QUARTERLY JOURNAL OF THE JAPAN WELDING SOCIETY
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PRINT ISSN: 0288-4771

QUARTERLY JOURNAL OF THE JAPAN WELDING SOCIETY Vol. 40 (2022), No. 1

  • Long-term Reliability of Nickel Micro-Plating Bonding by Using Resonant Type Fatigue Testing Machine

    pp. 1-8

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    DOI:10.2207/qjjws.40.1

    Recently, SiC (silicon carbide) has attracted attention as a new material to replace silicon power semiconductors. Since SiC is a wide-bandgap semiconductor and has high heat resistance, it can be operated in a high temperature environment. However, the conventional power module package is mainly joined by a low melting point material such as lead-free solder, and interconnection does not have enough heat resistance. Therefore, we have been proposing and experimenting with Nickel Micro-Plating Bonding (NMPB) using Ni (nickel), which has excellent heat resistance and strong bonding strength, as a new bonding method. Although various researches have been conducted on the bonding reliability of NMPB, long-term reliability evaluated from the aspect of metal fatigue has not been much discussed. In this study, a test has been conducted using a resonant type fatigue testing machine and examined whether acceleration of fatigue evaluation time and long-term reliability of NMPB were confirmed.
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  • Relationship between cross tension strength and carbon content of lower sheet in friction element welded steel joint

    pp. 9-17

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    DOI:10.2207/qjjws.40.9

    High-strength steels (HSSs) have been increasingly used in car bodies in order to simultaneously achieve the weight reduction and high collision safety of vehicles. In resistance spot welding, which is widely used for joining car bodies, low cross tension strength (CTS) of joints using HSSs is a problem. In this study, we focused on friction element welding (FEW) to enhance the CTS. To investigate the joint strength of steel sheets jointed by FEW, a pre-hole was provided in the upper sheets, and the element was passed through the pre-hole to joint with lower sheet and then cross tension tests of the joints were conducted. In addition, the microstructures of the joints were observed to investigate the fracture positions. In the cross tension test, three types of fracture mode were observed. When the carbon content of the lower sheet was as low as 0.07 to 0.15mass%, the joint broke at the head of the element. However, when the carbon content was increased to 0.20mass%, the joints were fractured at the softened area of heat affected zone in the thickness direction of lower sheets. While the carbon content was further increased to 0.30mass% (C30), the cracks propagated inside the area quenched from two-phase temperature region (inter-critically quenched area) of the lower sheets, and then fractured in the lower sheet thickness direction. Consequently the CTS was decreased. To clarify the mechanism of the decrease in CTS, the fracture surface was observed together with the hardness difference measurement between ferrite and martensite by nanoindentation method. As a result, it was clarified that the hardness difference between the two phases was significantly large, leading to the ductile fracture. Based on these findings, the low CTS of C30 was inferred to be caused by the poor local ductility in the inter-critically quenched area.
  • Examination of toughness and solidification cracking on 12% Cr martensitic stainless steel weld metal with Ni added

    pp. 18-26

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    DOI:10.2207/qjjws.40.18

    Toughness and weld solidification cracking in the weld metal of martensitic stainless steel containing Ni were investigated. The weld metal of martensitic stainless steel with low C and proper Ni can be applied without PWHT (Post weld heat treatment) due to the microstructure mainly consisting of soft martensite including retained austenite resulting in enough toughness though PWHT is necessary in conventional one because of high hardness as weld condition. However by the addition of Ni the susceptibility to weld solidification cracking can be higher.As the experimental evaluation was conducted in terms of solidification cracking test and Charpy impact test using the weld metal of 12/16%Cr-3/14%Ni steels. As the results the slight amount of ferrite phase caused the lowering of notch toughness within the weld metal including the same level of retained austenite. The excess Ni addition to decrease ferrite phase in solidification process increased the susceptibility to solidification cracking in during solidification process by austenite phase. In conclusion optimum Cr and Ni content to achieve the both of low susceptibility to solidification cracking and high toughness was clarified.
  • Evaluation of fatigue crack propagation behavior in low carbon steel and their simulated HAZ and fatigue life assessment of non-load carrying welded fillet joint.

