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Solidification of GTA aluminum weld metal: Part 1 - Grain morphology dependent upon alloy composition and grain refiner content

2014 , Schempp, P. , Cross, C.E. , Pittner, A. , Oder, G. , Neumann, R.S. , Rooch, H. , Dörfel, I. , Österle, W. , Rethmeier, M.

The solidification conditions during welding strongly influence the weld metal microstructure and mechanical properties of a weld. In the first part of this study, the grain morphology of gas tungsten arc (GTA) bead-on-plate welds was investigated for the aluminum Alloys 1050A (Al 99.5), 6082 (Al Si1MgMn), and 5083 (Al Mg4,5Mn0.7). The experiments revealed that increasing welding speed and alloy content allow the growth of small, equiaxed grains, particularly in the weld center. Furthermore, increasing grain refiner additions led to a strong reduction of the weld metal mean grain size and hence facilitated the columnar to equiaxed transition (CET). In addition, wavelength dispersive X-ray spectroscopy (WDS) and transmission electron microscopy (TEM) analysis revealed in the weld metal TiB2 particles that were surrounded by Al3Ti. This suggests the duplex nucleation theory for nucleation of aluminum grains in GTA weld metal.

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Characterization of microstructure and deformation behaviour of resistance spot welded AZ31 magnesium alloy

2012 , Babu, N.K. , Brauser, S. , Rethmeier, M. , Cross, C.E.

Resistance spot welds were prepared on 3. mm thick sheets of continuous cast and rolled AZ31 magnesium alloy. The microstructure and composition analysis of weld nugget, heat affected zone (HAZ) and base metal were examined using optical and scanning electron microscopy (HR-SEM and EDS/X). The resistance spot welded magnesium alloy joints consist mainly of weld nugget and HAZ. The nugget contains two different structures, i.e. the cellular-dendritic structure at the edge of the nugget and the equiaxed dendritic structure in the centre of the nugget. The structure transition is attributed to the changes of solidification conditions. In the HAZ, grain boundary melting occurred and grain boundaries became coarse. It has been shown that hardness reduction in the weld nugget and HAZ compared with base metal is evident due to dendritic microstructure and grain growth, respectively. The results showed that spot welded joints have failed in interfacial mode under torsion and te nsile-shear loading conditions. Digital image correlation during tensile-shear testing showed that low surface strains occur in the interfacial failure mode, because fracture and deformation happened primarily in the nugget area.

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Solidification of GTA aluminum weld metal: Part 2 - Thermal conditions and model for columnar-to-equiaxed transition

2014 , Schempp, P. , Cross, C.E. , Pittner, A. , Rethmeier, M.

In this study, the influence of solute content and heat input on microstructure was investigated for gas tungsten arc (GTA) bead-on-plate welding of the aluminum alloys 1050A (Al 99.5) and 6082 (Al SilMgMn). Temperature measurements in the solidifying weld pool showed that parameters such as solidification growth rate, cooling rate, local thermal gradient, and solidification time vary significantly along the solidification front (between weld centerline and weld interface). As a result, the obtained thermal data were used to explain the corresponding grain morphology from the first part of this study. On the basis of this comparison, an analytical approach was used to model the transition from columnar-to-equiaxed grain growth (CET). This model allows the prediction of critical values for both solidification growth rate and thermal gradient, at which the CET occurs.

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Weld metal grain refinement of aluminium alloy 5083 through controlled additions of Ti and B

2011 , Schempp, P. , Schwenk, C. , Rethmeier, M. , Cross, C.E.

The refinement of the weld metal gram structure may lead to a significant change in its mechanical properties and in the weldability of the base metal. One possibility to achieve weld metal grain refinement is the inoculation of the weld pool. In this study, it is shown how additions of titanium and boron influence the weld metal grain structure of GTA welds of the aluminium alloy 5083 (Al Mg4.5Mn0.7). For this purpose, inserts consisting of base metal and additions of the master alloy Al Ti5Bl have been cast, deposited in the base metal and fused in a GTA welding process. The increase of the Ti and B content led to a significant decrease of the weld metal mean grain size and to a change in grain shape. The results provide a basis for a more precise definition of the chemical composition of commercial filler wires and rods for aluminium arc welding.

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Influence of grain size on mechanical properties of aluminium GTA weld metal

2013 , Schempp, P. , Cross, C.E. , Häcker, R. , Pittner, A. , Rethmeier, M.

Grain refinement is an important possibility to enhance the mechanical properties such as strength, ductility and toughness of aluminium weld metal. In this study, grain refinement was achieved through the addition of commercial grain refiner Al Ti5B1 to gas tungsten arc weld metal of the aluminium alloys 1050A (Al 99.5) and 5083 (Al Mg4.5Mn0.7). The grain refiner additions led to a significant reduction of the weld metal mean grain size (Alloy 1050A, 86 %; Alloy 5083, 44 %) with a change in grain shape from columnar to equiaxed. Tensile tests showed for Alloy 5083 that the weld metal's ductility can be increased through grain refinement. No improvement in weld metal strength (i.e. yield strength and ultimate tensile strength) was observed. Furthermore, tear tests with notched specimens revealed that the resistance against initiation and propagation of cracks in the weld metal can be enhanced through grain refinement. The toughness was observed to increase clearly by grain refinement in weld metal of commercial pure Al (Alloy 1050A). In Alloy 5083 weld metal, the toughness was not improved through grain refinement, likely because of a semi-continuous network of brittle intermetallic phases that facilitate crack propagation.