Nondestructive Characterization Of Elastic Parameters Of Additive Manufactured Metal Parts Based On Multi-mode Ultrasonic Transducer

Selective laser melting（SLM）technology can rapidly form metal parts of any configuration,but the internal structure and material properties of the parts are significantly different from those of traditional parts,resulting in obvious anisotropy and inhomogeneity of mechanical properties.As one of the key mechanical properties to characterize the elastic deformation of solid materials,elastic constant is the premise of evaluating the mechanical behavior and failure analysis.Among the current elastic constant characterization techniques,ultrasonic nondestructive testing has attracted extensive attention because of its advantages such as wide range of adaptation,strong penetration and reliability.However,current ultrasonic characterization of elastic constants requires the replacement of ultrasonic transducers with different modes for the detection of longitudinal and transverse waves,which has low detection efficiency and is mainly oriented to isotropic materials.In addition,there are few studies on the variation and distribution of elastic constants of additive manufacturing parts with process parameters.In view of the above problems,the elastic parameters and their distribution of the additive manufacturing parts were characterized by designing and preparing multi-mode ultrasonic transducer.The main research contents are as follows:（1）Based on the propagation law of ultrasonic wave in solid medium and the material properties of additive manufacturing parts,the quantitative relationship between ultrasonic propagation velocity and elastic parameters under isotropic and transverse isotropic constitutive relations was studied respectively.Under the isotropic constitutive relation,the Young’s modulus and Poisson’s ratio in the orthogonal direction can be calculated by using the velocity of longitudinal wave and orthogonal shear wave at the same position.With the transverse isotropic constitutive relation,the elastic constants₁₁,₃₃,₄₄,₆₆,₁₂,₁₃and the Thomsen anisotropy parametersε,γcan be calculated by measuring the ultrasonic propagation velocities(1₁₁,(1₃₃,(1₁₂ and(1₁₃ in different directions.And the Young’s modulus and Poisson’s ratio in the principal direction are obtained by the inverse of the stiffness matrix.（2）The 316L stainless steel samples with key process parameters such as different laser power,scanning speed and hatch spacing were prepared based on selective laser melting forming technology.According to the requirements of multi-direction ultrasonic velocity measurement under isotropic and transverse isotropic constitutive relations,a dual-mode ultrasonic transducer and a three-mode ultrasonic transducer were designed and fabricated,and their performance was tested.Finally,based on the ultrasonic pulse-echo method and the distribution measurement requirements of elastic constants,an ultrasonic distribution measurement system of elastic constants was established,which was used for the rapid measurement and distribution characterization of the elastic constants of SLM samples.（3）The elastic constants of SLM samples were measured and characterized by the dual-mode ultrasonic transducer.Firstly,the longitudinal wave velocities of the SLM samples in the X,Y and Z directions were measured.The results show that the SLM samples are transversely isotropic.Then,under the isotropic constitutive relation,the distribution of the elastic constants in the two principal directions of the X-axis of SLM samples is realized,where the Young’s modulus₁₃ is greater than₁₂ and the Poisson’s ratio₁₃ is less than₁₂,and the distribution of the elastic constants between the melted layers is similar.Finally,the effect of process parameters on elastic constants is analyzed,the Young’s modulus and Poisson’s ratio decreased with the increase of scanning speed or hatch spacing.（4）The elastic constants of SLM samples were measured and characterized by the three-mode ultrasonic transducer.Firstly,the effect of mass density on longitudinal wave and shear wave velocities in X,Y and Z directions is analyzed,and the relationship between the velocity of S-wave and the angle of S-wave polarization direction is described.Secondly,under the isotropic constitutive relation,the distribution of elastic constants along the two principal directions of X-axis is in good agreement with the dual-mode ultrasonic transducer.Then,the elastic constants and Thomsen parameters of the SLM sample were characterized under the transverse isotropic constitutive relation,and the Young’s modulus₁ and Poisson’s ratio₁₂、₁₃ were obtained.Finally,the static tensile test of the SLM sample was carried out,and the tensile test was consistent with the results of the Young’s modulus measured by ultrasonic method,which verified the reliability of the results of the ultrasonic distribution measurement system for elastic constants.