| Magnesium and magnesium alloy materials with the advantages of lowdensity, high specific strength, high specific stiffness, recycling, etc., known asthe ‘green engineering materials’ in twenty-first century, have attracted greatattention and have applied in transportation, electronics and other fields. But thepoor mechanical properties at room temperature, corrosion resistance and wearresistance of the surface limit its broader development and widerapplication. The laser processing means ‘common manufacturing system infuture’ that, for high energy density, good controllability, energy saving andalmost no environmental pollution, is one of the ideal methods for magnesiumalloy surface modification.Based on the discussion of the interaction mechanism of laser andmagnesium alloy, the surface of AZ31B magnesium alloy was modified by CO2laser machine with high power and the wavelength of10.6m. Laser surfacemelting and surface cladding were carried out in two different cooling medium.The microstructure and surface properties of the modified layer were analyzedcontrastively, based on the theory and experimental data the microstructureevolution and the modification mechanism of laser surface modified magnesiumalloy were explained.In order to study the solidification and crystallization of magnesium alloy,experiment of magnesium alloy droplets cooled in different cooling medium wascarried out. The results indicated that, respectively cooled in argon gas, water,oil and liquid nitrogen, the grain size of magnesium alloy droplet decreased, andthe content of β-Mg17Al12also decreased gradually. The hardness of magnesium alloys droplets was also different with the different microstructure.Microhardness of magnesium alloy droplets cooled in the argon gas, water,quenching oil and liquid nitrogen was53.7HV,56.0HV,60.1HV and73.2HVrespectively. The microhardness of magnesium alloy droplet cooled in liquidnitrogen was1.36times,1.31times and1.22times of other droplets.In order to study the solidification and crystallization of laser modifying onmagnesium alloy under rapid cooling, laser surface melting was carried out intwo different medium which was argon gas and industrial liquid nitrogen. Thedepth of melted layer cooled in argon gas (CAL) or liquid nitrogen (CLNL) was580m and230m. The melting magnesium alloy grew with the oppositedirection of heat flow. From the bottom to the top of the melted layer, thedendrite grew, and dendrite arm spacing decreased, and the surfacemicrostructure grew to disorder direction. But in the CLNL, the grain size wasfiner, and the grains in the middle and the top of the melted layer were moreuniform, and the periodic microstructure was more obvious. The average grainsize of the original magnesium alloy and CAL and CLNL was about58.7μm,10.8μm and5.1μm. Melted layer and the original magnesium alloywere composed of α-Mg and β-Mg17Al12, but β-Mg17Al12in CLNL with morerapid cooling solidification reduced to almost nothing. Distribution andmorphology of precipitates were also different, precipitated phase appeared atgrain boundary and inside in the original magnesium alloy, while in the CALand CLNL they mainly appeared in the crystal. In these three kinds of materials,precipitated phases were in the form of spherical, rodlike respectively and strip.A mixed structure of nanocrystalline and amorphous was present in the CLNL.After laser surface melting, microhardness, wear resistance and corrosionresistance of magnesium alloy were improved, especially the improvingdegree of cooled in liquid nitrogen was the best. The average microhardness ofCAL and CLNL was2.98times and1.54times of that of as-receivedmagnesium alloy respectively. After rubbed under the load of2N for20min, thewear loss of as-received magnesium alloy, CAL and CLNL was10mg,8mg and 5mg respectively. The electrochemical corrosion results showed that, thecorrosion potential of CLNL was more positive about133mV and7mV than thatof as-received magnesium alloy and CAL, and corrosion current density waslower about2and1orders of magnitude, and the average corrosion rate was0.37times and0.78times of that of as-received magnesium alloy and CAL.The low temperature of liquid nitrogen cooling was used for laser surfacecladding of magnesium alloy, and a new composite powder cladding materialsof Al-Si alloy and Si3N4ceramic was adopted for magnesium alloy lasercladding. Si3N4ceramic would break down with high energy laser, and wouldinteract with the Al-Si alloy and surface of the substrate, a cladding layercomposed of Al, AlN, Al9Si and Mg2Si was formed. The microhardness of thecladded layer was increased about6.69times than the substrate. Thecorrosion potential was365mV, and the corrosion current density decreased byabout two orders of magnitude.On the basis of above studies, the solidification and crystallization behavior,the microstructure evolution mechanism and modification mechanism ofmagnesium alloy laser surface modification were analyzed. In the interactionprocess of magnesium alloy with laser, the surface of magnesium alloymelted. At the same time, with the effect of thermal super-cooling, moltenpool solidificated gradually and grew with the matrixunmelted grains on surfaceof magnesium alloy. The microstructure of the melted layer was columnardendritic finally, from the bottom to the top of melted layer, dendrite armspacing decreased, and the convergence of growth direction and thermalperturbations led to periodic microstructure. Grain refinement, high dislocationdensity and solid solution strengthening played a very important role on thesurface properties of laser surface modifying on magnesium alloy. |
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