This paper deals with the strengthening of spot welded lap joints by new hardened zone. Based on the equation to predict the strength proposed by past researchers, the factors necessary for the strengthening were extracted. And, based on these factors, the strengthening of spot welded lap joints was examined. First of all, in order to increase the hardened zone, the vicinity of the nugget edge was heated with a laser. Next, tensile shear tests were conducted by using spot welded lap joints which were heated by laser at various distances from nugget. Finally, the relationship between tensile shear strength and the distance from nugget center to the crack was clarified. As a result, by increasing the hardened zone, we found that the fracture position was far away from the nugget edge. And, we found that the tensile shear strength could be improved by 15%.
The purpose of this study was to examine the weldability of Si-solution strengthened ferritic ductile cast iron (SSFDI : EN-GJS-500-14) compared with a traditional ferrite-pearlite type ductile cast iron of similar strength (JIS-FCD500-7). The α+graphite→γ reverse transformation characteristics on heating process and the transformation characteristics on cooling were investigated by synthetic weld thermal cycle method. Heating SSFDI at 300°C/s (200-800°C), the reverse transformation started at 837°C and finished at 1149°C. The austenite arose around graphite nodules and the area fraction in matrix moderately increased to 100% as the temperature became higher. In the ferrite-pearlite type ductile cast iron FCD500 with 2.27%Si, the reverse transformation occurred in the temperature range from 763°C to 1105°C. The pearlite rapidly transformed to austenite and then the ferrite moderately transformed. The austenitization rate of SSFDI was also lower than the ferrite-pearlite type ductile cast iron at 1000°C. In case of the cooling parameter t8/5=25s, the martensite area fraction increased as the maximum temperature rose. Less martensite appears in SSFDI, indicating arc welding would give rise to less brittle heat affected zone (HAZ) in SSFDI.
The new welding method called as F-MAG, which has been developed based on CO2 gas-shielded arc welding method (MAG) increase deposition rate of weld metal. F-MAG welding method is combined with hot-wire, as having been used in TIG arc- and laser-welding methods. F-MAG is performed using hot-wire which is inserted into the rear part of weld pool made with leading electrode. Hot-wire melts by both electrical heating of itself and heat of weld pool. Multi-layered weld metals were prepared using F-MAG and MAG. Microstructures and mechanical properties of both as-weld and reheated zones in uppermost layer of the multi-layered weld metals formed with F-MAG and MAG were examined and the effects of hot-wire on microstructures and mechanical properties were analyzed. Both the as-weld and reheated zones of weld metal formed with F-MAG consisted of acicular ferrite (AF), equiaxed ferrite and so on. Both strength and elongation in as-weld and reheated zones formed with F-MAG were superior compared with those formed with MAG. It could be suggested that strength increased by refinement of AF due to increase in the concentration of alloying elements being contained in hot-wire. Larger elongation of weld metal in F-MAG compared with that in MAG could be analyzed in terms of the Aggregate of acicular ferrite Laths with nearly Parallel Slip systems between neighboring acicular ferrite laths (ALPS). The number of AFs contained in an ALPS formed with F-MAG is larger than that with MAG, in spite of the sizes of ALPSs formed with MAG and F-MAG being almost same. Deformation occurs over a lot of AFs in the case of finer AF formed with F-MAG through the rotation of tensile test piece during deformation, resulting in the larger elongation.
The dissimilar welds between aluminum (Al) alloy, A6061-T6, and rolled steel, SS400, had been fabricated by a friction stir welding (FSW) technique. The FSW tool was offset to the Al side and the probe was inserted only into Al plate. Based on the microstructural observation, small steel fragments were recognized in Al side due to the scratching of steel side surface by the probe. Subsequently, hardness, tensile and fatigue tests of Al similar and Al/steel dissimilar welds had been conducted. The hardness of stir zone (SZ) in Al side was lower than the base metal due to the dissolution of hardening precipitates during welding, where the lowest hardness located near the interface and thermo-mechanically affected zone (TMAZ) in Al side. The tensile strength of the dissimilar weld was lower than that of the similar one. Static fracture of the dissimilar weld occurred through the interface, where the hardness was the lowest, while through the TMAZ in the similar weld. The dissimilar welds exhibited lower fatigue strengths than the similar ones. The fractographic analysis revealed that fatigue crack initiated at the interface between Al and steel, but propagated through Al alloy near the interface. Based on the fracture mechanics, it was concluded that the lower fatigue strengths of the dissimilar welds could be attributed to the lower fatigue crack initiation resistance.
The influence of laser-induced plume on penetration is discussed in remote laser welding. Melt-runs accompanying plumes of various lengths were carried out in order to investigate the influence of plume on penetration using 4.5 kW YAG laser and air knife. Penetration depth obviously reduced on the conditions with long plume. It is considered that the decrease of penetration depth comes from attenuation of laser power in plume, through the decrease in weld metal cross-sectional area. Attenuation of laser power was estimated about 7% and focus shift was estimated to be about 0.7 mm on the experimental condition with plume of 90 mm length, compared with the condition with plume of 11 mm length.
