Inverse Analysis Of Elastic-plastic Constitutive Parameters Of Aluminum Alloy Laser Weldment Based On Digital Image Correlation

The laser welding technology has achieved the variety of structural design and material combination,improving the manufacturing quality in numbers of fields.However,the thermal effect of welding degrades the local material properties and make the overall strength at the weldment lower than the base metal,which brings difficulties to the evaluation of the structural performence.In this paper,the evolution of microstructures,hardness and elasto-plastic constitutive parameters at the welding fusion zone,heat affected zone and base metal zone were studied by the microscopic observation,hardness measurement and uniaxial tension test of laser welded aluminum alloy specimen.The influence of welding power on the above parameters at the three zones was discussed.Firstly,the initial physical parameter value of the mechanical behavior of the material is deduced from the experiment,which is the basic principle of the inversion analysis for material constitutive parameters.The Ramberg-Osgood model was chosen to inversely identify the constitutive parameters of aluminum alloy,and a quasi-Newton iterative method based on least squares fitting principle was deduced.The stress-strain relationship was established by the load data from the tensile test and the strain data measured by the digital image correlation technique.The constitutive parameters of the Ramberg-Osgood model were obtained as the elastic modulus of E = 43.42 GPa,the yield stress of ?0 = 121.52 MPa,the strength coefficient of ? = 0.26 and the hardening exponent of n = 17.81.Secondly,the microscopic observation and hardness measurement on the weldment of aluminum alloy 6061 were carried out.There were more compositions of Al and second phase Mg2 Si in base metal,and more dendritic Al-Si eutectic and some equiaxed grains in the weld zone.Most columnar crystals located at the fusion line,which increased with the welding power.The overall hardness distribution showed a significant "w" shape.The hardness of the base metal was highest,followed by the fusion zone,and the heat affected zone was the smallest.The hardness of the fusion zone decreased about 20% and the hardness value of the heat affected zone decreased about 30%.When the laser welding power was2400 W,the range of heat affected zone was about 0.75 mm,and the range of the fusion zone was about 1mm.When the welding power increased,the overall hardness value had little effect,while the heat affected zone range slightly increased.Finally,the uniaxial tensile test and the full field strain optical measurement of the aluminum alloy 6061 under the laser welding power of 1900 W,2400W and 2900 W were conducted.The Ramberg-Osgood constitutive parameters at 12 sub-regions were reversely identified by the least squares fitting principle.The elastic modulus and the yield stress under the three welding powers showed the "w" shape distribution.Compared with the base metal,the elastic modulus,the yield stress and the hardening exponent of the fusion zone and the heat affected zone were decreased.The strength coefficient of the fusion zone and the heat affected zone were increased.