| ZL114A is a heat-treated cast aluminum alloy independently developed in China.It is widely used in manufacturing large-scale components such as aeroengine gearbox with its excellent casting properties.However,due to the limitation of casting process,there are many casting defects and poor mechanical properties of the gearbox castings for a long time.The occurrence of these problems leads to the low yield of the gearbox,high manufacturing cost,and more serious is the poor mechanical properties of ZL114 A itself,the high accident rate and repair rate during the service of the gearbox,which seriously threatens the safety of the aviation aircraft in China.It restricts the further improvement of the performance of the aircraft.Therefore,the improvement of gearbox performance and casting defect repair have become an important problem to be solved.Under the support of the national "two machine special",the project focuses on the optimization of ZL114 A heat treatment and the repair welding method of casting defects.In this paper,the performance of the gearbox is improved by heat treatment method.The influence of heat treatment process on the structure and performance and the strengthening mechanism of precipitates during aging process are studied.It is found that the main reason for the low strength of ZL114 A is that the continuous distribution of sheet Si in the grain boundary seriously splits the aluminum matrix.After solid solution treatment,the lamellar eutectic Si spheroidization occurs,and the material strength and plasticity are significantly improved;Precipitated during aging β’’ The results show that the effect of strengthening is the best.The over aging temperature and the long time of aging will lead to the over aging.On this basis,the response surface method was used to optimize the heat treatment parameters.It was found that the best heat treatment process system should be 540 ℃ /12.7h +177.6℃/9.1h solid solution,and then the tensile strength of ZL114 A was 357 Mpa,which was about 190% higher than that before heat treatment(121MPa).The elongation was 5.6%.Based on the size specification of the actual casting defects,four types of openings are designed.The manual TIG welding is used to repair the casting defects.The influence of welding process parameters on the weld forming,joint structure and mechanical properties is studied.The results show that the size of the repair weld opening will affect the selection of welding process and the selection of welding process.The best welding process for the opening of Φ10 mm-5mm is 240 A welding current and single layer repair welding;The best welding current of the opening ofΦ20mm-10 mm is 220 MPa,and it is repaired in two times;The interference problem of welding tool exists in the repair welding of Φ25mm-20 mm opening,so it is impossible to get well formed joint.It is necessary to expand the size to the normal welding can only be carried out in Φ40mm-20 mm.At this time,the corresponding welding current is 220 MPa.In order to avoid weakening of joints,5 welding times are required.The strength of the weld area of the three kinds of repair welding joints(Φ10mm-5mm,Φ 20mm-10 mm and Φ40mm-20mm)is close and higher than the base metal,respectively 220.3MPa,210.4MPa and 215 MPa.After heat treatment of the joint,the mechanical properties of the weld and the base metal are significantly improved,the strength difference between the two is reduced,and the mechanical properties of the joint are more consistent. |
PDF Full Text Request