Study On Surface Strengthening By High-energy In Magnesium Alloy

In most cases, material failures occur on surfaces such as fretting fatigue, wear, corrosion and thermostability, etc. These failures are very sensitive to the structure and properties of the material surface. Optimization of the surface microstructure and properties is an effective approach to enhance the global behavior and service lifetime of materials. It has realistic significance for promoting wider application and giving full play to its unique performance advantages of magnesium alloys to strengthen the research on improving the wear and corrosion resistance. In this paper, the techniques of self-surface nanocrystallization induced by high-energy bombarding and laser alloying were used to achieve the purposes of improvement on surface properties of magnesium alloy.The technology of high-energy bombarding was investigated systematically. The process parameters were optimized to applicable to self-surface nanocrystallization of a magnesium alloy as follows:the pressure of N2 and O2 is 1.5 MPa, nitrogen and oxygen flow ratio of 7:5, kerosene flow rate of 4 L/h, bombing particle diameter ofφ0.5 mm, impact distance range of 290～320 mm, handling time in the range of 180～240 s. The surface nanocrystallization on magnesium alloy can be obtained successfully using the above technological parameters.Cross-sectional microstructure analysis of nano-layer revealed that the deformation layer presents gradient variation obviously, including the topmost surface layer of severe plastic deformation, deformation twins-based subsurface layer and a transition layer with slight deformation near the substrate. The inherent refinement mechanism of surface nanocrystallization on magnesium alloy was deduced through observation and analysis using transmission electron microscopy (TEM/HRTEM). Meanwhile, the grain refinement model of coarse grains converted into nano-grains was established under the condition of severe plastic deformation, i.e. the deformation model is dominated by mechanical twinning in the initial stage of magnesium alloy, and accompanied with the dislocation movement of basical plane (0001) and prismatic plane {10(?)0} or {11(?)0}. The coordination/competition between the deformation twinning and dislocation motion dominated the deformationprocess in the mid-stage of the severe plastic deformation. Subsequently, cross slip can be activated owing to a certain degree of grain refinement and temperature rising derived from high-energy bombarding, Dislocation movement dominates the competition in the later stage, and resulting in further fragmentation of the residual twins and micro-banded substructure. Then, the dynamic recrystallization occurred with sufficient driving force that coming from high-energy substructure with the increasing distortion, deformation storage increase, dislocation multiplication, annihilation and rearrangement. Eventually, the nano-grains formed with clear grain boundary, homogeneous distribution and random orientation.The behaviors of nano-layer were studied systematically. The microhardness of the most top surface layer is about twice comparison with the substrate, and it is gradually decreasing with the increase of the depth from the surface. The friction coefficient and wear weight loss of nano-layer were reduced significantly, and adhesive wear and abrasive wear is the main wear mechanism, meanwhile, coupled with oxidation wear. It is found that the deterioration of corrosion resistance occurred for nano-layer in acid, alkali and salt 3.5% NaCl solution with different PH values. Thermal stability experiments show that the critical temperature of stabilized existence for nanocrystalline of magnesium alloy is 330℃. The vacuum microwave oven was used for experiments of diffusion alloying Al-Si alloy in magnesium alloy before and after the surface nanocrystallization treatment. It's worth noting that the diffusion alloying of magnesium alloy utilizing microwave heating has not been reported. The results showed that the thickness of alloyed layer increasing with the processing temperature increases, and the thickness of nano-treated alloying layer is more 2 to 3 times than that without nano-treatment.Laser process of magnesium alloy by pulsed YAG laser surface alloying was investigated systematically. The optimized process parameters of laser surface alloying for Mg alloys are obtained as follows:pulse duration of 0.8 ms, frequency of 45 Hz, spot diameter of about 1.0 mm, current of 220 A and the scanning speed is 350 mm/min.The results of laser surface alloying for Al-Nb/Al-TiB2 and Al-(W, Ti)C system showed that in situ reaction produced in the alloyed layer between the element Al and Nb, and the matrix elements Gd, Y, and generated a high-temperature hard-phase such as Al2Gd, Al2Y and Al3Nb intermetallic compounds. Furthermore, strengthening phase TiB2 and (W,Ti)C did not decompose by XRD diffraction analysis of the alloyed layer. Surface macro-morphology shows that there has been some degree of nodulation phenomenon on the quality of laser surface alloying with the content of alloying element Al tapering off in the mixed powders.It is found that the entire cross-section is divided into three parts:alloyed layer, transition layer and matrix by SEM observation. The TEM observation shows that the grain refinement is significant and uniform distribution, furthermore, the new phases such as Al2Gd, Al2Y, Al3Nb, etc. of in situ synthesis and strengthening phase TiB2 and (W, Ti)C are uniformly dispersed in the alloyed layer, and most present nearly spherical shape, only a small part of that shows the quadrangle blocks like, and the size of most of intermetallic compounds is about 100 nm. It is worth noting that the morphology of larger size is not a single precipitates, but agglomeration of a number of fine particles by further observation.The performance of laser alloyed layer shows that the hardness of alloyed layer is to improve up to 4-6 times compared with the substrate. The friction and wear performance of the alloyed layer with different alloying elements and the mass ratio can be improved under the condition of dry friction and wear significantly, and the friction coefficient changed from about 0.52 to 0.25～0.35. The main mechanism of wear for the alloyed layer is the adhesive wear, abrasive wear and oxidation wear. The corrosion resistance of magnesium alloy in 3.5% NaCl solution can be improved by laser surface alloying.