| As a lightest structural material,magnesium alloy has a various desirable property,such as high strength,low density,good ductility and toughness,good conductivity and thermal conductivity,and easy to recycle.It is widely used in aviation,automobile,electronics and other fields.However,the corrosion resistance of magnesium alloys is poor,causing possible loss by corrosion in the applications.It is difficult and costly to enhance the corrosion resistance through the casting process.Surface modification is a suitable way to make the magnesium alloy to meet the requirements in industry.Therefore,considering the expense,the surface modification of magnesium alloys has developed rapidly in recent years.Compared with the traditional modification methods,surface modification by using laser has showed outstanding performance due to its high efficiency,high stability.By means of laser cladding modification,different powders can be selected to improve wear resistance,corrosion resistance,high temperature oxidation resistance and strength improvement of the treated materials.As is well known,the performance of alloys is mainly determined by the composition,microstructure and phase components.Therefore,selecting suitable laser treatment process and cladding materials to achieve the proper composition and phase components in the surface layer is important for laser cladding.The effect of heating temperature,solidification speed,cooling condition and alloy composition on the properties of the material is also crucial.Therefore,based on previous works,the purpose of this study is to improve the surface hardness and corrosion resistance by remelting and cladding composite powders on the AZ31 magnesium alloy.The main results obtained in this study is summarized as follows:The relationship of microstructure and properties in laser remelted AZ31 magnesium alloy was studied.XRD results showed that the remelted layer consists of only?-Mg,the diffraction intensity of?101?Mg was enhanced with the increasing of scanning speed,which means the rapid scanning speed caused the solidification texture.The grain size in the top of modified layer was refined with the increasing scanning speed,while the amount of precipitates decreased.The primary spacing of dendrite was decreasing with the increasing of scanning speed.The large columnar dendrite was present in the bottom of remlted layer.The measured average hardness of laser proccessed sample with 4mm/s laser speed reaches the maximum value of HV92.4,the laser processed sample with laser speed of 8mm/s showed better corrosion resistance over untreated sample with the corrosion potential increasing from-1.348V to-1.168V.The corrosion current density of clad sample with laser speed of 8mm/s was 1/4 of that of untreated sample.Al3Ti has the characteristic of corrosion resistance and high thermal stability,thus a Al3Ti containing layer with Ti B2 reinforcement was prepared by laser cladding.The results show that the cladding layer is mainly composed of flak Al3Ti,surrounding Al and Ti B2particles in agglomeration state.The thickness of Al3Ti phase on the top of clad layer is about5.6?m,and the size of Al3Ti is smaller in the bottom,whose morphology transforms into fine equiaxed shape.The eutectic layer appeared in the bonding zone on the matrix,which can improve the bonding strength between the clad layer and the matrix.The hardness and electrochemical tests showed that the average hardness of the treated sample with the power of 3k W reached HV387.8,which was 6.7 times of the hardness of the matrix;The corrosion potential increased from-1.348V for the matrix to-1.093V,while the corrosion current density decreased from 9.1 x 10-3A/cm2 to 1.3 x 10-3 A/cm2.The influence of different reinforce phase upon the microstructure and properties of laser cladding Al-Ti-Ti B2 was studied in this work.The XRD results showed that the clad layer consists of Al,Ti,Ti C phase with the laser power of 2.5k W.The calculation of mismatch indicates that the Al has near coherent lattice with Al3Ti,which can improve the corrosion resistance.The morphology of Al3Ti was mainly influenced by heat flow,causing the growth of Al3Ti perpendicular to the surface.Mg-Al eutectic was generated at the bottom of the clad layer because of the mixing of Al and Mg elements.Hardness and electrochemical results shows that the average hardness of the sample processed with the power of 2.5k W was 5 times of matrix;The corrosion potential increased by-195m V and the current density decreased from 9.1×10-3 A/cm2 for the laser clad sample to 3.2×10-3 A/cm2. |
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