Advanced Materials Research Vols. 652-654

Abstract: An arc-preheating wire GMAW process is proposed in this paper. The welding wire is preheated by a TIG arc to enhance the wire melting. As a result, the welding current is decrease. So the improved GMAW process is aimed at high efficiency welding with a low level heat input. In this paper, the welding system is designed. The wire temperature is measured by a pyrometer. The preheating current effect on the wire temperature is analyzed. In addition, the influence of the wire temperature on the weld current and weld formation are analyzed compared with conventional GMAW. The results show that the preheated wire can significantly enhance the wire melting. By increasing the temperature of the welding wire, the welding current decrease and the heat input of the base metal reduce accordingly. The shallow penetration and large reinforcement height are obtained at the arc-preheating wire GMAW process.

Abstract: The main aim of this research was to investigate the influence of processing parameters on the me-chanical properties and morphology of porous STS:TiH₂ composites made by using spark plasma sintering method. Mechanical parameters and structural analysis of the fabricated composite have been investigated by Rockwell hardness, density tester, optical microscope, XRD and FE-SEM. In particular the porous stainless steel (STS) and titanium hydride TiH₂ composites materials with op-timal ratio were prepared with using additional hollow glass microsphere and water glass for con-trolling of the porosity degree. It is found, that the porosity degree depended on addition of TiH₂ and hollow glass additive, resulting to modification of the pore shape and size. I was shown that the dispersion of STS/TiH2 composite which sintered by spark plasma sintering method without pressure result to increasing of mechanical properties (hardness and density). According to obtained results, the addition of TiH2 powder into STS improves the mechanical property of the synthesized specimens especially the specimens with addition 20 wt% and 50 wt% of TiH₂ demonstrated high hardness values and good density.

Abstract: A low-current GMAW experimental system is established using a constant current power supply and a controlled wire feed system. The metal transfer behavior and the droplet growing process in low-current GMAW are investigated by using a high-speed camera, and the welding current and voltage are monitored by a data acquisition set. The metal transfer in low-current GMAW can be classified into three modes: short-circuiting transfer, long-arc short-circuiting transfer and large globular transfer. The arc voltage is the primary factor that affects the metal transfer mode. The droplets growing in low-current GMAW are observed and analyzed. The maximum detaching size of the droplet and the corresponding growing time with different welding current are calculated from the recorded image sequences of metal transfer. Based on such results, a prediction model of the droplet size is thus established, which will be used in the future research on pulsed laser-enhanced GMAW.

Abstract: The manufacture core of CRH380 high-speed train is aluminum alloy welding technology. However, welding residual distortion which occurs in welding process brings unfavorable effect on the quality of high-speed train. As a result, welding distortion forecasting and control become an important and urgent research topic in railway vehicles. Using equivalent thermal load method based on inherent strain given by the formulae, the welding distortion of aluminum alloy side-wall was predicted by an Elastic FEM which consider actual welding conditions. The simulation results were compared with experimentally measured data to evaluate the validity of the model and to verify effectiveness of this method for large-scale welding structure which had long seams.

Abstract: The present paper describes the compulsively short circuiting transfer phenomenon of welding current in rotational arc MAG welding. Based on a previously reported model of spray transfer welding, a mathematical model of the short circuiting transfer of rotational arc welding is proposed. A series of welding experiments had been conducted and it’s results show that short circuiting transfer would take place when the welding gun electrode rotating to the sides of V groove. The relationship between the short circuiting transfer frequency and the arc rotational frequency also had been studied.

Abstract: Friction stir welding, which is considered to be a solid-state welding, possesses several advantages over conventional welding processes, is an effective approach to weld high-strength, large thickness and dissimilar materials. Material flow behavior on FSW was generally acknowledged to have effects on weld property. The material flow behavior in thickness direction of advancing and retreating side was analyzed by a numerical model established with cone-shape tool pin. Numerical results indicate that there exist material flow in thickness direction on both sides and the behavior was affected by welding and tool pin rotation speed. Decrease welding speed or increase rotation speed would make material deformation intensified, and increase the fluidity of the material.

