| The technological parameters of Selective Laser Melting(SLM)were determined by optimizing the density.The AZ91D magnesium alloy sample was prepared by SLM technology,then solution treatment(T4)and solution-aging treatment(T6)were conducted respectively.The microstructures,resistance and microhardness to corrosion of AZ91D magnesium alloy samples with SLM,T4 solid solution and T6 solution-aging were tested and examined through using metallographic microscope,scanning electron microscope(SEM),X-ray diffraction(XRD)and electrochemical workstation.The corrosion behaviors of SLM AZ91D magnesium alloy in 0.06 mol/L Na Cl solution(3.5%Na Cl solution),0.06mol/L Na2SO4 solution and 0.06 mol/L Mg SO4 solution were compared,and the effects of Cl–and SO42–on the corrosion property of SLM AZ91D magnesium alloy were studied.ZSimp Win was used to carry out the fitting analysis according to the electrochemical impedance spectrum,calculate the value of each equivalent element,establish the equivalent circuit model,and analyze their principles and change rules.The experimental results of corrosion under different heat treatment processes showed that,after solution treatment at 420°C for 6 hours,the?-Mg17Al12 phase of the SLM AZ91D magnesium alloy sample was basically solute in the matrix,and the grain size had increased from 5?m to about 60?m.After aging treatment at 200°C for 2 hours,the?-Mg17Al12phases were precipitated on the grain boundaries and matrix.As the aging time being extended from 2 hours to 6 hours,the amounts of precipitated phases increased gradually as well.After being immersed in a solution of 3.5%Na Cl for 120 hours,SLM sample had generated relatively small-sized corrosion pits on its surface.By contrast,larger corrosion pits appeared on T4 samples.In addition,the T6 samples demonstrated the most serious degree of corrosion,with corrosion pits of different sizes and depths appearing on the surface.Compared with 3.5%Na Cl solution in different heat treatment process,SLM samples in 0.06 mol/L Na2SO4 solution still have part of?-Mg phase on the surface,and the corrosion degree is relatively light.The polarization curve of the samples showed that,Jcorrof the SLM sample was(1.889±0.2)×10–5 A·cm–2 and was the smallest one(SLM sample also demonstrated the best resistance to corrosion).Jcorr of the samples which were treated first by solution and then aging for 2 hours were the largest one-(1.225±0.2)×10–4 A·cm–2.The sample demonstrated the worst resistance to corrosion.Regarding to the resistance to corrosion in 3.5%Na Cl solution,the ranking list of the three samples was:the SLM sample>the T4 sample>the T6 sample.The corrosion results in different salt solutions showed that,the overall polarization impedance value Rp of SLM sample in Na2SO4 solution was 2.61 times in Mg SO4 solution and 11.63 times in Na Cl solution.The polarization current densities of SLM samples in the three solutions were Jcorr(Na2SO4)=(7.047±0.04)×10–6A·cm–2,Jcorr(Mg SO4)=(1.269±0.15)×10–5A·cm–2,and Jcorr(Na Cl)=(9.166±0.10)×10–5 A·cm–2 respectively.The corrosion resistance of SLM AZ91D magnesium alloy in the three salt solutions was ranked as Na2SO4>Mg SO4>Na Cl.In addition,SLMAZ91D magnesium alloy was mainly uniform corrosion in Na2SO4and Mg SO4 solutions,while pitting corrosion occurred in Na Cl solution.The experimental results of microhardness under different heat treatment processes showed that,the grain size of SLM AZ91D magnesium alloy was only 5?m,which was about 10 times smaller than that of the as-cast grain size.After SLM samples were treated for 4 hours,a large number of?-Mg17Al12 phases were still distributed on?-Mg matrix.After the solution time was extended to 6 hours,8 hours,10 hours,12 hours and 14 hours,?-Mg17Al12 was completely dissolved in the matrix.However,the microstructure and grain size have little change.After 4 hours of solution,the microhardness value reached the highest(85.69 HV),and the holding time of solution reached 6 hours,and the microhardness value decreased.With the extension of solution treatment time,the microhardness value changed little.After SLM samples were treated with solution for 6 hours,solution+aging for 2 hours,4 hours and 6 hours,the microhardness of the samples changed significantly due to the dual effect of grain size and?-Mg17Al12 phase.The order of microhardness value is:the SLM sample>the T6+6 sample>the T6+4 sample>the T6+2 sample>the T4sample>the as-cast. |
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