In this study, the authors applied the ultrasonic welding method to weld A6061 heat treatable aluminum alloy and investigated the effects of clamping load and welding time on the properties of the weld. In addition, in order to improve the strength of the joint the effectiveness of the ethanol droplet on the faying surface was examined. The following results were obtained. The joint strength increased with clamping load and welding time. The fracture of the joint produced under the welding conditions of 1176N clamping load and 1.5s welding time occurred in the base metal. The ethanol droplet on the faying surface successfully produced the joint with the strength equivalent to that of the base metal under the welding conditions of smaller clamping load and shorter welding time than the case without ethanol droplet. The softening around the welded area that was performed with the ethanol droplet was smaller than that in the welded area produced by other methods such as TIG and FSW. The fracture surface of the joint welded with the ethanol droplet was remarkably irregular and rough. Dimple pattern was observed over the wide area, indicating that the welded area was significantly expanded. The ethanol droplet made the temperature of the weld area higher than that without ethanol, resulted in improving the joint strength with the increase of the plastic deformation at the interface.
In the large heat input weld joint of heavy gauge steel plate used for large container carrier, a brittle crack possibly propagates straightly along a weld joint without diverging to base metal. This phenomenon is being discussed for its application to the large heat input weld of heavy gauge plate to ship structures. In this study, to arrest a brittle crack at Tee joint embedding un-welded face, the effect of un-welded face on behavior of brittle crack propagation/arrest in Tee joint structure was investigated and analyzed. The ESSO test of Tee joint structure was carried out, supposing that a brittle crack which propagates along a weld joint of hatch coaming rush into Tee joint of hatch coaming and strength deck. The test results exhibited that the brittle crack arrested at the Tee joint embedding un-welded face, and the brittle crack arrested easier, the longer un-welded face was. The results of static FEM analysis showed that stress intensity factor of brittle crack was increased by un-welded face, until brittle crack reached a flange. However, when brittle crack propagate into the flange, stress intensity factor of brittle crack was decreased by un-welded face for instance. The crack-arrest effect of un-welded face appears remarkably after brittle crack propagates into the flange.
Application of aluminum alloy which is a typical lightweight material has been expected to achieve energy saving and prevention of pollution in many kinds of transportation vehicles. While the structure made of whole aluminum alloy, however, is lightweight, it still has problems, such as low mechanical strength and high cost. Hence, hybrid or joining structure made of aluminum alloy and steel seems to be reasonable because of its lightweight and higher strength. To make a hybrid structure for transportation vehicles, we examined welding by friction stirring between aluminum alloy and low carbon steel, which was welding without melted weld materials. As a result, welding between aluminum alloy and low carbon steel that had thin intermetallic compound at weld interface was obtained. In recent automobile manufacturing, zinc coated steel has been used as structural parts in general. On the welding between zinc coated steel and other materials such as aluminum alloy, existence of Zn layer between aluminum and steel have to be taken into account, to get high quality joint between the materials. In this study, the spot joining between aluminum alloy and several kind of zinc coated steels by friction stirring was carried out, and the effect of coated layer both on the weld strength and weld interface microstructure was investigated. As a results, joint between aluminum alloy and zinc coated steel was stronger than that between aluminum alloy and non coated steel, when coated layer was removed at weld interface by plastic flow of aluminum alloy.
Steel scrap is raw materials of the recycled steels refined in electric arc furnaces, and this recycle process is good for keeping our living environment. Authors focus in the toughness of the recycled steels that contain much tramp elements and nitrogen. Chemical compositions of specimens are 0.17C-0.20Si-1.15Mn,Cu : 0.08–0.50mass%,Ti : 0.006–0.031mass%. This report contains properties of HAZs (heat affected zones) in thermal cycled mother steels, and these HAZs were estimated a heat input of 45-600 kJ/cm. The following experimental results are obtained. TiN particles fasten on γ grain boundaries, and improve the toughness of HAZs. A grain boundary ferrite is restrained by Cu, and ferrite grains become fine, too.
