Fabrication Of Mg-Zn-Sn Based Wrought Magnesium Alloys And Study On Their Extrusion Deformation Performance

Magnesium alloys have been widely used in fields of automobile,rail transportation,aerospace,and defense as the lightest structural material,and are gradually becoming one of the most important alternative materials for lightweight structures.Magnesium alloys generally exhibit poor deformation ability due to their hexagonal close-packed crystal structure,which makes it difficult to manufacture magnesium alloy components.At present,most magnesium alloy products are formed by casting.However,the segregation,shrinkage and void defects of casting products seriously affect their mechanical properties.Magnesium alloy components with fine grain structure,high strength and high elongation can be obtained and casting defects can be eliminated by plastic forming process.However,there are few types of wrought magnesium alloys at present.Especially,the development and research of new magnesium alloys with low cost,excellent mechanical properties,and good formability are lagging behind,which restricts the application and promotion of magnesium alloys.Wrought magnesium alloys are usually processed by hot deformation.Amongvarious hot deformation processes,extrusion is a promising forming process with simple operation,high production efficiency and large deformation,so magnesium alloy components with fine grain structures and good mechanical properties are more easily to be obtained.However,in the field of the fabrication of wrought magnesium alloys and study on their extrusion deformation performance,there are still several fundamental scientific and technical issues in aspects of the effects of low-cost micro alloying elements on the extrusion formability,microstructure and mechanical properties of wrought magnesium alloys,the effects of extrusion process parameters on grain structure,second-phase particles,texture and mechanical properties of the extruded bars,and the influence mechanism of extrusion process parameters on the microstructure,texture and mechanical properties of longitudinal welds in hollow profiles.Therefore,it is of great theoretical significance and engineering application value to develop new wrought magnesium alloys and study their extrusion deformation performance.In this paper,a new type of low cost wrought magnesium alloy Mg-3.0Zn-1.0Sn-0.3Mn-0.3Ca(ZTMX3100)with good comprehensive mechanical properties and extrudability was developed by the addition and optimization of Mn,Ca,Al and Cu elements into Mg-Zn-Sn based alloy.The ZTMX3100 alloy ingots were fabricated by semi-continuous casting.The microstructure and mechanical properties of the ingot at different positions were investigated.The evolution of microstructure and mechanical properties of the alloy under different homogenization conditions were revealed.Based on the homogenized alloy,the effects of extrusion temperature and extrusion speed on microstructure,texture,and mechanical properties of the alloy were systematically studied.The age hardening behavior and the influence of aging time on the microstructure and mechanical properties of the as-extruded alloy were investigated,and the aging process parameters of the alloy were optimized.Hot extrusion experiments of hollow ZTMX3100 alloy profiles were carried out,and the microstructure and mechanical properties of the extruded profiles at welding area and reference area were studied.A novel method to quantitatively evaluate the welding quality in three-dimensional space was proposed,and the welding qualities of the longitudinal welds in the hollow profiles extruded at different ram speeds were evaluated accurately.The main research contents are shown as follows:(1)By adding the low-cost alloying elements Mn,Ca,Al and Cu to Mg-Zn-Sn based alloy,the effects of the alloying elements on surface quality,microstructure and mechanical properties of the extruded alloys were investigated.The results show that the addition of a small amount of Mn can obviously improve the extrudability,refine the grain structure,and enhance the mechanical properties of the extruded alloy.Compared with A1 and Cu elements,Ca element has more significant effect on improving surface quality,refining grain structure and enhancing mechanical properties of the extruded alloy,which makes ZTMX3100 alloy exhibit good extrudability and optimum comprehensive mechanical properties.(2)ZTMX3100 alloy ingot was produced by semi-continuous casting technology.The microstructure and mechanical properties of the ingot at different positions were studied.It was found that the mechanical properties of the alloy in axial direction are slightly better than those in the radial direction,and the sample at 1/2 radius of the ingot along the axial direction exhibits the optimum comprehensive mechanical properties.The relation between microstructure and mechanical properties of the alloy under different homogenization conditions was analyzed.The optimum homogenization process of ZTMX3100 alloy is confirmed to be 340?