Abdollah Bahador

Microwave sintering effects on the microstructure and mechanical properties of Ti−51at%Ni shape memory alloys

Ti−51at%Ni shape memory alloys (SMAs) were successfully produced via a powder metallurgy and microwave sintering technique. The influence of sintering parameters on porosity reduction, microstructure, phase transformation temperatures, and mechanical properties were investigated by optical microscopy, field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), compression tests, and microhardness tests. Varying the microwave temperature and holding time was found to strongly affect the density of porosity, presence of precipitates, transformation temperatures, and mechanical properties. The lowest density and smallest pore size were observed in the Ti−51at%Ni samples sintered at 900°C for 5 min or at 900°C for 30 min. The predominant martensite phases of β2 and β19′ were observed in the microstructure of Ti−51at%Ni, and their existence varied in accordance with the sintering temperature and the holding time. In the DSC thermograms, multi-transformation peaks were observed during heating, whereas a single peak was observed during cooling; these peaks correspond to the presence of the β2, R, and β19′ phases. The maximum strength and strain among the Ti−51at%Ni SMAs were 1376 MPa and 29%, respectively, for the sample sintered at 900°C for 30 min because of this sample’s minimal porosity.

Effect of Microwave Sintering Time and Homogenization Treatment on Biomedical Ti-51%Ni SMAs

Objective: The influence of microwave sintering time and Homogenization Treatment (HT) on the microstructure, density, phase composition, mechanical properties and phase transformation temperatures of biocompatible Ti-51%Ni Shape Memory Alloys (SMAs) are determined. Methods/Statistical Analysis: These alloys are fabricated at 900˚C for two different sintering times such as 5 min. and 30 min. Findings: The Field Emission Scanning Electron Microscopy (FESEM) micrographs show microstructure of needle-like morphology except for the sample which sintered at 900˚C for 30 min. without HT. Applications/Improvements: Sample synthesized at 900˚C for 30 min. without HT revealed the highest performance in terms of maximum compressive strength (1376 MPa) at 29% strain, Austenite finish temperature (Af) of 27˚C and Martensite finish temperature (Mf) of 47˚C at 18% porosity. They showed the Af temperature very close to the human body temperature, thus prospective for biomedical applications. During heating, the Differential Scanning Calorimeter (DSC) baseline shows multi-endothermic peaks, while during the cooling process there is only one exothermic peak.

Effect of Filler Metals on the Microstructures and Mechanical Properties of Dissimilar Low Carbon Steel and 316L Stainless Steel Welded Joints

In this paper, dissimilar joining of 316L stainless steel to low carbon steel was carried out using gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW). Samples were welded using AWS: ER309L welding electrode for GMAW and AWS: ER316L welding electrode for GTAW process. Determination of mechanical properties and material characterization on the welded joints were carried out using the Instron tensile test machine and an optical microscope respectively. The cross section area of the welded joint consists of three main areas namely the base metal (BM), heat affected zone (HAZ), and weld metal (WM). It was found that, the yield and tensile strengths of welded samples using ER316L filler metal were slightly higher than the welded sample using ER309L welding electrode. All welded samples fractured at low carbon steel base metal indicating that the regions of ER316L stainless steel base metal, ER316L filler metal and heat affected zone (HAZ) have a higher strength than low carbon steel base metal. It was also found that ER316L welding electrode was the best filler to be used for welding two dissimilar metals between carbon and stainless steel.