The Microstructure And Properties Adjustment Of Friction Stir Additive Manufactured Build For 7N01 Aluminum Alloy

Friction stir additive manufacturing(FSAM)is a solid state additive manufacturing technology based on the principle of friction stir lap welding,which uses a friction stir processing technique to superimpose multiple layers of materials in space and combines with milling processing to obtain the finally desired shape.Compared with laser,electron beam and arc additive manufacturing,it has the advantages of low heat input,small residual stress and thermal deformation,no coarse solidification structure and hot crack defects.However,multiple thermal cycles in the additive process cause the grain size and the strengthening precipitate particles to coarsened in the lower part of the build,and the mechanical properties decrease from the top to the bottom layer.It is necessary to improve the softening of the lower part of the build,then the problem of non-uniformity of structure and performance in the direction of the build can be solved.This study first proposed the idea of friction stir additive manufactured 7N01 aluminum alloy under water cooling condition.The microstructure and mechanical properties of the as-fabricated,natural aging and artificial aging builds fabricated in air cooling and water cooling conditions were systematically studied.The key factors affecting the structure and properties of the build were revealed,and the controlling of the microstructure and properties of the 7N01 aluminum alloy build was realized.In this study,the influence of process parameters on the formation of friction stir additive was investigated at the tool with the pitch of 1.0 mm and 1.6 mm,the rotational speed of 600?1200 rpm and the travel speed of 40?120 mm/min.The optimum process parameters are the pitch of 1.6 mm,the rotational speed of 1200 rpm,and the travel speed of 80 mm/min.The 7N01-T4 aluminum alloy builds with height of 40 mm were friction stir fabricated in air cooling and water cooling with the optimum process parameters.The following conclusions can be drawn by studying the microstructure and properties of the FSAM build:Under the air cooling condition,the grain size of the bottom layer of the build are larger than that of the top layer.The strengthening phases of the top layer are GP zones and finer ?precipitates(MgZn2),and the bottom layer are distributed with a large ? phase with size of 50?100 nm.The microhardness and the local tensile properties in the horizontal direction of the builds are reduced from the top to the bottom layer because of the grains and precipitated particles are coarsened in the bottom of the build.The tensile strength and elongation of the build in the building direction are lower than those of the horizontal layers,The interlaminar cold lap defect is the main factor affecting the tensile properties of the build along the building direction.The microhardness and tensile strength of the build are improved after natural aging and artificial aging,but they are also reduced from the top to the bottom layer.Under the water cooling condition,the difference between the grain size of the bottom layer(2.8 ?m)and the top layer(2.3 ?m)of the build is small.The strengthening phases of both the top layer and the bottom layer are GP zones and finer ? precipitates(MgZn2).Therefore,the microhardness of the build is uniformly distributed from the top layer to the bottom layer,and the layered tensile properties in the horizontal direction are not much different.The tensile strength of the build in the building direction of 325 MPa is comparable to the tensile strength of the horizontal layers,but the interlaminar cold lap defect of layers still cause the elongation in the building direction(19%)significantly lower than the elongation in the horizontal direction(34%).After natural aging and artificial aging,the microhardness and tensile strength of the build are improved relative to the as-fabricated,and uniformly distributed along the building direction.After natural aging for 90 days,the tensile strength of the building direction and each layer of the horizontal direction is about 400 MPa,which is equivalent to the base metal(BM).Under water cooling conditions,the growth tendency of grain and strengthening precipitate particles in the lower part of FSAM building affected by heat is effectively controlled.So that the microstructure and mechanical properties of the build from the top to the bottom layer are relatively uniform.In addition,FSAM under the water cooling condition can refine the grain size and improve the solid solution degree of the build,and further improve the age strengthening ability of the build.The underwater FSAM is an effective method to obtain high performance 7N01 aluminum alloy build.