Microstructure Evolution Of 2219-Al During GTA Based Additive Manufacturing And Heat Treatment

2219-Al is widely used in aerospace field for the advantages of low-weight,high-strength and low-temperature performance.The manufacturing process of hollow ring structures,such as rocket fuel tank,can be largely simplified by additive manufacturing.But the laser based additive manufacturing has the problem of high reflectivity in the production of aluminum alloy part,which is the mainstream in additive manufacturings at present.Because of the spheroidizing crystallization effect,the surfaces of additive manufactured aluminum alloy parts is usually coarse and hard to be metallurgically bonded.GTA(Gas Tungsten Arc)based additive manufacturing is developed from GTA welding.The problem of high reflectivity can be avoided by using arc source.In order to expand the application of 2219-Al in additive manufacturing,the microstructures and mechanical properties of as-deposited 2219-Al were first investigated.Then the thermal process during GTA based additive manufacturing was simulated to reveal the evolution of microstructures.Additionally,a new kind of heat treatment was proposed.The mechanical properties of 2219-Al after heat treatment were verified to be close to the properties of traditional manufactured 2219-Al.To begin with,technical studies were carried out on forming of aluminum alloy.By controlling inter layer temperature and implementing the strategy of decreasing heat input,single pass multi-layers 2219-Al parts were manufactured.By quadratic general rotary unitized design and stepwise regression,a model was obtained to produce designed-width part with GTA based additive manufacturing.Due to the volumes equal principle of feeding material and the manufactured part,another model was deduced to produce designed-height part with GTA based additive manufacturing.In the aspect of macro morphologies,the as-deposited parts exhibit features of regional division and layered division.The area of top region is smaller and the color is lighter.The middle and bottom regions are the main areas of the manufactured part and many light lines distribute uniformly on it.In the aspect of microstructures,the grains are inhomogeneous in both regional scale and layered scale.In top region,the grains are dendrite structures.The eutectics are discontinuously distributed exhibiting point-like and rod-like structures.The deposited layer segregation is severe.And few porosities occurs.In middle and bottom regions,though the deposited layers have similar features,a deposited layer inside is not uniform.In a deposited layer,the grains are clear and the sizes increase along the depositing direction.The eutectics distribute continuously and the sizes are extremely large in the interfaces.The deposited layer segregation is significantly reduced.And a lot of porosities occur along the interfaces exhibiting belt-like distribution.In the aspect of mechanical properties,the average yield strength,ultimate strength and elongation are 108 MPa,239MPa and 10.7%,respectively,which are 37.2%,57.6% and 107.0% of casting+T6 2219-Al.MSC.Marc was used to simulate the thermal process during GTA based additive manufacturing of 2219-Al.The analysis of temperature field shows that the steady state occurs in the moving coordinate system of scanning direction and depositing direction.The analysis of thermal cycles shows that,in steady state,the corresponding points from different additive layers have similar thermal cycles but the points in one additive layer inside have different.The relationship between thermal cycles and asdeposited microstructures was established.Melting-heat thermal cycles occur in top region resulting in solidified microstructure.Besides,melting-heat thermal cycles,post-heat and melting-point thermal cycles occur in middle and bottom regions leading to heat affected microstructures,including inner layer and interface microstructures.The inner layer microstructure is the product of melting-heat and post-heat thermal cycles.And the interface microstructure is the product of meltingheat,melting-point and post-heat thermal cycles.Finally,the effects of melting-heat,melting-point and post-heat thermal cycles were studied and the formation of solidificated microstructure and evolution of heat affected microstructure were clarified.Three kinds of conventional heat treatments were investigated.Consequently,the direct aging treatment is unsatisfied,the solution + aging treatment has slight improvement,the homogenization + solution + aging treatment is a success.Subsequently,the key factors to improve 2219-Al in heat treatment were concluded.First of all,a long-time holding near the temperature of eutectic's melting point is necessary to improve the concentration of Cu.It provides driving force for precipitated phase to separate out.Additionally,a proper time holding is necessary at aging temperature to provide high-level diffusion conditions.It promotes precipitated phases' nucleation and growth.Eventually,a new kind heat treatment of long-time solution + aging was proposed for as-deposited 2219-Al.The effectiveness was verified and the processing window was acquired.The microstructures,macro morphologies and mechanical properties were investigated on deposit+T6 2219-Al.For microstructures,the dendtire grains in top region transform to equiaxed grains.The deposited layer segregation is greatly relieved.The eutectics are almost dissolved.And many ?'-Al2 Cu phases are uniformly separated out.For macro morphologies,the features of regional division and layered division disappears and the entire part exhibits a uniform color.For mechanical properties,the average yield strength,ultimate strength and elongation are 266 MPa,391MPa and 8.8%,respectively.In addtion,the deposit+T6 2219-Al exhibits significant anisotropic properties,where mechanical properties in scanning direction are much better than depositing direction.In scanning direction,the yield strength,ultimate strength and elongation are 269 MPa,418MPa and 10.2%,respectively,which are 92.8%,100.7% and 102.0% of casting+T6 2219-Al.