Materials Science Forum Vols. 519-521

Abstract: One of the “show stoppers” in fusion welding of highly alloyed aerospace aluminium alloys is their susceptibility for liquation cracking in the weld heat-affected zone. Liquation cracking is a microscopic intergranular discontinuity, which occurs under the effect of welding thermal cycle and in the presence of stresses involved with the welding process. These intergranular discontinuities are often observed in welding of thick plates and extrusions, which usually have relatively coarse elongated grains, that are generally oriented parallel to each other. Friction Stir Welding (FSW) is a low temperature non- fusion process, which produces very fine equiaxed grain structure in the weld nugget for majority of Al-alloys. It was found that bead-onplate FS welds performed on alloy, which in fusion welding is susceptible to liquation cracking, were crack free. It was therefore proposed to use FSW for grain refining of the parent material by putting a number of overlapping FS welds onto the edges of both parent plates prior to joining by fusion welding. Experimentation has shown that there was no liquation cracking after the final weld was performed. This novel welding method has been successfully proven for Electron Beam Welding (EBW) of various Al-alloys including joining of dissimilar materials. The details of experiments as well as welded coupons test results are presented.

Abstract: The present study deals with the properties assessment of a new generation aluminium-copper alloy, developed by ALCAN and designed 2139, welded by the bobbin tool technique of the FSW process. Considering the best performances of this age formable alloy after T8 aging, both the T4 and T8 tempers prior to welding are assessed. Therefore, the behaviour under static and fatigue load conditions and also in damage tolerance including both fracture toughness and fatigue crack propagation of the FSW bobbin tool weld on 2139 after T8 post aging and on the T8 as welded condition are discussed. The corrosion resistance of the welds is investigated through an accelerated corrosion test with intent to evaluate the intergranular corrosion sensitivity of the weld and the impact of an artificial aging on the properties developed by the T8 as welded condition is also presented.

Abstract: Fusion welding of 7xxx aluminium alloy plates has been investigated for aerospace applications using autogenous laser welding and hybrid laser-MIG welding. Nd:YAG and Yb-fibre lasers have been used, with two different focussed spot sizes in each case. Autogenous and hybrid welding of 12.7mm thick plate using the Yb-fibre laser with a 0.6mm diameter spot was selected for further development, on the basis of penetration and weld quality achieved. These welds were acceptable to the highest quality class B (stringent) of BS EN ISO 13919-2:2001, with a porosity of only 0.3% of the cross-sectional area of the weld, and close to class A of AWS D17.1. Transverse proof strengths of ~60% of parent material were achieved. Development of hybrid welding is ongoing with novel fillers to refine weld metal grain structure and improve weld properties.

Abstract: A new welding method, magnetic pressure seam welding, was used to lap join dissimilar metals (Al-Fe, Al-Ni and Al-Cu). The circuit for magnetic pressure seam welding consists of a capacitor, an electric discharge gap switch, and a plate-type coil. The overlapped metal plates are placed over the coil. When an impulse current from an energy-storage capacitor bank passes through the coil, a high-density magnetic flux is suddenly generated around the coil. The generated high-density magnetic flux lines cross the end of the overlapped plates. Eddy currents are induced mainly inside the Al plate because it has a high electrical conductivity. Both the Joule heat generated in the plates and the magnetic pressure applied from the Al side promote the joining of the lapped plates. The welding is normally achieved within 10 μs. This results in very little microstructural change in the parent plates aside from the area around the weld interface. Strong lap joints were obtained for every metal combination and no tensile fracture took place in the weld region. A characteristic wavy morphology was observed at the weld interface. An intermediate phase layer was also observed at the weld interface. TEM observation revealed that the intermediate layer consisted of fine Al grains and intermetallic compound particles dispersed among the Al grains. The growth direction of the wave, the welding condition dependency of the wavelength and the amplitude of the interfacial wave were intensively investigated in order to clarify the welding mechanism of this method.

Abstract: From literature and own observations it is known that the clad and core alloys that make up aluminium brazing sheet can show severe interaction during the brazing cycle. This interaction leads to a complete re-distribution of elements, changing essential properties like strength and corrosion resistance. This interaction has been reported many times but up to present time no clear explanation is given why this interaction is actually occurring. There are a number of publications addressing the circumstances under which the interaction is more severe. Chemistry and low levels of strain applied before brazing have a significant influence on the severity of the interaction. As a yet possible mechanism behind the interaction Liquid Film Migration is mentioned. The observations done so far are in line with this described mechanism but no ultimate proof has been given so far. The question why the interaction takes place cannot be answered yet, clearly a change of free energy of the system is involved but the mechanism or mechanisms behind the change is unclear.

