In this paper, the effect of post heat treatment on fatigue behaviour of friction stir spot welded Al-Mg-Si aluminium alloy was investigated. The microstructure of the weld zone was classified into two regions: stir zone (SZ) and mixed zone (MZ), where fine equiaxed grains due to dynamic recrystallization were observed. Two kinds of post heat treatment, namely aging and T6 treatment, were applied to the as-welded joints. The grains in the SZ and MZ were extremely enlarged only by T6 treatment, but some fine grains still remained near the boundary of MZ. Fatigue tests were conducted using lap-shear specimens at a stress ratio R = 0.1. Post heat treatments exhibited little influence on fatigue strength, but fatigue fracture morphology was dependent on both load level and post heat treatment. At high applied loads, fatigue fracture took place through the MZ in the as-welded and aged joints, while along the boundary of MZ in the T6 treated joint. At low applied loads, fatigue crack initiated at the edge of the nugget and then propagated through the upper sheet in the as-welded joint, while the lower sheet in the aged and T6 treated joints. The dependence of fracture morphology on post heat treatment was attributed to the change of microstructures and hardness distribution around the nugget by post heat treatment.
The impact strength evaluation and fracture mechanism analysis in board level of Sn-3mass%Ag-0.5mass%Cu solder joints of BGAs using electrolytic Ni/Au plating were performed. The cause of impact strength degradation of BGA solder ball joints is the existence of low density defect, which contain organic materials, in the (Cu,Ni)6Sn5 intermetallic compound grain boundary formed in the solder joints. These organic materials are taken in by the Ni plating film at the time of Ni plating. To improve the impact strength of the Sn-3mass%Ag-0.5mass%Cu solder joint of the BGA, it is necessary to lower the concentration of these organic materials. The contamination prevention and Ni plating bath sanitization, solder mask material selection (to minimize Ni plating bath contamination) and higher current density of Ni plating are effective to keep lower concentration of organic materials in Ni plating film.
Recently, the demands of dissimilar metal joint of titanium to aluminum alloy have been arisen in industries, especially in transportation vehicle industry. However, it is well known that fusion welding of titanium to aluminum alloy is difficult because of generating the brittle intermetallic compound at the joint interface. Therefore, new welding processes with high reliability and productivity for these dissimilar materials are demanded. In the present work, Laser Roll Welding of titanium to aluminum alloy using a 2 kW fiber laser was tried to investigate the effects of the process parameters on the formation of interlayer and the mechanical properties of the joint. As a result, the cross-section of joint shows partial melting of the aluminum sheet and spreading of molten aluminum alloy on the titanium sheet occurs during the welding thermal cycle. Various types of intermetallic compound were confirmed at the interlayer of welded joint. The specimen with a bonding width of 2.8 mm was failure in the base metal of titanium in the tensile shear test. In Erichsen cupping tests, Erichsen value was 5.7 mm. This value was 89% of the base metal of aluminum sheet.
Measuring of arc length is important to obtain good welding quality in spite of variation of torch height. Therefore, it is necessary to detect arc behavior in the transient state in addition to the steady state. For this purpose, this paper proposes neural network models which output the present wire extension from the data relating to wire melting, such as welding current, current pickup voltage and wire feed rate in every sampling period. Since performance of the neural network model depends on threshold functions, authors investigate the performance of the neural network models based on both sigmoid function and radial base function.To confirm the validity of these systems, fundamental experiments were carried out. The arc was directly observed and recorded as image data using a high speed camera. The output data from the neural network were compared with the measured data which were obtained from every captured image. It was found that the neural network model based on the radial base function is useful than the sigmoid function to estimate the wire extension length in MIG welding because of better responses in the transient state and smaller steady state error.
CO2gas shielded arc welding is major arc welding method, but generates 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 0.058g/s to 0.005g/s).
The application of aluminum alloy, which is a typical lightweight material, has been expected to the construction of the transportation vehicle to achieve energy saving by reduction of its weight. However, the structure made of whole aluminum alloy had problems with low strength and high cost. Thus, hybrid structure made of Al alloy and steel is useful because of lightweight and higher strength. To construct the hybrid structure, it is necessary to weld between aluminum alloy and steel. However, the conventional welding methods, like brazing and mechanical fastening, have some problems such as low mechanical strength and low productivity. Also, it is difficult to weld Al alloy and steel by conventional fusion welding. In this study, spot welding between aluminum alloy and low carbon steel by friction stirring was carried out. Especially, optimization in welding conditions was carried out. Moreover, the effect of welding conditions on the joint strength and weld interface was studied. As a result, relatively higher tensile shear strength of the weld which was achieved at an optimum welding conditions was obtained. Temperature near weld interface measured by K type thermocouple during welding was found to be the value lower than the melting point of A5052. From the observation results on microstructure of weld interface, it was found that Fe/Al intermetallic compound layer was formed.
