Study On The Praparation Process,Compacting Mechanism And Mechancial Properties Of The Hollow Al Alloy Drive Shaft

The wheel-rail dynamic interaction directionally depends on the weight of the locomotive bogie especially the unsprung mass. With the increasing locomotive speed, the weight reduction of the entire body especially the unsprung mass is crucial to improve the safety and the flexibility of the high-speed locomotives, which is also the important content involved in the design and development of the locomotive bogie at present. The6-link hollow drive shaft is one of the main component in the unsprung structure. It is firstly proposed in the thesis that the high-strength wrought Al-Zn-Mg-Cu alloy can be used to fabricate the hollow drive shaft of the high-speed locomotive instead of the traditional steels, which is aimed at improving the wheel-rail dynamic interaction and the dynamic performance of locomotives by the weight reduction of the hollow drive shaft. However, the Al-Zn-Mg-Cu alloy exhibits poor castability and has high crack tendency. Therefore, squeeze casting is adopted to prepare the the hollow drive shaft of high-strength aluminum alloy and the praparation process, the compacting mechanism and mechancial properties of the hollow drive shaft squeeze castings are studied. The1:1trail product of the hollow Al-Zn-Mg-Cu alloy drive shaft (φthe end of shaft=780mm, φthe axle body=340mm,φthe inner-pore=230mm, h=1330mm) is prepared.(1) The microstructures of the1:1and1:5trail products of the hollow Al-Zn-Mg-Cu alloy drive shafts are comparatively studied. The former is characteristics of the semisolid solidification structure, while the latter is featured with the dendrite structure. It can be acribe to the height difference of the two type trail products. The pouring height of the1:1trail product is1500mm, much higher than that of the1:5trail product (300mm). The violent mechanical erosion and the strong convection of the aluminum melt are involved during the pouring of the1:1trail product, which lead to the breakup of the unsteady crystal bolck at the advancing front solidification and the dendrite or remelting, and even the shatter and collapse of the equiaxial chilled layer. Therefore, plenty of well-distributed, solid-state spherical particles are suspended in the aluminum melt. However, the conditions for the formation of such spherical particles are not existent during the preparation of thel:5trail product.(2) The condition and the process of the formation of the semisolid loosen layer during squeeze casting as well as its role in the exhaust mechanism of the die cavity are invesigated. A suitable increase in the pouring temperature of the alloy melt and the raised preheating temperature of the mold are beneficial to the prolong solidification time of the alloy melt in the mold. Therefore, no dense crust layer but the unsteady, semisolid loosen layer is formed during the initial stage of squeeze casting, which is convenient to the exhaust of the die cavity. Three exhaust mechanisms associated with the gap of squeezing head, the overflow groove and porous medium are involved during the preparation of the hollow drive shafts.(3) The1:1trail products of the hollow Al-Zn-Mg-Cu alloy drive shaft are prepared. The mechanisms of the timely retrievement of the mandrel, the temperature gradient preheating of the mold and the increase of pouring temperature during the preparation of such hollow shafts are explored. The effects of the temperature gradient preheating of the mold and the increase of pouring temperature on the microstructure and mechanical properties of the as-prepared hollow shaft are comparatively analysed. The as-prepared hollow shaft is characteristics of uniform microstructure, well-distributed fine spheric α-Al grains and lack of coarse dendrites. In comparison with that prepared without the temperature gradient preheating of the mold and the increase of pouring temperature, the average ultimate tensile strength, yield strength and elongation of the shaft increase40MPa,45MPa and2%respectively.(4) The effects of the process parameters such as the scheduled pressurizing start time t1, the pressure maintaining time t2, the specific pressure P, the pouring temperature Tm and the mold preheating temperature Td on the microstructure and mechanical properties of the1:5trail product are investigated. The optimised process parameters are as follows:ti=5s, t2-60s, P=160MPa, Tm=700℃. The as-prepared alloy is characteristics of densy, uniform and fine-grained microstructure and excellent mechancial properties, with the ambient tensile strength and elongation up to555MPa and9.0%repectively. The combined addition of Al-5Ti-1B and Al-10%RE exhibits a more obvious grain refinement effect on the Al-Zn-Mg-Cu alloy squeeze casting. The as-cast alloy is characteristics of the average grain size of18μm and enhanced ambient mechancial properties. The tensile strength and elongation are575MPa and10.1%respectively, which are6%and24%higher than those of the unmodified alloy.(5) The ambient tensile strength, the rotation bending fatigue strength and the static strength of the he1:1trail product are examined. No obvious difference in the tensile strength and elongation are detected along the altitude and the radial direction. The lowest measured value of the tensile strength is538MPa, while that of the elongation is7.0%. The σ-1(1x107) value of the smooth specimen is140MPa, while that of the notched specimen is69MPa. The highest measured stress value at all the testing positions is not beyond250MPa under the different testing conditions, which is much less than the lowest measured values of the ambient tensile strength (538MPa)and the yield strength (478MPa). The feasability in the preparation of the hollow Al-Zn-Mg-Cu alloy drive shaft by squeeze casting is demonstrated. The as-prepared Al-Zn-Mg-Cu hollow shaft can meet the operation requirements for the tensile strength, fatigue strength and the static strength of the hollow drive shaft under the different conditions, indicating the Al-Zn-Mg-Cu squeeze casting has the basic prerequisite for the replacement of the traditional steel shaft and thus the goal for the reduction of the unsprung mass can be effectively achieved.