Rinsei Ikeda

Development of advanced resistance spot welding process using control of electrode force and welding current during welding

In resistance spot welding of thin sheet–thick sheet–thick sheet joint, when the sheet thickness ratio is large (sheet thickness ratio = total thickness of sheet joint/thickness of the thin sheet positioned on the outside of the joint), how to stably secure the nugget between the thin sheet and the adjoining thick sheet is a key issue. If the sheet thickness ratio is so large, nugget formation between the thin sheet and thick sheet is extremely difficult. In order to control of the nugget (position of formation, shape, etc. of the nugget) during welding for three sheets joint with a high sheet thickness ratio, optimum welding process was investigated. The developed ‘two-step force, two-step current’ welding process was suitable for high sheet thickness ratio joint and relaxed the constraints on the sheet thickness ratio. In Step 1 (first part of welding period) of the welding process, a nugget is reliably formed between the thin sheet and thick sheet by applying conditions of low electrode force, short welding time, and high current. In the subsequent Step 2 (second part of welding period), a nugget is formed between the two thick sheets by applying high welding force and a long welding time. In the weld results of a three sheet joint (0.7+2.3+2.3 mm; sheet thickness ratio: 7.6) using mild steel GA (0.7 mm) as the thin sheet and 780 MPa high strength GA (2.3 mm) in the two thick sheets, ‘two-step force, two-step current’ spot welding process showed the wide available welding current range.

Improvement of HAZ Toughness for High Heat Input Welding by using Boron Diffusion from Weld Metal

In high heat input welding, such as 1-pass electrogas arc welding (EGW) of thick steel plates, HAZ toughness is deteriorated because HAZ microstructure coarsens significantly. TiN precipitates dispersed in steel plates are effective to suppress coarsening HAZ microstructure by pinning effect. However TiN precipitates resolve in high temperature region near the fusion line, where deterioration of HAZ toughness is unavoidable near the fusion line. As a solution of this problem, the authors paid attention to boron in the weld metal. Boron is known to be effective to prevent coarse ferrite formation at prior austenite grain boundaries, and to diffuse rapidly in steel. In this research, boron diffusion from weld metal to HAZ during EGW process was confirmed by SIMS analysis. It was certified that diffused boron in HAZ improved HAZ toughness because boron suppressed microstructure coarsening, and fixed free nitrogen originated by TiN resolution as BN near the fusion line. These research results proposed a new HAZ structure control technology considering combination of high boron bearing weld metal and TiN-treated steel plates.

Effect of electrode force condition on nugget diameter and residual stress in resistance spot welded high-strength steel sheets

This study examines the effect of the electrode force condition on the nugget diameter and residual stress in spot welded high-strength steel sheets. Numerical simulations of spot welding were performed to examine the nugget diameter and residual stress. The results indicate that adjusting the force profile changes the current density and stress state at the spot welds. Therefore, choosing an appropriate force profile extends the nugget diameter and reduces the residual stress.

Fracture behaviour and numerical study of resistance spot welded joints in high-strength steel sheet

Abstract Cross tension tests of resistance spot welded joints with varying nugget diameter were carried out using 980 MPa high strength steel sheet of 1.6 mm thickness. In proportion, as nugget diameter increased from 3√t to 5√t (where t is thickness), cross tension strength (CTS) increased while fracture morphology simultaneously transferred from interface fracture to full plug fracture. In cases of interface fracture, circumferential crack initiation due to separation of the corona bond arose at an early stage of loading. The crack opening process without propagation was recognized until just before fracture and then the crack propagated to the nugget immediately in a brittle manner around CTS. In full plug fracture, main ductile crack initiation from the notch-like part at the end of sheet separation occurred with the sub-crack initiated at an early stage. The ductile crack propagated toward the HAZ and base material to form full plug fracture. The mode I stress intensity factor was considered as a suitable fracture parameter because the circumferential crack behaved pre-crack for brittle fracture in the nugget region at the final stage. Based on the FE analysis, the mode I stress intensity factor was calculated as 116 MPa √m at CTS as fracture toughness for the nugget. With respect to full plug fracture, ductile crack initiation behaviour from the notch-like part was expressed by concentration of equivalent plastic strain. On the assumption that the ductile crack arose in critical value of equivalent plastic strain, the value was calculated as 0.34 by FE analysis. Reasonable interpretation for interface fracture and full plug fracture in the resistance spot welded joint was proposed due to first crack initiation by stress concentration, brittle fracture by using mode I stress intensity factor, and ductile crack initiation by using equivalent plastic strain.