Fracture toughness and microstructure of laser weld metal of 780MPa class steels are investigated and compared with those of SM490A and SM570Q. In SM490A and SM570Q, Charpy energy transition temperature of laser weld metals is 60-90°C higher than that of base metal, and upper bainite microstructures are observed in the laser weld metals. In 780MPa class steels, difference of Charpy energy transition temperatures between laser weld metal and base metal are only 10-30°C, and no upper bainite microstructures are observed in the laser weld metals. Hardness of the laser weld metals of 780MPa class steels is identical with that of martensite microstructure. A superior toughness of the laser weld metal of 780MPa class steels would be owing to the martensite microstructure resulted from a high carbon equivalent.
A friction stir spot welding process, in which a rotating tool without a probe was employed, was applied to a lap joint of low carbon steel plates with 0.5 mm thick. In this process, the rotating tool of 3.6 mm diameter, rotating at 18000 rpm, was plunged into the upper plate at a rate of 0.2 mm/s, and then kept at a maximum plunged depth of 0.05 – 0.25 mm for 0 – 1 s (dwell time). In the weld obtained by this process, a hole due to the impression of the plunged tool probe was not formed, although a slight depression by the tool plunging remained. At tool plunge depths of 0.05 mm or less, it was impossible to weld the plates. At tool plunge depths from 0.1 to 0.14 mm, joints were fractured at the interface in tensile test, and the failure load increased with tool plunge depth. At tool plunge depths from 0.16 to 0.22 mm, joints were fractured at an almost constant load along the periphery of the depression, leaving a part of the upper plate on the bottom plate surface. The maximum tensile failure load of 1.8kN was obtained at a plunge depth of 0.2 mm. Based on the observation of the weld microstructure and measurement of the thermal cycle at various spots in the weld, controlling factors of the joint strength are discussed.
It was found that the characteristics of welding distortion and residual stress generated by laser beam welding had the same tendency as ones by arc welding but that the absolute value was totally small. Moreover, nevertheless the process of laser beam welding is quite different from that of arc welding, the validity and generality of the dominant factors to predict high accuracy welding distortion and residual stress generated by welding could be verified by the high accurate prediction of welding distortion and residual stress which had been proposed base on arc welding. Inherent force was proposed as a parameter which could indicate the magnitude of compressive residual stress, which might influence the load-carrying capacity. Although inherent forces with laser beam welding was 40% that of arc welding, it was found that control of welding heat input was important so as to improve the load-carrying capacity.
This study examined development of the stir zone and plastic flow of the material during friction stir spot joining of aluminum alloys. The development of the stir zone was discussed through the observation of macrosections of the joints produced at various process times. The friction stir spot joining trial with insert of Au foil to lap surface and the dissimilar lap friction stir spot joining showed that the movement of the lap surface was attributed to the downward plastic flow from the upper plate to the lower plate around the probe. Additionally, it was clarified that the direction of the plastic flow was roughly the same as the direction of the tool rotation. The dissimilar butt friction stir spot joining suggested that the plastic flow occurred not only within the stir zone but also in larger area below the tool shoulder. The present study revealed that the threads on the probe surface were important for producing the plastic flow of thickness direction during friction stir spot joining.
Bulk metallic glass (BMG) alloys exhibit excellent nature in mechanical, chemical and magnetic properties. Because random atomic arrangement is stable in liquid state, glass phase of BMG alloys can be obtained easily from liquid even at much lower quenching rate, like 0.1–100K/s. That is, they don't need to be cooled down at more than 106K/s from liquid state, such as an ordinal amorphous alloy. Thus, thick metallic glass coatings have been successfully fabricated on the substrate by regular thermal spraying methods. However, glass forming mechanism in thermal spray process hasn't been understood clearly up to today. In this study, in order to elucidate the formation mechanism of glass phase in iron-base metallic glass, in-flight temperature history of the particles was estimated by comparing the results between Fe43Cr16Mo16C15B10 metallic glass powder and its crystallized powder by pre-heat treatment. Moreover, comparison of the particles behavior between atmospheric plasma spraying (APS), which is characterized as high temperature low velocity process, and high velocity oxy fuel spraying (HVOF), which characterized as low temperature high velocity process, was conducted. Consequently, it was revealed that the thick coating with glass phase of BMG can be easily obtained in HVOF process by depositing the particles at super-cooled liquid state without heating to the liquid phase during spraying.
Crystal growth behavior in the surface-melted region of Ni-base single crystal superalloy CMSX-4 was investigated. The surface melting was conducted using 2kW diode laser or gas tungsten arc. Laser power and scanning speed in laser surface melting were varied while defocusing distance and shielding gas (Ar) flow rate were fixed. For arc surface melting, arc current and welding speed were varied. Specimen surfaces were arranged parallel to the (001) of the base metal. The microstructure and crystal orientation in the melted region were analyzed by using optical microscopy, scanning electron microscopy(SEM) and electron backscattered pattern analysis(EBSP). The microstructure and crystal orientation in laser surface-melted region were markedly influenced by heat input. The laser surface-melted region under low heat input conditions was found to be solidified as the single-crystalline state with unidirectional dendrites which grew along the  directions. The laser surface-melted region under intermediate heat input conditions was also single-crystalline with dendrites which grew along the ,  and  directions. In contrast, in the case of high heat input conditions, the laser surface-melted region was consisted of polycrystal with stray crystals. Such tendencies were also observed in the melted region with gas tungsten arc. These results demonstrate that heat input significantly affects single-crystallinity regardless of the type of heat source, such as laser and arc. Furthermore, stray crystal formation in surface-melted region was investigated in detail by using SEM and EBSP. From the results of these experiments, it is revealed that stray crystal formation was affected by constitutional supercooling and dendrite growth direction.