A novel resistance brazing method aided by a numerical simulation, in which the brazing is completed through several preliminary heating and a subsequent final heating aided by the numerical simulation, is presented. The preliminary heating is performed with a relatively low electric energy input so that the uniformity of the surface contact condition between two parts can be improved due to local melting and subsequent solidification, and so that the electric current data can be acquired for preparing analytical conditions necessary to the numerical simulation. The final heating is performed with an energizing condition determined by the numerical simulation in advance. To prove the efficacy of the resistance brazing method aided by the numerical simulation, Ti-Ni alloy and type 304 stainless steel wires with diameters of 96μm both was butt-joint brazed using Au-Cu brazing filler metal supplied with the individual metal plating. The brazed joints had the tensile strengths ranging from 74 to 448MPa in accordance with the energizing conditions.
Measurements of arc plasma are important for determining the associated physical properties. Such measurements usually involve the use of spectroscopic techniques to measure temperature distribution in free-burning arcs. Most of the studies have reported the temperature of axially symmetric arc plasma using Abel inversion. This method cannot be used for axially asymmetric arc plasma such as two-electrode TIG arc plasma. It is complex phenomenon because the arc plasma generated from each electrode is affected by the other due to electromagnetic force. We measured the temperature distribution of tandem TIG arc plasma three dimensionally with one camera rotation system. The measurement method includes the multi-directional detections and the image reconstruction technique: Maximum Likelihood-Expectation Maximization (ML-EM). We evaluated the influence of number of detection directions on the reconstruction by virtual intensity distribution and found that the sufficient direction number is 6. As a result, we could measure the temperature distribution of the inclined, drawn and coupled arc plasma provided by two-electrode TIG arc.
Plate temperature and heat input in oxyfuel gas cutting process with H2/LP gas and LPG flame are calculated by 3-dimensional FE heat conduction analyses. FE analyses are performed by using moving coordinates, and cutting groove temperature is determined by iterative calculation. The 2-dimensional groove temperature distribution determined by Matsuyama's theory is chosen as the initial values in this iterative calculation. The heat transfer properties of the preheating flame are determined by using the GA-based heat transfer estimation technique proposed in the previous report. The validity of the proposed numerical procedure and the accuracy of the determined groove temperature are examined by comparing the calculated and measured plate temperature and HAZ sizes. Heat input due to preheating, qG, and that due to self burning of steel, qB, are estimated in these analyses, and they are compared with the heat inputs estimated by Wells' and modified Wells' equations. The relation between the heat transfer characteristics of the preheating gas flame and plate temperature distribution is examined, and the cutting performance improvement mechanisms of Hydrogen preheating are discussed. As results, followings are found: 1) The 3-dimensional groove temperature distribution can be calculated by performing the iterative analyses procedure proposed in this study; 2) The critical cutting speed can be estimated once the gas heat transfer parameters are known; 3) It is not appropriate to evaluate the magnitude of cutting thermal deformation only from the preheating gas's total calorific value; 4) Under the conditions chosen, the heat generated by self burning is inadequate to maintain the cutting process, and it is essential to supplement heat by preheating; 5) The faster cutting speed and smaller total heat input of H2/LP gas are results of the larger local heat transfer coefficient below the gas ejection hole. It is supposed that the improvement in oxyfuel gas cutting performance can be achieved by modifying the heating apparatus so that the local heat transfer coefficient becomes larger.
The occurrence of solidification cracks in laser welds of type 310 stainless steels was predicted by numerical analyses of the solidification brittle range (ductility curve for cracking) and thermal strain in the weld metal. The solidification brittle range in laser welding was estimated from that in arc welding based on the numerical analyses of supercooling (for calculating dendrite tip temperature) and segregation (for calculating completely solidified temperature) during rapid solidification. The calculated solidification brittle range was reduced with an increase in the welding speed because of the enhanced supercooling and the inhibited solidification segregation. The thermal strain analysis by FEM suggested that solidification cracks would occur in SUS310S welds at laser travelling velocity of 60mm/s applying the initial strain of 1.5%, while no solidification cracks in SUS310EHP welds at any laser travelling velocities applying the higher initial strain of 2.2%. The cantilever type cracking test in laser welding revealed that the predicted results of occurrence of solidification cracks were consistent with experimental ones.
Micro welding of titanium eyeglass frames requires sound strength, good exterior appearances and stable production which were solved narrow HAZ (heat-affected zone), contactless method and stable production, which suit micro laser welding. The objective of this work is to fundamentally investigate butt welding conditions with a pulsed YAG laser beam for exterior appearances and joint strength. Eleven factors of the welding conditions which influence joint strength were evaluated from the viewpoint quality engineering. It was found that two factors of defocused distance and processing of welding surface of the welding parts led to fluctuation on the joint strength. Therefore sub-millimeter scale control of the laser irradiated point along optical axis and removal of impurities lead to brittle metallic compound on the surface produced sound strength of the joints. Furthermore, the optimized laser condition is applied to fabrication of components with a real shape of titanium eyeglass frames. The qualities of the obtained joints were confirmed by stripping test, cyclic bending test, metallographic observation and hardness determination. It was revealed laser welding method is more reasonable than conventional resistance brazing method.