Investigation On The Material Design And Fabrication Technique For Magnesium Alloy Cladding And Repairing

During actual use,various local damages,such as corrosion and wear on the surfaces of magnesium alloy structural parts or components,are inevitable.These damages severely affect the overall performance and service life of magnesium alloy materials.Surfacing welding,a type of cladding technology,which can be used to perform effective and quick repairs of local damages on the surfaces of a magnesium alloy equipment,which has high importance for the service life extension of metal equipment and the maintenance period shortening.By contrast,the mechanical and corrosion properties of surface welded magnesium alloys are usually unsatisfactory,which severely restricts the magnesium alloy surfacing welding technology development.In addition to surfacing welding,other welding processes for magnesium alloys are also proven necessary,such as butt welding.As filling material,magnesium welding wires have good application prospect.Adversely,the plastic forming ability of magnesium is limited at room temperature.As a result,the traditional extrusion and cold drawing are difficult to produce magnesium wires.To increase the deformation degree and avoid frequent annealing,the hot-drawing technique was exploited.Furthermore,the current research was not systematic and most research objects was AZ31 alloy.In conclusion,two important problems exist in the magnesium alloy welding technology development.Firstly,the forming property of magnesium alloys is poor.It is consequently difficult to realize the industrial production of magnesium alloy wires.Secondly,the welded magnesium alloy performance is poor,especially in the aspects of wear and corrosion resistance.To solve these problems,the hot-drawing technique of a traditional Mg-Al-Zn alloy was exploited.The preparation of magnesium alloy wires for industrialization was investigated,which was combined with the characteristics of drawing and extrusion.In order to improve the properties of surface welding alloy,the contents of the main alloy elements were adjusted,various rare earth elements were added and the post-weld heat treatment was studied.The main research contents and results are as follows:(1)The effects of homogenizing annealing on the microstructure and properties of the as-cast AZ61 alloy,along with the effect of extrusion and surface quality of the AZ61 extrusion rod were studied.The cause of surface cracks was analyzed.The results demonstrated that in the magnesium alloy extrusion,the differences in temperature and force of the inner and outer layers resulted in the additional stress of the outer metal.When the additional stress exceeded the tensile strength of the metal,the crack morphology was produced.The reasonable homogenizing annealing(400℃/12 h)was obtained.The extrusion parameter(380℃,1.3 mm/s)was selected,through which,the prepared rod was not required to be annealed.(2)During the single pass hot-drawing,the effects of drawing temperature,speed.deformation degree and cooling mode on the drawing fracture phenomenon,along with the microstructure and properties of the AZ61 wires,were studied.The parameter range for stable hot-drawing was summarized.The results demonstrated that the excessive drawing temperature and the deformation degree resulted in the fracture of wires,due to the drawing force exceeding the maximum tensile strength the wire could sustain.As the drawing temperature decreased and the deformation degree increased,under the water cooling condition,the wire strength increased and the ductility decreased.In the parameter range of this experiment,the drawing speed change had no apparent effect on the AZ61 magnesium alloy wire performance.(3)The maximum accumulated deformation degree of AZ61 wires without annealing during drawing was studied.The reasonable technological parameters of multi pass hot-drawing of AZ61 wires were formulated.The evolution rule of the structure of the wires during multi pass hot-drawing was investigated.The results demonstrated that the change of channel deformation had low effect on the maximum accumulated deformation when the drawing temperature was invariable.By contrast,when the channel deformation was invariable,the maximum accumulated deformation increased as the temperature increased.The multi pass hot-drawing of the AZ61 magnesium alloy superfine welding wire(Φ0.72 mm)with cumulative deformation of 98.6%was exploited,which was the lowest-sized magnesium alloy welding wire reported in the current literature.The AZ91 welding wire(Φ3.00 mm)preparation plan was formulated with reference to the research results of the AZ61 welding wire.(4)Through the utilization of Mg-Al-Zn magnesium alloy wires for surfacing welding,the effect as well as the action mechanism of the Al,Zn and Mn contents along with heat treatment,subsequently to welding,on the structure and properties of the Mg-Al-Zn surfacing weld alloy were studied.The friction and wear mechanisms of the magnesium alloy under different experimental loads were analyzed..The results demonstrated that the Al and Zn content increase had positive effect on the mechanism and corrosion properties of the surfacing magnesium alloy.Moreover,the increase of Mn had low effect on the mechanism properties and apparent positive effect on the corrosion resistance.The heat treatment resulted in the dissolution of the massiveβ-Mg17Al12 phase,which was believed to reduce the stress concentration generated in the particle-to-matrix interface as well as to improve the wear resistance of alloys.Following heat treatment,the thickness of the deformation layer in the subsurface of the surfacing alloy was decreased and the wear resistance was improved.With the tribological system presented in this paper,the dominant wear mechanisms were abrasion and oxidation in the load range of 5-50 N,which were related to mild wear;delamination and plastic deformation occurred mainly at high loads(50-100 N),corresponding to severe wear.(5)The effects as well as the action mechanisms of the Y,Gd and Ce rare elements on the structure and properties of the AZ91 surfacing alloy were studied.The effects of tribological parameters on the friction and wear behavior of the surfacing alloy were also investigated.The results demonstrated that the addition of rare-earth element reduced the size and amount of the coarse β-Mg17Al12 phase,which resulted in the decrease of macrohardness as well as the increase of wear and corrosion resistance.The AZ91+0.5%Gd alloy had the best wear resistance property which was 39.2%lower than the commercial AZ91 welding wire sold on the market.The AZ91+1%Y alloy had the best corrosion resistance property,which was 95.2%lower than the commercial AZ91 welding wire.As the sliding speed increased,the abrasive wear degree of the magnesium alloy was reduced,the degrees of oxidation wear and severe plastic deformation increased,while the friction coefficient and wear rate decreased.At the load range of 10～20 N and the sliding speed range of 0.5-1.25 m/s,the surfacing AZ91 magnesium alloy with 1.5%Ce had the lowest friction coefficient and wear rate.