Materials Science Forum Vols. 783-786

Abstract: Shielded metal arc (SMA) and gas tungsten arc (GTA) weldments were investigated to study the welding effects on the mechanical behavior of 308L austenitic stainless steel weldments, respectively. Both SMA and GTA weldments showed dendritic microstructure. The observed austenitic stainless steel welds solidified to give primary ferrite and secondary austenite as the ferritic-austenitic solidification mode (FA-mode) solidification. However, the lower heat input with larger Cr-versus-Ni ratio in SMA weld process led to lathy ferrite morphology and more residual ferrite in the SMA welds, while vermicular ferrite morphology was shown in GTA weldments. The yield strength of the welds significantly increased with decreasing elongation, which was mainly due to the dual phase strengthening effect after rapid solidification during welding

Abstract: The development of 3D integration is necessarily required for high speed, high density, small size, and multi-functional electronic devices. Through silicon via (TSV) technology has been rapidly developed to fulfill the demand of the next generation of multifunctional electronic systems as one of the most alternative applications for 3D packaging. In this study, low cost and high speed molten solder-filling of TSV and bonding process by using micro bump was investigated. Micro bumps were formed with two step, Cu pillar bump and Sn-Ag cap bump by using electroplating. The size of micro-bumps was 10 and 20um and the precise content of the bump was developed with good planarity by adding reflow process. Additionally, the SiC nanoparticle composite solder was fabricated for low CTE filling material. From the results, it is possible that the CTE of composite solder added 1.0wt% SiC nanoparticles had decreased until 15.0ppm/°C. Comparing CTE of pure Sn, which was about 24.0ppm/°C, it was very low CTE and lower then CTE of Cu (16.5ppm/°C). Even if the electrical resistance of nanopowder composite solder is increased, the increasing rate is very slow until 1.0 wt%. However, the resistance changing rate is rapidly increased over the 1.0wt%. From the result, it is expected that nanopowder composite solder can contribute high reliability of molten solder filling TSV.

Abstract: Electronics devices consist of silicon chips, copper leads, substrates and other parts which are jointed to each other with solder, conductive adhesive or other materials. Each coefficient of thermal expansion is different and it causes strain concentrations and cracks. We analytically investigated the stress reduction structure at the edge of the joints such as Sn-Ag-Cu solder or Cu/Sn alloy between the silicon chip and copper lead. At first, we examined the influence of the joint thickness and fillet at the joint edge on the stress. In the joint without fillet, the stress at the end of the joint increased depending on the thickness of the joint. The fillet of the joint increased the stress of the Cu/Sn alloy joint and the stress was increased depending on the thickness, though the fillet decreased the stress of the solder joint. We suggested the copper lead with slits to reduce the force of constraint. We compared the effects of the structure parameters of the slits on the stress reduction. The height was a more effective parameter than the width and the pitch. In the case of solder joint, the slits of the copper lead reduce the stress more effective in the thick joint than the thin joint. However, in the case of Cu₃Sn joints, the slits reduced the stress more effectively in the thin joint than thick joint.

Abstract: A fusion reactor is expected as one of the new electric power sources in next generation. Reduced activation ferritic/martensitic steel F82H is planned to be used as a structural material for the blanket modules set on the inner wall of the reactor. However, especially in the case of laser beam welding (LBW), the weldability of the steel was not completely clarified. On the other hand, although post weld heat treatment (PWHT) should be conducted for the welds of the steel in accordance with general standards for chrome steels, the heat treatment conditions were uncertain. Therefore, adaptability of LBW as a joining method for the steel and the applicable PWHT conditions for the welded joints were investigated in this study. The effect of LBW conditions on weld penetration behavior were ascertained by observation of cross sections in the welds. The adequate PWHT conditions were confirmed in consideration of both hardness distributions measured in welds and ductile-brittle transition temperatures (DBTT) evaluated using Charpy impact test. Full penetration without weld defects such as hot cracking, porosity etc. was obtained for plates with the thickness of 4mm of the steel by control welding conditions. That means laser beam is one of useful welding heat sources to realize sound weld joints of the steel. In addition, due to select appropriate PWHT conditions, the hardness in welds was suppressed to the level of base metal and the toughness in the welded joints was improved to a practical level without the damage to base metal.