Abstract: To study the impact of the welding process parameters on the spot weld surface molding, mechanical performance and the stir zone microstructure, we use friction stir spot welding without keyhole method which designed and developed by ourselves to carry on the lap welding between 1mm sheet of AZ31B magnesium alloy and 3mm sheet of 5052 aluminum alloy with magnesium on top and aluminum bottom. The result of the orthogonal test shows that the shoulder friction time have a greater impact on the forming of the surface of the welding joint; When the rotating speed of 1000 r/min, shoulder friction time of 4s, pin length of 3.5mm, the tensile shearing force of the welded joint achieves its maximum value of 2.46 KN.Through observating the microstructure of the welding joint’s section stirring zone, we found the typical joint microstructure of Dissimilar metal. Through analyzing the microstructure of the tensile fracture, we found that fracture was cleavage fracture.We also found the fracture mechanism of the welding joint was cleavage fracture in microcosm but brittle fracture macrocosm.

Abstract: The experimental investigations on resistance spot welding are presented for 316 stainless steel. The influence of spot welding parameters (welding time, electrode force and welding current) on the tensile shear load and the diameter of nugget have been researched, based on an orthogonal test and analysis method. The results show that welding current has significant influence on the tensile shear load and diameter of nugget, and then is electrode force, welding time in turn. The optimum parameters are as follows: welding time is 5 cycles, electrode force is 3.5KN and welding current is 5.5KA. And the maximum tensile shear force of joint is up to 13.55KN.

Abstract: Recently, the fuel oil of diesel engine of marine ships has been changed with heavy oil of low quality as the oil price is getting higher and higher. Therefore, the wear and corrosion against all parts of the engine, such as cylinder liner, piston crown, and spindle and seat ring of exhaust valves has predominantly increased due to its consumption. Thus, the repair welding of the piston crown is a unique method to prolong its life in a economical point of view. In this case, filler metals having a better corrosion and wear resistance are mainly being used for repair welding. However, the piston crown on the ship’s job site is often actually being welded with mild filler metals. Therefore, in this study, the mild filler metals, such as E5016, E1216, and E2316, were welded to the SS401 steel as the base metal, and the corrosion properties of their weld metal zones with and without post weld heat treatment were investigated with some electrochemical methods in 0.1% H2SO4 solution. The weld metal zone welded with E1216 filler metal exhibited the best corrosion resistance among the filler metals with irrespective of heat treatment, however, its corrosion resistance was somewhat decreased due to the post weld heat treatment(annealing:625oC, 2hr). In particular, the weld metal of E2316 exhibited relatively a good corrosion resistances by the post weld heat treatment.

Abstract: Recently a fuel oil of the diesel engine in the merchant ship has been changed with heavy oil of low quality as the oil price is getting higher and higher .Therefore, the wear and corrosion against all parts of the engine are significantly increased with using of heavy oil of low quality. In particular, the degree of wear and corrosion in between spindle and seat ring of exhaust valve are more serious compared to the other parts of the engine due to severe environment such as the high temperature of exhaust gas and repeating impact. Thus, repair welding is a unique method to prolong their lifetime in an economical point of view. In this study, an effect of welding methods to corrosion and mechanical properties of both weld and base metal zone was investigated with some electrochemical methods such as measurement of corrosion potential, cathodic and anodic polarization curves, cyclic voltammogram and AC impedance etc. in. The hardness of the weld metal zones exhibited comparatively higher values than that of the base metal zone. And their corrosion resistances were also increased compared to the base metal. The weld metal zone of P1F( plasma welding with 1 pass) showed a relatively good corrosion resistance as well as the hardness compared to the other welding methods. Consequently, it is considered that plasma welding method is a desirable method compared to other welding methods for both better corrosion and hardness properties.