Motomichi Yamamoto

Material flow and intermixing during dissimilar friction stir welding

Abstract Material flow and intermixing during dissimilar friction stir spot welding and friction stir seam welding are investigated. During friction stir spot welding, a ribbon of contiguous dissimilar lamellae is produced during each rotation of the tool and the number of intermingled lamellae contained in the intermixed region is determined by the tool rotational speed setting and the dwell time applied. When the rotating tool moves across the component, the ribbon of dissimilar contiguous lamellae continues to be produced and the linear distance in the traversing direction between dissimilar lamellae corresponds with the pitch distance [the travel speed (mm s−1) divided by the tool rotational speed (Hz)]. The material flow pattern produced when a threaded tool moves across a component is therefore a variant of that produced during the touch down period when the rotating tool is held stationary. It is suggested that the onion ring structures observed in similar and dissimilar friction stir seam welds made using threaded tools are produced by material incorporation from the locations beneath the tool shoulder and the bottom of the rotating pin and the creation of a helical vertical rotational flow within the intermixed region formed beside the periphery of the rotating pin.

Mechanism of cracking in AZ91 friction stir spot welds

Abstract The mechanism of liquid penetration induced (LPI) cracking is investigated in AZ91 friction stir spot welds. Liquid penetration induced cracking results from the following sequence of events: melted eutectic film formation in the periphery of the stir zone region, engulfment of melted eutectic films when the stir zone grows in width during the dwell period, penetration of α-Mg grain boundaries in the stir zone extremity and crack propagation when torque is applied by the rotating tool. Liquid penetration induced cracking occurs early in the dwell period during AZ91 spot welding and almost the entire stir zone is removed when the rotating tool is withdrawn. However, tool withdrawal does not provide the driving force for cracking during AZ91 spot welding; the upwards axial movement of the rotating tool at the end of the spot welding operation merely separates sections which are already cracked. The cracking tendency (the amount of the stir zone material produced during spot welding, which is removed when the rotating tool is withdrawn) is determined by the competing effects resulting from dissolution of melted eutectic films at α-Mg grain boundaries and the incorporation of eutectic rich material into the stir zone.

Cracking in dissimilar Mg alloy friction stir spot welds

Abstract Cracking during dissimilar friction stir spot welding of thixomoulded AM60 and AZ91 sheet materials is investigated. Liquation cracking occurs in the thermomechanically affected zone (TMAZ) region during friction stir spot welding when AZ91 material is the lower sheet in the dissimilar AM60/AZ91 sandwich. A TMAZ microstructure comprising elongated α-Mg grains and aligned Mg17Al12 is created during the tool penetration stage in spot welding and melted eutectic film formation is facilitated since the temperature in the stir zone (504°C) is much higher than the α-Mg+Mg17Al12 eutectic temperature (437°C). There is no evidence of liquation cracking in the stir zone or the TMAZ region of AM60 sheet when it is the lower sheet in the dissimilar sandwich. However, liquation cracking is observed in the upper sheet material (AZ91) in the location beneath the tool shoulder close to its periphery.

Local melting and cracking in Al 7075-T6 and Al 2024-T3 friction stir spot welds

Abstract Local melting of eutectic films and cracking is found in Al 2024 and Al 7075 alloy friction stir spot welds. Dissolution of melted films removes all evidence melted film formation in spot welds made using typical welding parameter settings. For this reason friction stir spot welding is carried out at a rapid plunge rate of 10 mm s−1 and an extremely short dwell time of 0·05 s and after tool retraction, the welded samples are rapidly quenched using a mixture of methanol and liquid nitrogen at a temperature of −80°C. Eutectic films rich in Zn and Cu are formed in Al 7075 spot welds while melted Al2CuMg particles promote the formation of α-Al+Al2CuMg eutectic films in Al 2024 spot welds. Melted eutectic formation and cracking is also observed beneath the tip of the rotating pin during Al 7075 friction stir spot welding and is consistent with the occurrence of melt wear in this location.

The effect of welding conditions on solidification cracking susceptibility of type 310S stainless steel during laser welding using an in-situ observation technique

Solidification cracking occurs easily at high welding speeds, and should therefore occur more easily during laser welding. Both the solidification behavior and thermal strain change depend on the welding speed, and therefore, the critical strain for solidification cracking must be measured to clarify the factors influencing the solidification cracking susceptibility. However, the critical strain required for solidification cracking under high welding speed conditions has not yet been determined. The aim of this work was to investigate the effect of welding speed on the solidification cracking susceptibility of Type 310S stainless steel. U-type hot cracking tests were conducted using a developed in-situ observation technique with high-speed camera, and the critical strain for solidification cracking was evaluated quantitatively. The critical strain for solidification crack initiation decreased with increasing welding speed. The distribution of residual liquid depended on the microstructure, and the morphological distribution of the residual liquid changed from a droplet to a thin film with increasing welding speed. The transition in distribution morphology of the residual liquid implies the material is susceptible to solidification cracking.

