Numerical Simulation And Experimental Investigation On Laser Heated Friction Stir Welding Of Dissimilar Metals Steel And Aluminum

Lightening the weight of vehicles is an effective way to reduce oil consumption and pollution,so more and more light metal materials such as aluminum alloys,magnesium alloys and so on are used in automobile and aerospace industry.Among these light metal materials,aluminum alloys are the main one kind of light metal materials with extensive applications because of their excellent performances of light weight,corrosion resistance,plasticity and mechanical properties.Steel,as a main kind of engineering material,is still widely applied in various industries because of its excellent properties such as high hardness and strength.So,the welding of steel and aluminum can not only guarantee the quality of product but also reduces its weight.However,due to the great differences in physical and mechanical properties such as hardness,melting point and poor weld ability between aluminum alloys and steels,it is difficult to obtain a sound joint between aluminum alloys and steels using conventional welding methods.As a promising solution to the above problems,laser assisted friction stir welding is proposed in this thesis.In laser assisted friction stir welding,the strength and hardness of steel can be reduced obviously by means of a focused laser beam with high intensity in front of the rotating tool in the steel side to preheat the steel work-piece,which can realize quick softening of steel and aluminum alloy and improve the flowability and mixture of the materials.In this thesis,numerical simulation and experimental investigation on laser heated friction stir welding of dissimilar metals steel and aluminum are conducted.First of all,a mathematical model of coupling multiple heat sources is developed,in which such three kinds of heat sources as the laser beam,the friction heat between the workpiece and the tool shoulder,and the friction heat between the workpiece and the tool pinis are considered.The temperature field preheated by laser beam at different laser powers is calculated,and the profile of the softened region in the cross section of workpiece is determined.The proper laser power can be obtained by comparing and analyzing the calculation results.Therefore the effects of other welding parameters on the temperature field of the workpiece are studied,and the welding parameters are optimized,which lays a foundation for the proceeding experimental investigation on laser heated friction stir welding of dissimilar metals steel and aluminum alloys.Then,the forming mechanism of main defects including fins,keyhole,furrow,surface peeling and tunnel defect in laser assisted friction stir welding of steel and aluminum is analysised,and the corresponding methods of controlling these defects is put forward.The quantitative relationship between the welding parameters&the structural parameters of the stiring tool and the tunnel defect is induced,which provides a theoretical basis for controlling effectively tunnel defect.In adition,the experiments of laser assisted friction stir welding of dissimilar metals steel and aluminumalloys are carried out and the pre-hole offset of the welding is proposed.The effects of such welding parameters as laser power,rotation speed,traveling speed,downward force and pre-hole offset distance on the properties of the welding joint during laser-assisted friction stir welding of Q235 steel and 6061-T6 aluminum alloy are studied.The tensile strength of the joints obtained at different welding parameters are tested and compared.SEM,EDS and optical microscope are used to analyze the compositions of the inter-metallic compounds and the metallographic structure of the joints,respectively.Results show that a sound joint can be obtained under such conditions as the laser power of 900W,rotation speed of tool of 950rpm,traveling speed of 23.5mm/min and pre-hole offset distance of 0.8mm.Finally,the inflences of different filling alloying elements on the the formation of inter-metallic compounds on the steel/aluminum interface of the joints are studied.Such two kinds of alloying elements as Ni and Zn are selected as filling material in LFSW of Q235 steel and 6061-T6 aluminum alloy.SEM is used to analyze the profile of the inter-metallic compound layer and the fractography of tensile fracture surfaces.In addition,EDS is applied to investigate the types of the inter-metallic compounds.Results show that a thin smooth layer of iron-abundant inter-metallic compound forms when Ni is added,but a thick rough layer of aluminum-abundant inter-metallic compound forms when Zn is added.What is more,ductile fracture occurs after tensile test when Ni is added,but brittle fracture occurs when Zn is added,so the tensile strength of the welds with Ni added is higher than that with Zn added.