Key Engineering Materials Vols. 297-300

Abstract: In Gas Metal Arc Welding (GMAW), electromagnetic force is one of the most important factors that effects the metal transfer, short-circuit rate, spatter generation rate and mechanical properties for welding metal. Shielding gas and welding current have a great influence on metal transfer mode in GMAW. In this paper different ways for external electromagnetic forces are applied by attaching a cylindrically rounded conducting wire solenoid on touch tip holding. It was conformed that the intensities of electromagnetic force and kinds of shielding gases influence on the droplet transfer mode. With the applied electromagnetic field, the arc transfer mode changes from normal mode to rotating mode. The rotating direction changes with the change of electromagnetic current direction. Applied electromagnetic field intensity varied by electromagnetic current influences on the spatter generation rate in CO2 welding. In MIG welding, the influences of electromagnetic force on the spatter generation showed different tendency as in the CO2 welding. This paper is for the purpose of discussing these factors.

Abstract: An experimental study of dissimilar friction welding was conducted using 15mm diameter solid bar in chrome molybdenum steel (SCM440) to carbon steel (S45C) not only to optimize the friction welding conditions, but also to investigate the fatigue performance. The main friction welding parameters were selected to endure good quality welds on the basis of visual examination, tensile tests, Vickers hardness surveys of the bond of area and HAZ, and microstructure investigations. In this study, the specimens were tested as welded. For fatigue strength, the notched specimens for the optimal conditions were rotary bending fatigue tested. The results were compared with S-N curves for the base metals.

Abstract: Ductile fracture occurs due to micro-void nucleation, growth and finally coalescence into micro-crack. In this study a new ductile fracture condition that based on the microscopic phenomena of void nucleation, growth and coalescence was proposed. Using this condition and combining with finite element model to predict the fracture locations in bulk metal forming. The macroscopic behavior of the material is described according to the flow rules of Levy-Mises. An idealized spherical void within an finite matrix is assumed. The void volume is calculated by taking the increasing volume of the continuum, caused by plastic straining, incorporated in the yield functions. In the model there includes the strain-hardening coefficient of the Ludwik-Holomom stress-strain relationship and concentration of stress. The accumulated damage value is a phenomenon in this model. The results show that it is in close accordance with observations of some experimental specimens. However, in order to obtaining the high trustiness many experiments have to be carried out.

Abstract: The investigation aimed at the invalidation mechanism analysis and reliability evaluation of aluminum alloy welded joint under space environment will provide spaceflight designing epartments and manufactures with important technology gist and data so as to greatly reduce paceflights manufactured cost and improve spaceflight working reliability. In this paper, the tensile properties and microstructures of LF6 aluminum alloy welded joint under vacuum thermal cycling have been studied by means of vacuum thermal cycling system. The influence of vacuum thermal cycling on the tensile properties of the welded joint has investigated. Results show that the strength and the enlongation reach their maximum values at about 75 times cycling. The changes of the tensile properties are regarded as resulted from the changes of grain and the motion of dislocation.

Abstract: The spot welded structure operates in diverse climatic situations because of temperature, humidity and precipitation. In addition, factors of environmental pollution such as acid rain, that causes corrosion, have the tendency to increase. But spot welded structure strength is affected by dampness and environment temperature. Therefore, it is important to evaluate the effect of temperature of the spot welded part. In this study, the strength distribution of spot welded plates is evaluated for the environmental temperature of zinc coated steel plates and the test is conducted with the welded part immersed in distilled and synthetic sea water. Specimens are immersed into water for 10, 100, 500 and 1000hours to evaluate the effects of water immersion time on tensile-shear strength under the conditions of -40, 0, 20 and 50°C. Strength is evaluated by using the tensile-shear test. The conditions of spot welding are 240kgf electrode force, 10kA welding current with 0 and 5mm clearance. In this study, spot welded specimens with clearance have lower tensile-shear strength in the distilled water or synthetic sea water compared with spot welded specimens without clearance. And they have lower tensile-shear strength under -40°C and over 50°C.

Abstract: Conventionally, shield metal arc welding (SMAW) process was applied to join pipes of RCL, which caused lot of loss in time and cost due to excessive heat input and defects in joining section. Recently, narrow-gap welding (NGW) process was introduced to overcome the disadvantages of SMAW. However, the application of NGW to nuclear power plant is not yet common because safety of NGW process is not proven. In present paper, the welded coupons are manufactured under different welding processes in carbon steel. Then, microstructure observations and various mechanical tests are performed. It is verified that the mechanical properties of carbon steel are greatly changed after repair welding process due to applied heat flux, and that the effect of post-welding heat treatment is beneficial.

Abstract: The application of fracture mechanics is being increased gradually to assess the safety of welding crack-containing structures. The joint parts of vehicle frame are mostly made up of welding joint. Due to the impact and fatigue load, cracks initiate and propagate in the welding joint part, as a result, the frame comes to a final failure. Therefore it is necessary to improve the welding performance and fracture resistance of welding part in structure. This study presents a most suitable way of welding process for lap joints by CO2 arc welding which is widely used in the the vehicle structure. Also this paper makes an evaluation to tensile strength and fatigue strength under various welding condition

Abstract: An influence of a surface finish of a Cu electrode on joint properties of a lead-free solder joint with Sn-3mass%Ag-0.5mass%Cu has been investigated. As the surface treatment method, Ni/Au electroplating, Au electroplating and organic solderability preservative (OSP) treatments were conducted to Cu electrodes. A heat exposure treatment was conducted at 150°C up to 500h in order to investigate the reliability of the solder joint under heat exposure conditions. Ball shear test was performed to examine joint strength. Microstructural observation was conducted to investigate growth kinetics of a reaction layer formed at a joint interface and microstructural revolution in the solder layer.

Abstract: The residual stress that occurs in fiber-reinforced thermosetting composite materials during cure is one of the severe factors that can deteriorate the performance of composite structures. To investigate residual stresses occurring in laminated composites during cure, an incremental viscoelastic constitutive equation is derived as a function of temperature, degree of cure and chemical shrinkage. A finite element program is developed on the basis of a 3-D degenerated shell element and the first order shear deformation theory. Experiments were performed to measure the coefficients of chemical shrinkage of the Hercules AS4/3501-6 composite during cure. Residual strains were measured using strain gages during cure and compared with the results of finite element analysis. Good agreement is found between numerical and experimental results. It is found that the chemical shrinkage seriously affects the residual strains of the composite during cure.

Abstract: This study investigated the microstructure and mechanical properties of a wide-gap region brazed with various process variables. The IN738 and DF 4B alloy powders were used as additive and filler metal powder for the brazing process. The wide-gap brazing process was carried out in a vacuum of 2×10-5 torr. The wide-gap region brazed with 60wt.% IN738 additive powder had a microstructure consisting of IN738 additive and (Cr, W)2B. The fracture strength of the wide-gap region (60 wt.% additive and 40 wt.% filler metal powder) brazed at 1230°C for 30hr was as high as 862MPa (93% of base material strength). It was observed that the brazing temperature was the main process variable affecting the mechanical properties of the wide-gap brazed region. The creep rupture life of the region brazed with 60wt.% additive and 40 wt.% was longer than that of other brazed samples. The Cracks in the wide-gap brazed region initiated at the (Cr, W)2B and propagated through them. It was found that the (Cr, W)2B and the pore in the brazed region are important microstructural factors affecting the mechanical properties of the wide-gap brazed region.