Effects of Postweld Heat Treatment on the Phase Transformation and Mechanical Behavior of NiTi Ultrasonic Spot Welded Joints With Al Interlayer

2020 ◽  
Vol 142 (10) ◽  
Author(s):  
Chunjie Li ◽  
Sansan Ao ◽  
J. P. Oliveira ◽  
Zhi Zeng ◽  
Huijie Cui ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Phase Transformation ◽  
Welded Joints ◽  
Niti Alloy ◽  
Postweld Heat Treatment ◽  
Transformation Temperatures ◽  
Lap Shear ◽  
Lap Shear Test ◽  
Postweld Heat ◽  
Spot Welded

Abstract The influence of postweld heat treatments (PWHTs) on the phase transformation characteristics, microstructural evolution, and mechanical properties of ultrasonic spot welded NiTi alloy with Al interlayer was investigated. At room temperature, the as-welded joints were fully austenitic, while the microstructure of welded joint after PWHT at 450 °C consisted of a mixture of martensite and austenite. The transformation temperatures were found to increase after the PWHT. Transmission electron microscope analysis shows that Al3Ti intermetallic compounds were formed on the Al side of the Al/NiTi interface after the PWHT at 450 °C for 90 min. Furthermore, finely dispersed nanoscale Ni4Ti3 precipitates were observed in the joint. These nanoscale precipitates resulted in an increase in the transformation temperatures and improved the mechanical properties of the PWHT material. During lap shear test, the as-welded samples failed in a brittle manner in the NiTi alloy, while the PWHT samples failed in the Al interlayer and exhibited ductile-like fracture characteristics.

scholarly journals Through-Thickness Residual Stresses, Microstructure, and Mechanical Properties of Electron Beam-Welded CA6NM Martensitic Stainless Steel after Postweld Heat Treatment

2020 ◽  
Vol 2020 ◽  
pp. 1-16
Author(s):  
Sheida Sarafan ◽  
Priti Wanjara ◽  
Jean-Benoît Lévesque ◽  
Javad Gholipour ◽  
Henri Champliaud ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
Stainless Steel ◽  
Electron Beam ◽  
Residual Stresses ◽  
Tensile Properties ◽  
Martensitic Stainless Steel ◽  
Postweld Heat Treatment ◽  
Image Correlation ◽  
Postweld Heat

In this study, the integrity of electron beam- (EB-) welded CA6NM—a grade of 13% Cr-4% Ni martensitic stainless steel—was assessed through the entire joint thickness of 90 mm after postweld heat treatment (PWHT). The joints were characterized by examining the microstructure, residual stresses, global mechanical properties (static tensile, Charpy impact, and bend), and local properties (yield strength and strain at fracture) in the metallurgically modified regions of the EB welds. The applied PWHT tempered the “fresh” martensite present in the microstructure after welding, which reduced sufficiently the hardness (<280 HV) and residual stresses (<100 MPa) to meet the requirements for hydroelectric turbine assemblies. Also, the properties of the EB joints after PWHT passed the minimum acceptance criteria specified in ASME sections VIII and IX. Specifically, measurement of the global tensile properties indicated that the tensile strengths of the EB welds in the transverse and longitudinal directions were on the same order as that of the base metal (BM). Evaluation of the local tensile properties using a digital image correlation (DIC) methodology showed higher local yield strengths in the fusion zone (FZ) and heat-affected zone (HAZ) of 727 MPa and 740 MPa, respectively, relative to the BM value of 663 MPa. Also, the average impact energies for the FZ and HAZ were 63 J and 148 J, respectively, and attributed to the different failure mechanisms in the HAZ (dimples) versus the FZ (quasi-cleavage consisting of facets and dimples). This study shows that the application of PWHT plays an important role in improving the weld quality and performance of EB-welded CA6NM and provides the essential data for validating the design and manufacturing process for next-generation hydroelectric turbine products.

Effect of Rotational Speed and Dwell Time on Mechanical Properties of Dissimilar AA1050-AA3105 Friction Stir Spot Welded Joints*

2014 ◽  
Vol 56 (10) ◽  
pp. 818-825
Author(s):  
Hakan Aydin ◽  
Oguz Tuncel ◽  
Celalettin Yuce ◽  
Mumin Tutar ◽  
Nurettin Yavuz ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Welded Joints ◽  
Rotational Speed ◽  
Dwell Time ◽  
Friction Stir ◽  
Spot Welded
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