Development of a low-spatter CO2 arc welding process with a high-frequency pulse current

In CO2 arc welding of solid wire, metal transfer phenomena and spatter generation are investigated with rectangular pulse current, and a low spatter CO2 arc welding process with high frequency pulse current is developed. The optimal conditions of high frequency pulse CO2 arc welding are in the range of peak current: 450–550 A and pulse frequency: 450–750 Hz. These high frequency pulse currents have the effect of droplet oscillation due to resonance between applied pulse frequency and the natural frequency of the droplet. The droplet is transferred consistently every 9–11 pulses and the average interval of metal transfer is about 16 ms which is reduced to half that of conventional CO2 arc welding. This average droplet weight is 34 mg, showing a large reduction in comparison with that of the conventional method. As a result, total spatter weight is reduced by 70% in comparison with the conventional method, and especially, large spatters more than 0.5 mm in diameter are reduced from 1.5 to 0.2 g/min.

Influence of minor elements in electrode wire on spattering phenomena in CO2 gas shielded arc welding

The CO2 gas shielded arc welding method was developed in the 1950s and through the improvements in welding power and consumables it has now developed to become the mainstream arc welding method. However, because a large amount of spatter is generated during CO2 gas shielded arc welding, even now the improvement in the procedures of welding execution in terms of spatter generation is still a task to deal with. Studies into the effect of welding wire composition on the welding arc phenomenon have already been reported4,1,2 and they have shown that, in CO2 gas shielded arc welding, the reduction of C content and the increase of Ti content in the welding wire induce the restriction of short-circuiting and the reduction of spatter as a result2 . However, it cannot yet be said that the degree of spatter reduction has been sufficient. The authors also achieved a reduction of spatter generation by developing the high frequency pulsed CO2 gas shielded arc welding method 3, but, even with this welding method, to reduce the generation of spatter sufficiently it is essential to optimise the composition of the welding wire. Therefore, in this paper on the reduction of spatter in CO2 gas shielded arc welding, an investigation was made of changes in arc mode and spatter generation by adding Ti, REM (rare earth metal), Ca and K to the welding wire.

Improvement of Weld Metal Toughness in High Heat Input Electro-Slag Welding of Low Carbon Steel

In recent years, important steel structures of high-rise buildings such as the box columns required high toughness of welded joints considering the shockproof against the large-scale earthquake. In Japan, there are some specifications that demand Charpy impact energy more than 70 J at 0 °C in any welded joints. However in high heat input electro-slag welding, the microstructure of the weld metal becomes very coarse and weld metal toughness is remarkably deteriorated. In this paper, authors tried to form fully acicular ferrite microstructure in the weld metal of high heat input electro-slag welding to improve the weld metal toughness. High heat input (approximately 100 kJ/mm) electro-slag welding was carried out in the 60 mm thickness steel plate joint. Applying high Ti bearing welding wire and low basicity welding flux provided the weld metal with acicular ferrite microstructure, because large amounts of Ti containing oxide inclusions which were known as effective nucleation sites for acicular ferrite formations were dispersed in the weld metal. B addition to the weld metal suppressed the formation of coarse grain boundary ferrite when B/N ratio in the weld metal ranged between 0.5 and 0.8. When B/N ratio in the weld metal exceeded 0.8, the weld metal toughness was deteriorated due to an increase of M-A constituent. Consequently it was possible that fully acicular ferrite microstructure was formed in the weld metal of high heat input electro-slag welding and achieved the excellent weld metal toughness (vE0 > 100 J).

Effect of REM addition of wire on CO2 gas shielded arc phenomenon

Carbon dioxide (CO2) gas shielded arc welding is the main arc welding method, but it generates a large amount of spatter during welding. The root cause of spatter lies in the fact that the droplet undergoes repeated irregular shaking. To solve this problem, spatter generation modes were clarified and the effects of polarity and rare earth metal (REM) addition of the wire on CO2 gas shielded arc welding were investigated. As a result, when welding is performed with an electrode negative (DCEN) polarity using REM added wire, it was found that a conical arc plasma is formed, and the droplet which is transferred from the wire tip to the molten pool is fine and continuous, in what is termed ‘spray transfer’. Thus, spatter generation was reduced to 10% of amount of the conventional CO2 gas shielded arc welding (from 0.058 to 0.005g/s).