×10 h +520 ?×16 h.After optimum homogenization treatment,the yield strength of the alloy is reduced and the elongation is increased obviously,and the microstructure uniformity is further improved,which is more favorable for the subsequent hot deformation processing.(3)The mechanism of the extrusion temperature's effect on microstructure and mechanical properties of ZTMX3100 alloy was systematically investigated.The optimum extrusion temperature was achieved and the quantitative relationship between the yield strength and extrusion temperature was obtained.As a whole,the grain size is remarkably refined after extrusion.The grain size increases gradually and grains distribute more uniformly in the alloy with the increase of extrusion temperature.The alloy extruded at 300 ? presents the optimum comprehensive mechanical properties due to the combined effect of relatively small grain size,uniformly distributed particles,and weak basal texture.On the basis of optimum extrusion temperature,the evolutions of grain structure,second phase particles,texture and mechanical properties of the alloy extruded at different ram speeds were studied.The extrudability of the alloy was also investigated and explored.It was found that,with the increase of extrusion speed,the average grain size increases,the fraction of CaMgSn phase decreases,and the maximum basal texture intensity of the as-extruded alloy increases gradually.The alloy extruded at the ram speed of 0.2 mm/s presents the highest strength and elongation.ZTMX3100 alloy has excellent extrudability due to moderate alloy content and high incipient melting temperature.Therefore,the alloy extruded at high extrusion speed of 10 mm/s(die-exit speed 18 m/min)still maintains good surface quality and relatively high comprehensive mechanical properties.(4)The age hardening behavior of ZTMX3100 alloy was studied and the aging process parameters were optimized.The evolution of microstructure and mechanical properties of the extruded alloy under different aging time was analyzed.The results show that a large number of second phase particles precipitate in the alloy under the peak aging condition of 175 ?/24 h.These densely and evenly distributed particles exhibit short phase spacing and present good strengthening effect.The effects of different extrusion speeds on grain structure,second phase particles,precipitates and mechanical properties of peak aging alloy were clarified.It was found that the size of precipitate after peak aging is relatively small and the precipitates distribute densely and uniformly in the alloy at low extrusion speed,which makes the alloy exhibit the best mechanical properties.At high extrusion speed,the aging strengthening effect is still remarkable and the alloy maintains high strength and elongation after peak aging.(5)Extrusion experiments of hollow ZTMX3100 alloy profile were carried out.The microstructure and mechanical properties of the profiles extruded at different ram speeds at welding area and reference area were investigated.The fracture morphology and fracture mode of the welds in hollow profiles extruded at different ram speeds were analyzed.At reference area of the hollow profile,with the increase of extrusion speed,the average grain size increases and the fraction of CaMgSn phase decreases gradually.The yield strength increases firstly and then decreases,and the elongation decreases gradually.At welding area,the grain size grows but the second phase particles distribute uniformly on two sides of the welds as the extrusion speed increases.The mechanical properties of the longitudinal welds at different positions have various changing trends with the increase of extrusion speed.The weld fracture morphologies at good welding zone show typical dimple structures at different extrusion speeds,while the weld fracture at poor welding zone presents various morphologies at different extrusion speeds.As the extrusion speed increases,the fracture morphology changes from large area of groove structures to dimple structures gradually.(6)Aiming at the quantitative evaluation of welding quality in hollow magnesium alloy profiles,a method on the basis of K criterion was proposed for quantitative evaluation of welding quality in three-dimension.The application of K criterion is extended from two-dimension to three-dimension,and from a welding line to the whole welding plane.Based on the hot compression tests,the equation of ZTMX3100 magnesium alloy was established.The extrusion processes of hollow profiles extruded at different ram speeds were simulated by numerical simulation software,and the physical field variables of the welding points on the whole welding plane were obtained.Then the three-dimensional welding quality factor K3D and non-dimensional welding quality factor K*were calculated.Finally,the accurate quantitative welding quality evaluation in three-dimension of the longitudinal welds in hollow magnesium alloy profiles was achieved.