Abstract: A detrimental parameter which is always connected with welding processes is the distortion which may occur in welded components. The reduction of distortion is a very costly production factor and in complex constructions the disposal of the distortion might be very complicate or even impossible. On the other hand we know that every step which is done to increase the stiffness of a welded component in order to avoid distortion is connected with the possibility of the generation of higher tensile residual stresses.

Abstract: The mechanical properties and interfacial microstructure of slices of friction stir welded aluminum alloy/stainless steel dissimilar lap joints were characterized. In an FSWed A3003 aluminum alloy/SUS304 steel lap joint, the strength on the advancing side was larger than that at the retreating side. TEM observation indicated that a sound joint that fractured at the base metal can be obtained from the stage of the formation of the amorphous layer owing to the mechanical alloying effects before the formation of intermetallic compounds. This lap joining technique was also successfully applied to A6061/T6 aluminum alloy-grooved SUS304 plates. Equiaxed aluminum grains were observed at the interface of the specimen after it was fractured, indicating that the interface deformed only slightly during the microtensile test. It was found that tensile strength of the joint was increased by aging at 433K, considering that precipitation occurred at this temperature. In addition, it was confirmed that the joint heated at 723K for 1.8ks still fractured at the aluminum matrix, assuming that intermetallic layers at the interface would slightly grow in this heating condition.

Abstract: The laser welding can provide flexible processing, and ultrashort pulsed laser with high pulse repetition rates enabled locally selective welding of monocrystalline silicon and glass with comparable processing performance to anodic bonding method. The 20ps laser pulse of 1060nm was absorbed at monocrystalline silicon through glass plate, and its surface temperature reached its boiling temperature. In addition, it was considered that the absorption of laser energy to glass was occurred, and the temperature of glass increased more than the forming temperature. Thus the convection of silicon and glass was caused by the recoil pressure of evaporation, which led to mild mixture of silicon and glass. This phenomenon produced anchor geometry at the interface between silicon and glass without gap generation. The number of laser shot in the laser spot had an influence on shearing strength of weld joint, and higher shearing strength of weld joint could be performed at proper number of laser shot in the laser spot.

Abstract: Friction Stir Welding (FSW) is one of the most recent welding processes, invented in 1991 by The Welding Institute. Recent developments, mainly using polycrystalline cubic boron nitride (PCBN) tools, broaden the range of use of FSW to harder materials, like steels. Our study focused on the assembly of high yield strength steels for naval applications by FSW, and its consequences on the metallurgical properties. The main objectivewas to analyze the metallurgical transformations occurring during welding. Welding tests were conducted on three steels: 80HLES, S690QL and DH36. For each welded sample, macrographs, micrographs and micro-hardness maps were performed to characterize the variation of microstructures through the weld.

Abstract: Flux applications prior to the convention Gas Tungsten Arc Welding (GTAW) is known to improve weld penetrations and improve process competitiveness. This paper summarizes the investigations on aluminum, plain carbon steels, stainless steels and titanium. The importance of flux composition, homogeneity and profile of its application are shown to be primordial in determining the weld depth to width ratio of weld pools. The mechanisms that lead to improved penetrations along with some industrial case studies are presented.Key Words: ATIG, Weld penetrations, Steels, Titanium, Aluminum

Abstract: The effect of strain rate on tensile properties of several lead-free solder was investigated using miniature size specimens. High-temperature lead-free solder which are Sn-Cu and Sn-Sb alloys were prepared. Moreover, low-Ag lead-free solder which are Sn-1Ag-0.7Cu (SAC107, mass%) based alloys were prepared. Sn-3Ag-0.5Cu (SAC305) was also prepared for comparison. Tensile strength is proportion to the logarithm of strain rate in all solder investigated. Although 0.1% proof stress decreases at high strain rate in high-temperature solder, it scarcely changes in low-Ag solder. Elongation somewhat increases with increasing strain rate in high-temperature solder. It increases with increasing strain rate in low-Ag solder although it is lower than that of SAC305. Chisel point fracture mainly occurred except Sn-13Sb. In Sn-13Sb, brittle fracture occurred and thus elongation was lower than those of other solder. Sn-8.5Sb and Sn-1Ag-0.7Cu-1Bi-0.2In show mechanical properties similar to SAC305.