Abstract: To meet the requirements of weight-saving and low-cost production of components for future transport vehicles, the concept of multi-material mix is of increasing importance. In this context aluminum-iron compounds produced by means of compound casting are considered to be of particular importance. An essential and critical aspect of such compound castings is the formation of intermetallic phases (IMP) at the Al-Fe interface. Both the nature and the kinetics of potential IMPs are not well understood and require a systematic investigation. In this paper we document the interface formation of pure Al and binary Al-alloys on a mild steel substrate by means of isothermal wetting experiments. Tests were carried out employing the sessile droplet method in a controlled atmosphere. Using pure Al and Al7Si, Al7Cu, and Al7Zn alloys the interface reactions were investigated by quantitative metallography (LOM, SEM/EDX). Special attention was paid to the influence of the alloying elements on the type and sequence of IMPs at the interface.

Abstract: The effect of processing parameters on mechanical and microstructural properties of dissimilar AA6082-AA2024 joints produced by Friction Stir Welding was analysed in the present study. Different samples were produced by employing a fixed rotating speeds of 1600 RPM and by using the advancing speeds of the tool of 80 and 115 mm/min. All the welds were produced in direction perpendicular to the rolling one for both the alloys and by changing, for all the processing conditions, the alloy positioned on the advancing side of the tool. The mechanical properties of the joints were evaluated by room temperature tensile tests. Fatigue tests on the welds were carried out by using a resonant electro-mechanical testing machine under constant loading control up to 250 Hz sine wave loading. The fatigue tests were conducted in the axial total stress-amplitude control mode with R=smin/smax=0.1. The microstructural evolution of the material was analysed by optical observations of the welds cross sections and SEM observations of the fracture surfaces.

Abstract: The liquid film re-growth behaviour resulting from pulsed laser surface melting (LSM) has been investigated for typical 2xxx, and 7xxx aerospace alloys, both on parent plate and friction stir welded (FSW) joints. In Zr free alloys, as a result of the high growth rate and steep thermal gradient, the melted layer re-grew with a stable front, epitaxially, from the parent subsurface grains. This caused a thin coarse grained solidified layer to form over the parent material, thermomechanically affected zone (TMAZ) and heat affected zone (HAZ), and fine columnar grains to develop over the FSW nugget zone of the same order in width as the nugget grain size. In the case of the Zr containing alloys, a very fine columnar grain structure was found over the entire surface, independent of the subsurface grain structure. This has been shown to occur by growth selection from a band of nanoscale Al grains epitaxially nucleated on Al3Zr dispersoids, at the melt solid interface, that had not fully dissolved in the melt.

Abstract: Creep and creep rupture tests were carried out for friction-stir-welded (FSW) joints of 5052 aluminum alloy plates at temperatures between 573 and 723 K. The results were compared with those of the base metal. 5052-O plates of 20 mm in thickness were joined by FSW and round bar creep specimens were machined out of the welded plates. Tensile tests were also conducted at RT, 623 and 723K for both FSW joints and base metal. The tensile strength of the joints was almost the same as that of the base metal at room and elevated temperatures. However, the FSW joints showed appreciably higher minimum creep rate and shorter rupture time than the base metal at all the tested temperatures and initial creep stresses. Creep rupture of the joints always occurred within the plastically stirred zone with lower contraction of cross-sectional area. Thus, FSW joints of 5052 alloy plates showed lower creep strength than the base metal.

Abstract: The effect of friction stir welding (FSW) and post weld heat treatment (PWHT) on the second phase particle distribution and cross weld hardness profile in AA7449 plate has been investigated. The alloy was received in an underaged condition, welded, then PWHT to give an overaged condition (in the parent material) . The effect of this complex treatment on the precipitate distribution in the weld and parent plate has been investigated over a range of length scales using small angle X-ray scattering (SAXS), TEM and FEGSEM. It is shown that the PWHT does not improve the hardness in the heat affected zone (HAZ), which is the location of the strength minimum after welding, but it does reduce the difference between the hardness in the HAZ and the nugget and parent hardness. The reduction in nugget strength after PWHT is particularly marked and is due to replacement of fine GP zones formed on post weld natural ageing by coarse overaged precipitates.