    pp. 27-35

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    DOI:10.2207/qjjws.40.27

    Generally, it is considered that fatigue life can be divided into two separate contributions: crack initiation life and crack propagation life. Recent years, the authors studied on a method which predicts the fatigue crack initiation life by using local strain approach and extended to the assessment method of crack propagation life handled as continuous behavior of crack initiation life using an unconventional elasto-plasticity model called the Fatigue SS Model (hereafter, FSS model). The FSS model is based on the Subloading Surface theory, which was enriched by including the elastic boundary and cyclic damage concepts for the description of strain softening behavior within macroscopically elastic stress state. The FSS model was used to investigate the inelastic response of the material under different cyclic loading conditions. The aim of this paper is to study the effect of heat affected zone (HAZ) on fatigue crack initiation and propagation life of welded joint by using the above-mentioned fatigue life prediction method. In this study, finite element analysis was conducted to assess the fatigue crack growth rates of base metal, FGHAZ and CGHAZ. Subsequently, fatigue crack initiation and propagation life of a non-load carrying welded fillet joint are investigated using two FE models with and without considering distributions of material. The result shows good agreement with the experimental results by considering HAZ.
  • Cyclic elasto-plasticity behavior and fatigue crack initiation lives of low carbon steels and their simulated HAZ.

    pp. 36-43

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    DOI:10.2207/qjjws.40.36

    Fatigue is one of the major causes of structural failure. In particular, the crack initiation and propagation are usually located in correspondence of the welded parts of a structure. Therefore, the correct evaluation and prediction of the fatigue life of welded components is a crucial aspect to avoid unexpected failure and to improve the design and durability of structural parts. The present study focuses on the cyclic stress-strain behavior in the weld heat-affected zone (HAZ), where mostly fatigue cracks are found. In order to investigate the cyclic plasticity behavior of HAZ including cyclic hardening and softening together, experimental and numerical campaigns are carried out. The experimental part of this study deals with the fatigue experiments conducted on test specimens that have undergone the same temperature history to which the heat-affected zone of a weld is exposed (hereafter, simulated HAZ) and the softening and hardening response of base metal, CGHAZ and FGHAZ are observed. Numerical investigations aim to reproduce the cyclic stress–strain behavior simulating the nonlinear material behavior utilizing an elasto-plasticity model called the Fatigue SS Model (hereafter, FSS model). The main feature of the FSS model is the ability to describe the cyclic softening behavior within a macroscopically elastic stress state. The cyclic stress-strain behavior predicted by numerical simulation showed good agreement with the experimental result. Finally, fatigue crack initiation life criteria of SM490A is proposed based on experimental result.
  • Heat deformation analysis using a finite element model on gas pressure welding for railway rail

    pp. 44-55

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    DOI:10.2207/qjjws.40.44

    We developed heat deformation analysis model based on that of steel bar using Gaussian distribution in order to quantitatively understand deformation behavior of gas pressure welding of rail. The validity of the developed analysis model was confirmed by the variation of temperature, transition of upset length and geometry of budge through gas pressure welding test of rail. Therefore, it was confirmed that this model could simulate that with high accuracy. We found that the deformation degree at the center of the rail base and the jaw part of the rail head are smaller than that of other parts from results of heat deformation analysis. It is estimated that the deformation degree depends on the shape of rail. This paper describes the developed heating deformation analysis model of gas pressure welding of rail, and its validation test results.
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  • Investigations for improving fatigue strength by low energy laser peening via bending fatigue test

    pp. 56-65

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    DOI:10.2207/qjjws.40.56

    Laser peening can introduce compressive residual stresses on the surface of various materials, thereby effectively prolonging their fatigue lives. In this study, the effects of laser peening with pulse energies of 20mJ and 70mJ on residual stress and fatigue life were investigated on two types of HT780 box-welded specimens with different sizes, considering the realization of a portable laser peening system equipped with a small laser device with low pulse energy. As a result of such low pulse energy laser peening, it was found that the depth of compressive residual stress becomes shallower compared to the current laser peening with a pulse energy of 200mJ, while the residual stress on the surface remains at the same level. Bending fatigue tests were conducted with the stress ranges at 100MPa and 150MPa, which revealed that the fatigue life of the specimens with low pulse energy laser peening was at least 50 times and 8 times longer than that of the as-welded specimens, respectively. Comparing the fatigue test results of different size specimens, it is clear that laser peening with low pulse energy is effective in extending the fatigue life of HT780 box-welded specimen regardless of its size.

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