A welding distortion prediction method based on inherent strain concept was presented. In the proposed method, welding distortion of large welded structures could be estimated by elastic analysis using the result of thermal-elastic-plastic analysis result of smaller welded joints or components. Thermal-elastic-plastic analysis is performed to calculate residual plastic strain distribution, which is the input data for the elastic analysis of welding distortion. The obtained residual plastic strain distribution is mapped to non-deformed finite element models to calculate welding distortion by elastic analysis. The mapping procedure is done in different ways for welding start/end parts and the rest of weld length in order to take into consideration of unsteady strain distribution at start/end of welds. For start/end parts, strain distribution used is identical with thermal-elastic-plastic analysis. For the part except start/end parts, strain distribution obtained by thermal-elastic-plastic analysis is extracted from the center of weld length and is extruded along welding direction. The proposed method was applied to the welding distortion prediction of joints with weld length 900 mm and 1200 mm based on thermal-elastic-plastic analysis result of a joint with weld length 600 mm. The estimated results were in good agreement with the thermal-elastic-plastic analysis results of models of corresponding weld length to show the validity of the proposed method.
The microstructures of an Inconel 718 alloy subjected to rapid thermal and stress cycles have been investigated to explain those observed in the friction welded joint of the alloy. The thermal and stress cycles were simulated with a Gleeble thermal and mechanical simulator. It turned out that the microstructural changes caused by the rapid heating cycle to peak temperatures of 1253-1553 K were almost in accordance with those reported in previous papers about the solidification process of the alloy and phase diagram of Inconel 718 calculated by Thermo-Calc: (1) dissolution of film carbide and needle precipitates of δ phase at grain boundaries at temperatures from 1253 to 1373 K, (2) dissolution of massive carbide (NbC) at 1373-1533 K, (3) liquation due to the eutectic reaction between γphase and NbC at 1443-1533 K, and (4) melting of γphase at 1553 K or higher. In the specimens heated to peak temperatures above 1473 K, γ/Laves eutectic were formed which were not detected in the as-received base metal. When a pressure of 350 MPa was applied during the heating cycle, most of liquid phase were expelled into the flash, leaving fine grain zone involving liquid phase areas much narrower than those observed when the pressure was not applied. When the pressure was applied to a specimen involving no liquation areas (peak temperature < 1443 K), a microstructure characterized by jaggy grain boundaries was found at peak temperature below 1253 K and a microstructure that consisted of coarse grain and fine grain distributing along the grain boundary region of the coarse one was found at peak temperature above 1253 K, with the fine grain area increasing with peak temperature up to 1443 K.
Microstructures forming in the friction welding of Inconel 718 alloy have been investigated in order to understand phenomena occurring during the welding process and to determine the factor controlling the joint performance from a metallographic point of view. In the interfacial zone, liquation microstructures characterized by a eutectic structure consisting of γ and Laves phases, and Nb-rich microstructures along the grain boundary (Nb-rich G.B. microstructures) were observed, and their amounts increased with the friction time and pressure. The volume fractions of these liquation structures were greater in the flash than in the interfacial zone, suggesting that the liquid phase was preferentially expelled into the flash by friction pressure, compared with the solid phase. Since the liquid phase was rich in Nb, this preferential expulsion of the liquid phase caused the depletion of Nb, a major element for the precipitation hardening of the alloy. The depletion of Nb brought about a decrease in the hardness in the interfacial zone after a post weld heat treatment for precipitation hardening. Thus, although the friction bonding is a solid state welding process, the liquation occurs in the weld of Inconel 718 alloy having a wide solid-liquid coexisting temperature range, and has a significant influence on the joint performance.
The influences of welding parameters on tensile properties of friction-welded joints of Inconel 718 alloy (subjected to a post weld heat treatment consisting of a solution treatment at 1253 K, and double aging treatments at 993K and 893K) have been investigated to reveal the controlling factor of the joint performance. All joints obtained were fractured near the bond interface at smaller elongations and area reductions than the base metal on tensile tests, although most of them showed tensile strengths comparable with that of the base metal. The observations of fractured surface and its cross-sectional microstructure suggested that an interfacial fine grain zone including numerous fine Laves phase particles 30-100 nm in size was responsible for a low ductility fracture at shorter friction time and lower friction pressure. As the friction time and pressure were increased, the fine grain zone was disappeared, and a reduction in hardness near the bond interface became significant, causing a rather ductile fracture near the bond interface. With an increase in friction time, coarse Laves phase particles a few μm in size remaining near the bond interface increased, and they acted as a crack nucleation site of ductile fracture. An increase in the solution treatment temperature during the post weld heat treatment enhanced the dissolution of the coarse Laves phase in the low-hardness region, and enabled us to obtain joints that were free from unacceptable grain growth and fractured in the base metal at a solution treatment temperature of 1323 K.
