| The magnesium alloy is known as "21st century green engineering material" because of its advantages, such as, low density, good casting properties and high damping, etc. However, due to the low surface hardness, low wearing resistance, and poor corrosion resistance, the development of magnesium; alloy is obstructed seriously.In this study, AZ31B magnesium alloy is considered as the research object and treated on the surface with laser in order to improve the properties of magnesium alloy. Metallography, X-ray diffraction analysis, hardness test, friction-wear test and electrochemical corrosion test are used to research the influences and mechanisms of alloy surface improvement layer treated by four different cooling-down methods as air-cooling, deep-cooling, extreme-cooling and the combination of deep-cooling and extreme-cooling on the macro-and microstructure, phase composition, hardness, wear resistance and corrosion resistance of alloy.It is found that the combination of melting layer and substrate is good without any cracks, holes and other defects extremely after the laser surface melting treatment on AZ31B magnesium alloy. In this study, laser power and scanning rate are chosen as the parameter to research the influences on the alloy melting layer. The optimal combination of melting layer and substrate is created when the laser power and scanning rate reach the values of3300w and360m/s respectively. In this case, the melting layer is composed of a-Mg and β-Mg17Al12. The grain is refined obviously in comparison to the substrate. The wear resistance and corrosion are improved in a way. Corrosion potential shifts0.093V. Corrosion electric current density is reduced by an order of magnitude. The maximum friction coefficient and the average friction coefficient are reduced by0.047and0.029respectively.Based on the wide application of liquid nitrogen deep-cooling treatment in the conventional steel material and the improvement of wearing resistance, hardness and working life, etc, by means of deep-cooling treatment, the melting layer is still composed of a-Mg and β-Mg17Al12when using deep-cooling treatment to process the laser melting treatment of magnesium alloy. However, due to the effect of deep-cooling, the strengthening phase β-Mg17A112in this case is more than it in the base metal and air-cooling laser melting layer, the hardness, corrosion resistance and wearing resistance are all improved. In addition, the corrosion potential in this condition is-1423mV, which is larger than the corrosion potential of base metal (-1575mV) without any treatment. The corrosion electric current density is reduced by two orders of magnitude. In comparison to original magnesium alloy, deep-cooling melting layer and average friction coefficient are reduced by0.011and0.017respectively.It is confirmed that extreme-cooling technology has a great improvement on the microstructure and properties of laser melting layer. In this study, the contrast of extreme-cooling technology with the same parameters shows up that the surface grain in the extreme-cooling laser melting layer is smaller than that in the air-cooling laser melting layer and the deep-cooling laser melting layer. The melting layer is still composed of a-Mg and β-Mg]7Al12. The surface hardness in the melting layer can reach up to150HV0.05. The wearing resistance and corrosion resistance are also improved. The electrochemical polarization curve test shows that in the extreme-cooling condition, the melting layer surface is inactivated obviously, and the corrosion electric current is6.98E-4, which is larger than that in the deep-cooling laser melting layer of2.14E-4. When comparing to the magnesium alloy substrate and air-cooling laser melting layer, the corrosion electric current in the extreme-cooling melting layer is reduced by around1to2orders of magnitudes. In addition, the friction-wear test shows that in the extreme-cooling laser melting layer, with the same power parameter and friction condition, the maximum friction coefficient is0.261, the average friction coefficient is0.168, the friction coefficient is the smallest within the different cooling technology. However, from the surface topography and SEM enlarged view, it is found that the wearing resistance is improved because of the lubrication from the oxidation film produced.Since both of deep-cooling and extreme-cooling technology can improve the performance of the laser melting layer partly, it is possible to find an optimal condition by using the combination of the two technologies. It is confirmed that in comparison to the extreme-cooling, the organization of the extreme-cooling+deep-cooling laser melting layer is more uniform. Due to the increase of cooling rate, there is nearly no dendrite but some uniform and fine chilling isometric crystals formed in the surface layer. The phase is still composed of and β-Mg17Al12, however, because of the solid thermal expansion and contraction, the content of precipitated phase β-Mg17Al12is relatively more, the highest hardness can reach up to180HV0.05, the hardness of melting layer increased more than around233%. From electrochemical polarization test (3.5%(wt%) NaCl), it is found that extreme-cooling+deep-cooling laser melting layer is inactivated twice, the corrosion potential is-1512mV, the corrosion electric current is1.04E-2A, which is smaller than that in the substrate (1.60E-2A), air-cooling laser melting layer (5.06E-3A), deep-cooling laser melting layer (2.14E-4A), extreme cooling laser melting layer (6.98E-4A). Based on this point, it is concluded that the optimal improvement in the corrosive performance of magnesium alloy are able to be achieved in extreme-cooling+deep-cooling condition. What’s more, the friction-wear analysis test shows that in the extreme-cooling laser melting layer, with the same power parameter and friction condition, the grinding trace width in the laser melting layer is around600μm, the average wearing capacity is0.003g, the maximum friction coefficient is0.345, and the average friction coefficient is0.238. In the extreme-cooling+ deep-cooling condition, the narrowest grinding trace width, the smallest average wearing capacity, the smallest maximum friction coefficient and the smallest friction coefficient are achieved. Hence, the extreme-cooling+deep-cooling technology can improve the wearing performance of AZ31B magnesium alloy laser melting layer obviously. |
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