Crystal growth behavior in the laser surface-melted region of Ni-base single crystal superalloy CMSX-4 have been investigated to clarify the effect of heat input on the conditions for achieving single-crystallization and/or preventing the formation of stray crystals. The crystal growth in the surface-melted region was numerically analyzed based on the geometric model and CET (columnar to equiaxed transition) theory in combination with an inverse heat conduction analysis. A theoretical analysis of the dendritic growth direction suggested that the surface-melted region consisted of ,  and  dendrites, when the laser beam was scanned along the  or  direction on the (001) surface. Furthermore, it was predicted that the occurrence of the  dendrite could be observed at the bottom of the melted region and the  and  dendrites existed only at the upper part of the melted region. On the other hand, theoretical analysis of the stray crystal formation indicated that stray crystals tended to form under conditions of higher laser power and lower scanning speed, i.e. higher heat input conditions. These calculation results agreed with the metallographic observation. As the results, it can be said that the conditions needed to suppress the formation of stray crystals and to promote single-crystallization in the surface-melted region with epitaxial dendritic growth of  orientation and/or the dendritic growth of  and  orientations is predictable by calculation.
Crystal growth behavior in the overlay welded regions of Ni-base single crystal superalloy CMSX-4 has been investigated. The Ni-base superalloy René142 was used as the filler metal for overlay welding. The surface of the specimens was prepared as to be coincident with the (001) plane. The overlay welding was conducted using a 2kW diode laser along the  direction. Microstructural observation revealed that the overlay weld metal grew epitaxially on the substrate and stray crystal tended to form in the conditions of higher power and (Vf /Vw) ratio (Vf : wire feeding speed, Vw : laser scanning speed). Furthermore, the dendritic growth direction and the single-crystallized condition were analyzed by the theoretical model with combining the geometric model, CET (columnar to equiaxed transition) model and CFD (computational fluid dynamics) analysis. The predicted results of the dendritic growth direction and the stray crystal formation in the overlaid regions corresponded approximately with the experimental ones, which indicated that the current theoretical calculation can be used to predict the conditions to suppress the formation of stray crystals and to promote single-crystal production in the overlay welded region with epitaxial dendritic growth of  orientation and/or the dedritic growth of  and  orientations. On the basis of the condition by the theoretical prediction, a single crystalline layer of 3 mm thickness on a single crystal substrate could be achieved by laser overlay welding with ten passes.
Recrystallization behavior in postheat treatment of the surface-melted regions of Ni-base single crystal superalloy CMSX-4 have been investigated. In, addition, mechanical properties of surface-melted regions after postheat treatment were evaluated. The surface melting was conducted using 2kW diode laser. The microstructure and crystal orientation in the melted regions were analyzed by using optical microscopy, scanning electron microscopy (SEM) and electron backscattered pattern analysis (EBSP). The effect of postheat treatment condition on microstructure was investigated using the specimens whose surface-melted regions were single-crystallized. The surface-melted regions were recrystallized when the temperature of postheat treatment was higher than 1513K. On the basis of this result, the partial solution treatment condition was proposed as 1493K×3.6ks to maintain single crystal structure without recrystallization in the surface-melted regions. Furthermore, the mechanical properties of surface-melted regions after partial solution treatment and subsequent aging were investigated by the creep rupture test. The creep rupture strength of surface-melted specimens was comparable to those of the base metal. It could be concluded that the crystalline controlled repairing technique would be applicable to the repair of single crystal superalloy turbine blades.
Gas Tungsten Arc (GTA) is suitable as a heat source device for many applications because it can stabilize high temperature arc plasma easily by employing shielding gas. In many cases, (Magneto Hydro Dynamic) MHD simulation model assuming the local thermodynamic equilibrium (LTE) is utilized for analyzing property of GTA. Although the LTE assumption is effective to evaluate high temperature region in the arc column, it is difficult to apply it to low temperature region such as the fringe of the arc column or the sheath regions closed to the electrodes due to decrease of energy exchange caused by collisions. Especially, in order to consider the influence of chemical reaction between the arc plasma and the material surface such as oxidation, non-equilibrium property of the arc plasma should be considered without the LTE assumption, since it is required to understand precise property of the arc plasma closed to the material surface. We have developed a simulation model of GTA assuming chemical and thermal non-equilibrium. In this paper, as a first step of the study, heat source property of argon GTA employing a water-cooled copper anode was simulated and dependence of non-equilibrium plasma property on argon GTA on arc current was investigated.