The Forming Process And Structure-property Relationships Of Al-Mg Aluminum Alloy Utilized As Feedstock For GMAW-based Additive Manufacturing

The GMAW-based additive manufacturing technology,using the gas metal arc as the heat source and metal wire as the the deposition material,fabricates metal parts by layer-by-layer.Compared with the additive manufacturing technology using laser and electron beam as the heat source,it has the advantages of low manufacturing cost,high deposition rate,and large size of manufacturing parts.At present,There is a lack of systematic quantitative study of the influence of forming process parameters on the forming quality and microstructure of forming straight wall.In-depth research on the microstructure and properties alloy parts has rarely been reported,especially the lack of experimental data and theoretical support for testing and analysis of fatigue properties and wire composition optimization.Al-Mg and Al-Mg-Mn alloy has good corrosion resistance,high strength and ductility.They are most widely used in aerospace,high-speed trains,automobiles,mechanical engineering and other fields.This paper systematically focuses on the forming process of Al-5Mg alloy produced by GMAW arc additive and the microstructure and mechanical properties of Al-5Mg and Al-5Mg-1.0Mn alloy forming straight wall,aiming to improve the arc additive forming parts.This paper aims at providing experimental basis and theoretical basis for improving the performance of arc additive forming parts and the composition design of wire used for additive,and promoting the engineering application process of the GMAWbased additive manufacturing technology.Based on the self-designed GMAW-based additive forming system,the Al-5Mg alloy straight wall（length 270mm × width 9-12mm × height 65mm）was successfully fabricated.A systematic study is researched about the heat input,interpass temperature and additional pulse frequency parameters to the influence of Al-5Mg alloy straight wall forming quality,microstructure and mechanical properties.The results show that as the heat input increases from 171（J·mm-1）to 395（J·mm-1）the width of the Al-5Mg alloy straight wall increases,and the effective width factor decreases.With the increase in the heat input,grain size and porosity raise;the tensile strength of the tensile specimens in the parallel surfacing direction is in the range of 277MPa to 279MPa;the tensile strength and elongation of the tensile specimens in the vertical surfacing direction decrease.As the interlayer temperature increases from 60℃ to 140℃,the width of the straight wall is about 9.0mm,the effective width coefficient is about 0.85;the grain size is 56.1 μm,the porosity is 0.13%.The tensile strength of the tensile specimens in the parallel surfacing direction（X axis）and in the vertical surfacing direction（Z axis）is at about 278MPa,and the elongation does not change significantly.When the interlayer temperature increases to 180℃,the forming quality of the straight wall decreases;the grain size and porosity increase;the mechanical properties deterioration.The application of additional pulses is advantageous for refining the grains and reducing the propensity of the pores.As the additional pulse frequency increases from 0Hz to 9Hz,the width of the Al-5Mg alloy straight wall increases,and the effective width coefficient decreases;the grain size and porosity decrease initially and increase afterwards;The tensile strength of the tensile specimens in the parallel surfacing direction（X axis）and in the vertical surfacing direction（Z axis）is at about 275MPa;the elongation in the vertical surfacing direction（Z axis）increases initially and decreases afterwards.It is the best when the additional pulse frequency is 3Hz.Based on the above research results,when the heat input is 171（J·mm-1）,interlayer temperature is in the range of 60 to 140℃,and additional pulse frequency is 3Hz.It is concluded that the forming process of Al-5Mg alloy is optimal.The tensile strength of the Al-5Mg alloy straight wall with optimized process parameters in the parallel direction（X direction）and vertical surfacing（Z direction）is about 275MPa,but the ductility is anisotropic.The yield strength and elongation of tensile specimens in parallel surfacing direction are 116MPa and 45.50%,respectively.The yield strength and elongation of tensile specimens in vertical surfacing direction are 105MPa and 36.13%,respectively.The yield strength and elongation of the former are 10MPa and 9.37%higher than that of the latter,respectively.The tensile specimens in the vertical surfacing direction mostly break at the junction between layers,mainly in the form of crystal fractures.The fatigue limit of the samples taken in the parallel surfacing direction（X direction）and the vertical surfacing direction（Z direction）of the straight wall is 118MPa and 104MPa,respectively.And the former is 14MPa higher than the fatigue limit of the latter.Fatigue source originates from pores,impurities and liquefied phases.The α-Al matrix of the Al-5Mg alloy straight wall along the vertical surfacing direction（Z direction）exhibits the characteristics of alternating columnar and equiaxed regions.In addition,coarse phase particles such as Al（FeMnSi）at the interlaminar junction are of large size,and there are micropores and liquified phase at the grain boundary,making the interlaminar junction a weak area of mechanical properties.This is the fundamental reason why the mechanical properties in vertical surfacing direction（Z direction）are worse than those in parallel surfacing direction.The yield strength and tensile strength in the parallel surfacing direction（X direction）of Al-5Mg-1.0Mn alloy prepared by optimized process parameters are 124MPa and 310MPa,respectively,which are 9MPa and 33MPa higher than the Al-5Mg alloy straight wall.The yield strength and tensile strength in the vertical surfacing direction（Z direction）are 111MPa and 293MPa,respectively,which were 5MPa and 19MPa higher than the Al-5Mg alloy straight wall.It is considered that a part of Mn element is solid-dissolved in the α-Al matrix,which enhances the solution strengthening effect;the rest exists in the form of Al6Mn phase in the structure,which acts as a dispersion strengthening.This is the main reason why the Al-5Mg-1.0Mn alloy straight wall has better mechanical properties than Al-5Mg alloy straight wall under the same process conditions.