The Research Of Electroless Ni-P And Ni-P-SiC On AZ91D Magnesium Alloy

Magnesium is one of the most abundant elements on the Earth. Because of its low density, high specific strength and specific stiffness, and good performance of casting and machining properties, magnesium alloys are widely used in aerospace, electronic communication, hardware, automobile and defense fields etc. AZ91D has been becoming into the focus by its anti-aging, shock resistance, good cushioning ability and dimensional stability, good mechanical behavior and good electromagnetic shielding.Although magnesium alloys has such outstanding advantages, its poor corrosion resistance caused by active chemical property of magnesium element has been limited their application. Electroless plating Ni-P(EN) is one type of environmental protection surface treating technology, it not only has simple process and low costing, but also could remarkably improve corrosion resistance and wear resistance properties of materials. The composite coating made by EN solution with hard particles can significantly raise wear resistance of magnesium alloys. At present, study of electroless plating on magnesium alloys is more and more extensive both in domestic and in international. While many problems still exist, such as the poor stability of electroless plating solution; the unmanageable quality of coating that sensitive to the composition of electroless plating solution and process parameters, and the unsystematic tribological properties. In this thesis, the optimal process parameters and plating time of EN and composite plating on AZ91D, and heat treatment process to achieve coating for good corrosion resistance and wear resistance were investigated systematically.Through orthogonal test, immersion test, the experiments were done with equipments such as optical microscope, SEM, hardness tester, TE66 micrograin-abrasion testing machine and electrochemical analyzer, etc. The mainly conclusions were:⑴For AZ91D alloy, the best parameters of EN process were: pH value and temperature of EN solution was 6.0 and 90℃separately; plating time was 60min, and the stabilizer was KIO3 + thiourea. The best parameters of electroless plating Ni-P-SiC process were: concentrations of SiC particles and surfactant(lauryl sodium sulfate + polyethylene glycol(6000) ) was 3g/L and 60mg/L separately; SiC particles size was 1-2μm; stirring speed was 18r/s; and plating time was 120min.⑵Surface morphologies was very important to corrosion resistance properties of the Ni-P-SiC coatings. Coatings with uniform and dense cellular structure had good corrosion resistance properties, while, on the contrary, coatings with rough and loose cellular structure had bad corrosion resistance properties.⑶With increasing of revolution speed, firstly friction coefficient of Ni-P-SiC composite coating shows increase trend, but then decreased. With increasing of loading value, friction coefficient shows increase trend. As time goes on, wear volume lose rate shows growing trend and at plating time 120min layer got the best optimal tribological properties. With increasing of heat treatment temperature, friction coefficient of Ni-P-SiC composite coating shows increase trend, and its abrasion mechanism was mainly grain-abrasion.⑷Considering to hardness, corrosion resistance and wear resistance of Ni-P-SiC composite coating, the best plating time was 120min, and its heat treatment temperature was 320℃.