Experimental investigation of material flow during friction stir spot welding

Abstract Material flow during friction stir spot welding is investigated by experimental approaches in this study. Different kinds of fine wires and WC powder are used as trace material and disposed in the interface between upper material and lower material. After spot welding, detailed microscopic observations are carried out in the various sections. Continuous distributions of tracer materials are investigated during friction stir spot welding. Three-dimensional material flow model is proposed by the combination of the results, which are obtained in the present study and previous studies.

Cracking and Local Melting in Mg-Alloy and Al-Alloy During Friction Stir Spot Welding

Although it is generally assumed that friction stir seam welds and friction stir spot welds are free of many of the defect formation issues commonly associated with fusion welding, liquid penetration induced (LPI) cracking in the stir zone have been recently found in friction stir spot welds of AZ91. In the present study, cracking during friction stir spot welding of Mg-alloy (AZ91, AM60 and AZ31) sections is examined. Both liquation cracking and liquid penetration induced (LPI) cracking are observed in Mg-alloy friction stir welded joints. Local melting and cracking is also apparent in Al 7075-T6 friction stir spot welds produced with the precise objective of limiting dissolution of melted eutectic films in the high temperature stir zone and when spot welds cool to room temperature. Based on these test results there is no need to assume that the stir zone temperature during friction stir spot welding is less than that required for formation of melted eutectic films or for spontaneous melting of second-phase particles contained in the as-received base material.

Bead formation and wire temperature distribution during ULTRA-HIGH-SPEED GTA WELDING using pulse-heated hot-wire

The melting of filler wire is investigated in detail to obtain the precise temperature distribution of the filler wire during ultra-high-speed gas tungsten arc (GTA) welding and develop ultra-high-speed GTA welding with a pulse-heated hot-wire system for three kinds of materials. The melting phenomena of the filler wire were observed using a high-speed camera, and temperature distributions of the filler wire were measured using a radiation thermometer. The results show that ultra-high-speed GTA welding can be achieved with suitable welding conditions for each material. Ultra-high-speed GTA welding requires suitable wire current to obtain a suitable temperature distribution and melting position of the filler wire. Moreover, the temperature distributions of three kinds of filler wire could be estimated using a proposed simple estimation method.

Development of Evaluation Method for Solidification Cracking Susceptibility of Inconel600/SUS347 Dissimilar Laser Weld Metal by In-Situ Observation

This study was carried out on the development of the evaluation method for solidification cracking susceptibility of Inconel600/SUS347 dissimilar weld metals during laser welding. Some dissimilar weld metals which have different ratios of Inconel600/SUS347 were prepared by TIG welding and then were remelted on the U-type hot cracking tester by laser. Solidification cracking behavior during hot cracking test was observed by a high speed camera and the dynamic strain, close to the solidification crack, was evaluated. It appeared that local critical strain, for the initiation of solidification crack, was obtained by this strain measurement method. So the solidification cracking susceptibility could be directly evaluated based on the critical strain for different dissimilar joint. By using this method, it was discovered that solidification cracking occurred most easily when the ratio of Inconel600/SUS347 is 40%/60%, in the case of the Inconel600/SUS347 dissimilar laser welded joints.

Development of Structural Steel With Superior Resistance Against Fatigue Crack Growth

The fatigue crack problems that broke out at the end of the twentieth century were fatigue crack properties in hull structures. This motivated research on a lot of technologies against fatigue fracture. It was clarified that the detection of fatigue crack initiation in complex welded structures like hulls is quite difficult, and that the crack length at recognition is mostly long compared to mechanical parts. From these research results, not only stress reduction at critical areas by improvement in design but also newly developed materials with excellent resistance to fatigue crack growth has been desirable for structural integrity. The newly developed structural steel, in which fatigue crack growth resistance is controlled by microstructures, will be introduced in this report. Various fatigue properties of the base steel plates in air and in synthetic sea water are compared. And the fatigue life extension effects by FCA is observed in fatigue tests of welded joints and welded structural models.Copyright