The Preparation Of Gradient Porous Mg-Mn Alloy And Its Surface Modification Research

Gradient porous Mg-Mn alloys were prepared by powder metallurgy method withNH4HCO3as pore-forming agent and Mg powder, Mn powder as raw material. The effects ofNH4HCO3distribution, particle sizes of Mg powder, Mn content, pressing pressure andsintering temperature on pore characteristic, sintering shrinkage compressive strength andmicrohardness of sintered products were studied. The effect of the addition of Mn element oncorrosion resistance of gradient porous Mg-Mn alloys was observed. The gradient porousMg-Mn alloys were micro-arc oxidated in the Na2SiO3electrolyte. The voltage-time curvesduring micro-arc oxidationg process were analyzed. The effects of oxidation current and time,content of Mn element on voltage-time curves, morphology and thickness of the oxidationlayers were studied. The phase composition of the oxidation layers was analyzed andcorrosion resistance of the porous Mg-Mn alloys samples was measured.The results show with the decrease of pore-forming agent in the inner layer and theincrease of sintering temperature, the porosity of sintered products decreases, but the sinteringshrinkage and compressive strength increases. With the decrease of the particle size of Mgpowder and the pressing pressure increase, the porosity of sintered products decrease, but thesintering shrinkage increases. The compact degree of the grains in the pore cell of gradientporous Mg-Mn alloys increases and the grain sizes decrease with the increase of the Mncontents. But the compact degree of the grains in the pore cells of gradient porous Mg-Mnalloys decreases slightly when Mn content is beyond2wt%. XRD and EDS analysis show thegradient porous Mg-Mn alloys are composed with a simple phase of α-Mg solid solutionwhen Mn content is2wt%and two phases of α-Mg solid solution and α-Mn, when Mncontent is3wt%. With the increase of Mn content, the compressive strengths of gradientporous Mg-Mn alloys first increase and then decrease, but the micro-hardness of the gradientporous Mg-Mn alloys always increases.The corrosion resistance shows the Ecorrincreases,Icorrand corrosion rate of the gradient porous Mg-Mn alloys both decrease at the same time.The corrosion resistance of gradient porous Mg-Mn alloys increases. The gradient porousMg-Mn alloys with2wt%Mn, NH4HCO3distribution of15wt%-5wt%-15wt%, pressingpressure of100MPa are sintered at640℃, the compressive strength, microhardness,corrosion rate of sintered products are32.5MPa,49.5HV,0.178mm/a, respectively.The results show with the increase of oxidation current density and oxidation time, U1、U2、U3during micro-arc oxidation process and the thickness of the oxidation layers alsoincrease. With the increase of the Mn content, U1、U2、U3also decrease, but they all increasewhen Mn content is beyond2wt%. With the increase of the content of Mn, the thickness ofoxidation layers first decrease and then increase. The oxidation films of the thickness of39.6μm have fine and uniform pores and best quality with current density of9.5A/dm2, oxidationtime of2min and Mn content of2wt%. XRD and EDS analysis show there are MgO and Mg2SiO4phases in the oxidation films. Corrosion resistance measurement shows the gradientporous Mg-Mn alloys without micro-arc oxidation have lower Ecorr, higher Icorr, bigger theamount of hydrogen evolution and weight loss. For the samples with micro-arc oxidation, theEcorrincreases and Icorrdecreases. The weigh loss of the oxidated samples first increases andthen decreases. But the weight loss and amount of hydrogen evolution are both less than thoseof samples without micro-arc oxidation, which shows the corrosion resistance of gradientporous Mg alloys is improved after micro-arc oxidation. When Mn element is added in Mgmatrix, the corrosion resistance of gradient porous Mg-Mn alloys is best.