This study employs nonlinear ultrasonic techniques to track microstructural changes in additively manufactured metals. The second harmonic generation technique based on the transmission of Rayleigh surface waves is used to measure the acoustic nonlinearity parameter, β. Stainless steel specimens are made through three procedures: traditional wrought manufacturing, laser-powder bed fusion, and laser engineered net shaping. The β parameter is measured through successive steps of an annealing heat treatment intended to decrease dislocation density. Dislocation density is known to be sensitive to manufacturing variables. In agreement with fundamental material models for the dislocation-acoustic nonlinearity relationship in the second harmonic generation, β drops in each specimen throughout the heat treatment before recrystallization. Geometrically necessary dislocations (GNDs) are measured from electron back-scatter diffraction as a quantitative indicator of dislocations; average GND density and β are found to have a statistical correlation coefficient of 0.852 showing the sensitivity of β to dislocations in additively manufactured metals. Moreover, β shows an excellent correlation with hardness, which is a measure of the macroscopic effect of dislocations.

中文翻译：

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非线性超声技术表征添加剂材料的微观结构

本研究采用非线性超声技术来跟踪增材制造金属的微观结构变化。基于瑞利表面波传输的二次谐波生成技术用于测量声学非线性参数β。不锈钢标本通过以下三个过程制成：传统的锻造制造，激光粉末床熔合和激光工程网成形。所述β参数通过退火热处理的连续步骤测量旨在降低位错密度。已知位错密度对制造变量敏感。与用于二次谐波产生中位错-声非线性关系的基本材料模型一致，β在重结晶之前的整个热处理过程中，每个试样中的液滴均会下降。几何学上必要的位错（GND）是通过电子背散射衍射测量的，以此作为位错的定量指标。平均GND密度和β具有统计相关系数0.852，表明β对加法制造的金属中位错的敏感性。而且，β显示出与硬度的极好的相关性，这是位错的宏观效应的量度。