| Summary: | 碩士 === 國立成功大學 === 機械工程學系碩博士班 === 90 === A split Hopkinson bar is used to investigate the effects of strain rate and welding current mode on the dynamic impact properties of plasma arc welded 304L stainless steel, and these results are correlated with microstructure and fracture characteristics. Annealed 304L stainless steel is welded by plasma arc welding(PAW) process variations, namely continuous current and pulse current, then machined as cylindrical compression specimens. Dynamic mechanical tests are performed at strain rates ranging from 1200 to 7700 at room temperature.
Results indicate that the mechanical properties and the microstructure largely depend on impact loading. Increasing the strain rate of impact loading increases both flow stress and strain rate sensitivity. However, the inverse tendency is observed for the activation volume. The results also show the greater flow stress, work hardening rate and strain rate senstivity of pulse current welds compared to those of continuous current welds. By using the proposed constitutive equation proposed by Zerrilli-Armstrong with the experimentally determined specific material parameters, the flow behaviour of pulse current welds and continuous current welds can be described successfully for the range of test conditions.
The effect of loading rate on mechanical response and impacted substructure of 304L stainless steel PAW welds are found directly related to dislocation density and the amount of martensite. OM and SEM fracture feature observations reveal that adiabatic shear band formation is the dominant fracture mechanism of both continuous and pulse current PAW welds of 304L stainless steel. Adiabatic shear band is initially formed near the fusion line and then crack occurs along the direction of maximum shear stress and induces specimen fracture. Microstructural observations reveal that the morphologies and characteristics of both dislocation substructure, mechanical twins, micro-shear bands and martensite formation, are strongly influenced by welding current mode and strain rate. At higher strain rate, greater dislocation density and more martensite transformation are observed, but a decay of twin density is showed. The microstructrues of continuous and pulse current welds are compared and their dynamic properties correlated. There are greater dislocation density and more martensite transformation in the pulse current welds and correspondingly they have higher flow stress under the dynamic deformation. Significant strengthening is found to result from dislocation multiplication and martensite transformation.
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