| Summary: | 碩士 === 國立成功大學 === 機械工程學系碩博士班 === 90 === The dynamic impact response and microstructural evolution of 304L stainless steel GTAW and SMAW joints were studied by means of split-Hopkinson bar and microscope (OM, SEM and TEM). Experiments were performed at room temperature under strain rates from 1500 to 7500 . The Zerilli-Armstrong constitutive equation with the experimentally determined specific material parameters successfully describe the flow behaviour of the material for the tested conditions. Results indicate the material properties of 304L stainless steel GTAW and SMAW joints depend strongly on strain rate. Flow stress and strain rate sensitivity increase with increasing strain rate, but decrease for fracture strain and activation volume. SEM fractography shows HAZ and weld metal fractures both dominated by microcrack formation in adiabatic shear bands. Increasing dislocation density, twinning and martensite transformation are observed as strain rate increases, and are identified as the strengthening mechanisms of 304L stainless steel weldments. However, martensite can be detected only minimally in the weldments of deformed specimens. 304L stainless steel GTAW joints are found to have higher flow stress but lower fracture strain than SMAW joints.
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