The Influence Of Friction Stir Processing On The Wear Resistance And Corrosion Resistance Of AZ31 Mg Alloy

With UMT-3 type multifunctional friction and wear testing machine, the friction and wear property of friction stir processed AZ31 magnesium alloy FSP-AZ31 and the base metal AZ31 magnesium alloy was comparatively studied by the ball-block dual reciprocating dry sliding friction and wear test method. The influence law of the load and time on the friction coefficient and wear volume of the two was explored. The corrosion resistance of the two was studied using salt bath soak method. The variation law of the corrosion rate with the immersion time was analyzed. With optical microscope, scanning electron microscope and energy disperse spectroscopy, the microstructure of FSP-AZ31 and AZ31, the morphology characteristics and chemical composition of the wear surface, wear debris, corrosion surface and the dual friction surface were analyzed. Both of the polarization curve and electrochemical impedance was measured with electrochemical testing instrument. Based on the experimental results and theoretical analysis, the improved mechanism of FSP on the friction and wear property and corrosion resistance of the AZ31 magnesium alloy was discussed. The main research content and results are as follow:1、The influence law of the load and time on the friction coefficient and the wear volume of FSP-AZ31 and AZ31 magnesium alloy was explored. Under the constant load, the instantaneous friction coefficient of FSP-AZ31 and AZ31 gradually decreases with time extension in the first 0-200s time period. The starting friction coefficient of the former is slightly smaller than that of the latter. Subsequently, both of the friction coefficient gradually tend to be close to the steady state and consistent. Under the same friction time, the average friction coefficient of FSP-AZ31 and AZ31 increases with the increasing of the load but the increasing amplitude gradually decreases. The average friction coefficient of FSP-AZ31 is smaller than the base metal. With the increasing of the time or the load, both of the wear volume show increasing momentum. But the wear volume of FSP-AZ31 is smaller than AZ31 which means that FSP can improve the friction and wear properties of AZ31 magnesium alloy surface.2、The morphology characteristics and the chemical composition of the wear surface, abrasive debris, and the friction pair of FSP-AZ31 and AZ31 magnesium alloy were analyzed. And the wear mechanism in the process of friction and wear was discussed. Both of the wear surface show the morphology features such as furrow, oxidization, flaw, and delamination. With the increasing of the time or the load, more serious plastic deformation and material peeling occured on the wear surface of the base metal AZ31. And a lot of flaws and potholes are formed on the wear surface. The abrasive debris of FSP-AZ31 and AZ31 turns from graininess to bulk. But the size of the latter is even bigger. A large amount of wear debris adhere to the friction surface of the friction pair which illustrates that, in the process of the reciprocating friction, a lot of magnesium alloy material transfer to the friction pair part. FSP-AZ31 and AZ31 have experienced from abrasive wear to oxidation wear, ending up into the serious adhesive wear and peeling wear at last. But the process of FSP-AZ31 into adhesive wear and peeling wear is slower.3、The influence law of the corrosion morphology characteristics and the corrosion rate of FSP-AZ31 and AZ31 with the immersion time was explored, and the variation law of the polarization curve and electrochemical impedance of the two were studied. In the solution containing Cl", both of the two show bit corrosion and the corrosion degree deepen with the corrosion time. But the pitting corrosion rate of FSP-AZ31 is much less than AZ31 and the corrosion degree is lower. After 24h corrosion time, a lot of corrosion crack and flake appear on the surface of AZ31, but the FSP-AZ31 only formed corrosion crack on the local scale. The self corrosion potential of FSP-AZ31 is-1675mv, which is slightly higher than-1681.7mv of AZ31, the corrosion current density is about an order of magnitude lower, and the former impedance better than the latter. It suggests that FSP improved the corrosion resistance of AZ31 magnesium alloy.4、The influence of FSP on the microstructure of AZ31 magnesium alloy was studied. Based on the microstructure evolution characteristics of FSP-AZ31, the improved mechanism of FSP on the friction and wear property and corrosion resistance performance was explored. The base metal AZ31 has coarse grains with uneven shape and size, there is much second phase discrete distribute in the grain boundary and transgranular. Under the effect of friction heat and deformation heat together with strong shear and extrusion deformation in the FSP process, the AZ31 magnesium alloy sheet produced obvious dynamic recrystallization behavior. Uniform fine grain structure is formed in the FSP-AZ31, the average grain size is about 10μm, and the second phase significantly get reduced because of solid solution, which is the reason that corrosion resistance getting enhanced. Grain boundaries increased in small grains, surface hardness increased is the reason that the friction and wear performance getting enhanced.