer is about700μm, the surface residual compressive stress is as high as-125MPa. The surface nano grain size of AZ31B magnesium alloy and its TIG weldment by LSP is about20nm and35nm, respectively.Thirdly, in the aspect of data processing, modified fitting method of seven data is put forward to obtain fatigue crack growth rate equations in different probability of AZ31B Mg alloy with and without LSP. It is found that crack growth rate of samples by LSP is obviously smaller than as-received samples. For the first time mathematical statistics method is used to analyze the value of fatigue life, crack length and crack growth rate. It is found that the standard deviation of crack length between sample with and without LSP has no obvious difference, however, the standard deviation of crack growth rate and fatigue life between sample with and without LSP has obvious difference. Fatigue life of sample by LSP is1.85-2.37times longer than sample without LSP under95%reliability, and the statistical phenomenon is explained by the mechanism of LSP leading to improvement of fatigue resistance.