Dissimilar metals joints of Zn-coated steel (GA-steel) and commercially available pure aluminum (A1050) sheets were produced by changing the laser power and the roller pressure by the laser pressure welding method. In this method, the YAG laser beam was irradiated into a flare groove made by these dissimilar metals sheets. In addition, the laser beam was scanned at various frequencies and patterns through the fθ lens using two dimensional scanning mirrors. Then the sheets were pressed by the pressure rolls to be joined. The compound layers in the weld interface were observed by optical microscope, and the layer thicknesses were measured. The thicknesses were in the range of 7 to 20 μm. The mechanical properties of welded joints were evaluated by the tensile shear test and the peel test. In the tensile shear test, the strengths of the joints produced under the most welding conditions were so high that the fracture occurred through the base aluminum sheet. In the peel test of the specimens subjected to the laser beam of 1200 to 1400 W power under the roller pressure of 2.94 kN, the specimen fracture took place in the base aluminum sheet. Even if the compound layer was thick, high joint strength was obtained. On the other hand, the specimen fractured in the weld interface at the laser power of 1500 W. The results of XRD on the peel test specimen surface identified that the intermetallic compound on the GA steel side was Fe2Al5Zn0.4. Moreover, the aluminum parts adhering to the GA steel side were confirmed. These results suggest that the fracture in the peel test occurred between the compound layer and A1050 and partly in the base aluminum. Micro-Vickers hardness test was performed to examine the hardness distribution in the compound layer. The hardness values near A1050 and GA-steel were about 100 and 470 Hv, respectively. It suggests that the compound layer should not necessarily consist of brittle intermetallic compounds. It is therefore concluded that the laser pressure welding could produce high strength joints of GA-steel and A1050 dissimilar materials.
The laser pressure welding was conducted by changing the laser power and the roller pressure in the previous experiment. It was revealed that dissimilar metals welding of GA steel and pure aluminum was feasible in a wide range of welding conditions. When the roller pressure was more than 1.96 kN at the laser powers equal to or less than 1400W, the joint strengths were so high that the specimens in the tensile shear and the peel tests fractured in the A1050 parent metal. In order to know the reason for such high strength of joints with thick compound layers and the joining mechanism, the compound layer was observed by the HR-TEM. The TEM observation results revealed that the main phase in the compound layer was the solid solution of Al+Zn. Moreover, the intermetallic compound was identified as FeAl, Fe2Al5, Fe4Al13 and Fe2Al5Zn0.4 phase by electron diffraction. The Fe3Zn10 (Γ phase) of Fe-Zn intermetallic compound was confirmed on a Fe base material. It is guessed that the joining areas were heated in a range of 782°C more than 665°C, a melting point of the Al, by laser irradiation because the δ1K phase aspect was not confirmed. Because the surfaces of A1050 and Zn plated layer were melted thinly, the layer was over 10μm thicker. The reason for the production of high strength joints with the relatively thick intermetallic compound layer was attributed to the formation of (Al + Zn) phase with finely dispersed intermetallic compounds.
The multilayer welding is considered as accumulation of two pass welding, and it was clarified that the basic factors of welding deformation produced by two pass welding. From the comparison between the experimental results and numerical analysis, it was found that the numerical analysis method used in this research was very accurate to predict the welding deformation. Also, the influences of the welding heat input and the distance between two passes on the welding deformation were examined by using the numerical analyses. As a result, it was found that the longitudinal shrinkage made from each pass was decided by the inherent strain distribution parallel to the weld line. The inherent strain distribution parallel to the weld line after two pass welding was larger value at one pass or two pass welding. On the other hand, it was revealed that transverse shrinkage made from each pass was decided by the integration of inherent strain distribution perpendicular to the weld line. In the case of the inherent strain distribution perpendicular to the weld line after two pass welding, it was clarified that the inherent strain was almost same as the sum of inherent strain distribution perpendicular to the weld line made from each pass.
The sealing of pores by a veined oxide in a thermal sprayed alumina coating with some kinds of metal particle was investigated by heating in air. Also, the oxide grew from the metallic substrate into an artificial pore in the alumina block was observed after heating in air. The sealing of pores arose by the spinel oxide in the alumina coating containing Ni, Co or NiCrAlY particles after heating at 1173K for 100h in air. The oxide grew from Ni substrate into the artificial pore in alumina block after heating at 1273K for 100h in air. The oxide formed from the Ni substrate in the pore consisted of NiO (core) and NiAl2O4 spinel oxides. Also, the NiAl2O4 spinel formed from the top of the grown oxide on the surface of the pore. The spinel oxide preferentially forms on the surface of the pore and the primary oxide of the metal occupies the remaining space in the pore.