Material characterization of duplex stainless steel by superplastic free bulging test
Macroscopic superplastic behaviour of metallic or non metallic materials is usually represented by the strain rate sensitivity, and it can be determined by tensile tests at elevated temperature. However, a uniaixal stress state occurs in the test whilst a multiaixal stress state is induced in actual superplastic forming processes. The difference of the stress state might affect the reliability of the strain rate sensitivity obtained from the tension test. In the current study, the material characterization of duplex stainless steel, UNS S31803, has been carried out to determine the strain rate sensitivity throughout superplastic free bulging test under constant pressure. The measured height of bulged plate during the test was transformed into strain rate and stress corresponding to height at each time by theoretical considerations. The strain rate sensitivities were calculated for each pressure level. After calculating the strain rate sensitivity and strength coefficient for each test case, the flow stress equations have been employed in the finite element simulation to verify the experiments. The bulged height obtained from the simulation showed a reasonable agreement with the experimental findings. In the future, if a study on the effect of fillet radius of test tool and microstructure evolution is carried out, it is thought that the reliability of the material characterization by superplastic free bulging test can be enhanced.
Material characterization of Inconel 718 from free bulging test at high temperature
Macroscopic superplastic behavior of metallic or non metallic materials is usually represented by the strain-rate sensitivity, and it can be determined by tensile tests in uniaxial stress state and bulging tests in multi axial stress state, which is the actual hot forming process. And macroscopic behavior of Non-SPF grade materials could be described in a similar way as that of superplastic materials, including strain hardening, cavity and so on. In this study, the material characterization of non-SPF grade Inconel 718 has been carried out to determine the material parameters for flow stress throughout free bulging test under constant temperature. The measured height of bulged plate during the test was used for estimation of strain-rate sensitivity, strain-hardening index and cavity volume fraction with the help of numerical analysis. The bulged height obtained from the simulation showed good agreement with the experimental findings. The effects of strain-hardening and cavity volume fraction factor for flow stress were also compared.
ROOM- AND HIGH-TEMPERATURE MAGNETIC PROPERTIES OF Sm(CobalFexCu0.088Zr0.025)7.5 (x=0.30) SINTERED MAGNETS
The effect of Fe content on room- and high-temperature magnetic properties of Sm(CobalFexCu0.088Zr0.025)7.5 (x=0-0.30) sintered magnets, prepared by the conventional powder metallurgy method, has been systematically studied. With increasing Fe content, the remanences BT and maximum energy products (BH)max at room temperature gradually increase and reach optimal values of 1.055 T with x=0.27 for BT, and 205 kJ/m3 with x=0.21 for (BH)max, respectively, and then drop sharply with further Fe increase. An excessive addition of Fe (x≥0.24) results in rapid deterioration of permanent magnetic properties due to the formation of a considerable amount of FeCo soft magnetic phase. It was found that the effect of Fe content on high temperature stability is huge. The lower the Fe content, the smaller the temperature coefficient of coercivity. When x=0.07, a temperature coefficient of coercivity of -0.14%/K (293-723 K) can be achieved, as compared with -0.23%/K for x=0.21. BT of 0.725 T, bHc of 517 kA/m, iHc of 764 kA/m, and (BH)max of 95 kJ/m3 at 723 K were obtained, and the B-H demagnetization curve remains a straight line.