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.

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 Si1MgMn). 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.

One important aspect of a fusion weld is the weld metal microstructure. On the one side, the microstructure has a significant influence on the mechanical properties of the weld. For instance, several studies have shown that small, equiaxed grains (instead of large, columnar grains) can improve weld properties such as strength, ductility, and toughness (Refs. 1–4). On the other side, weld metal grain refinement is an important possibility to reduce the susceptibility to solidification cracking in aluminum welds (Refs. 5–8). Small, equiaxed weld metal grains can be achieved through the addition of a commercial grain refiner to the filler metal (Refs. 3, 7, 9). Such master alloys usually consist of the systems Al-Ti, Al-Ti-B, Al-TiC, or Al-B (Ref. 10), where Al-Ti-B master alloys are considered to be more efficient than Al-Ti or Al-B alloys (Ref. 11). Al Ti5B1 is one of the most important grain refiners (Ref. 12). Titanium and boron form particles such as TiB2 (Ref. 13) and Al3Ti (Ref. 14) that act during the solidification of the weld pool as heterogeneous solidification nuclei for aluminum grains. TiB2 particles were found in castings at the center of Al grains (Refs. 15, 16) where they nucleate aluminum grains (Ref. 17). In contrast, it was argued that Al3Ti is a more potent nucleant than TiB2 (Refs. 18, 19) because of the low atomic lattice mismatch between Al3Ti and α-Al. Furthermore, Al3Ti has more atomic planes that can nucleate aluminum grains than TiB2 (Refs. 15, 20). Others argued that, regarding Ti/B additions, AlB2 is the most efficient nucleus for Al, even though it dissolves quickly in the melt (Ref. 21). One widely accepted approach to explaining the exact role of each particle is the duplex nucleation theory (Refs. 20, 22, 23) that was developed from the peritectic theory (Ref. 14). The duplex nucleation theory suggests that the insolvable TiB2 particles are covered in liquid aluminum by a thin Al3Ti layer. Afterward, the peritectic reaction Al3Ti + AlL → AlS takes place on these particles. Accordingly, the Al3Ti layer reacts with liquid aluminum (AlL) to form a further layer of solid aluminum (AlS). This reaction converts such particles into efficient solidification nuclei for aluminum grains (Refs. 14, 20, 23). Properties that make the above particles favorable for nucleation of aluminum grains are, for example, their size and size distribution (Refs. 24, 25), and shape and atomic lattice (Ref. 26). Consequently, additions of grain refiners such as Al Ti5B1 to the aluminum weld pool can provide an increased number of active solidification nuclei and thus a fine, equiaxed weld metal grain structure. An important influence on nucleation, subsequent grain growth, and hence the resulting weld microstructure is the chemical composition of the weld metal. During solidification of the weld pool, the alloying elements partition in the melt and provide constitutional undercooling (Ref. 27), which is needed to activate the abovementioned particles for nucleation of Al grains (Ref. 28). Titanium is supposed to provide the highest degree of constitutional undercooling of all elements (Ref. 29). This explains why excess solute Ti (which is not tied up in particles) plays an important role in aluminum grain refinement and why commercial Al grain refiners usually contain Ti (Ref. 19). Furthermore, the aboveSolidification of GTA Aluminum Weld Metal: Part I – Grain Morphology Dependent upon Alloy Composition and Grain Refiner Content

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.

Grain refinement provides an important possibility to enhance the mechanical properties (e.g., strength and ductility) and the weldability (susceptibility to solidification cracking) of aluminum weld metal. In the current study, a filler metal consisting of aluminum base metal and different amounts of commercial grain refiner Al Ti5B1 was produced. The filler metal was then deposited in the base metal and fused in a GTA welding process. Additions of titanium and boron reduced the weld metal mean grain size considerably and resulted in a transition from columnar to equiaxed grain shape (CET). In commercial pure aluminum (Alloy 1050A), the grain-refining efficiency was higher than that in the Al alloys 6082 and 5083. Different welding and solidification parameters influenced the grain size response only slightly. Furthermore, the observed grain-size reduction was analyzed by means of the undercooling parameter P and the growth restriction parameter Q, which revealed the influence of solute elements and nucleant particles on grain size.

Gas-tungsten arc (GTA) welds were prepared on 3 mm thick sheets of continuous cast and rolled AZ31 magnesium alloy, welding over pre-placed die-cast inserts of magnesium alloys (AM20, AM50, AM60 and AZ91) in order to systematically vary the aluminum content in the weld metal. Microstructure and compositional analysis of the weld metal was examined using optical and scanning electron microscopy (HR-SEM, EBSD and EDS/X). The results showed that the average weld metal grain size decreased from 74 to 41 mu m as the aluminum content increased from 2.4 to 5.0 wt.%. This observed grain refinement is believed related to the increased constitutional undercooling associated with higher aluminum content. Weld strength and hardness, and to limited extent ductility, were also observed to increase with aluminum content, as determined from cross-weld tensile tests.

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 tensile-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. (C) 2012 Elsevier B.V. All rights reserved.

Grain refinement is an important possibility to enhance the weldability of aluminium weld metal that is usually defined by its susceptibility to solidification cracking. In this study, grain refinement was achieved through the addition of commercial grain refiner containing titanium and boron to the GTA weld metal of aluminium alloy 6082. The weld metal mean grain size could be reduced significantly from about 70 μm to a saturated size of 21 μm with a change in grain shape from columnar to equiaxed. The grain refinement prevented the formation of centreline solidification cracking that was present only in welds with unrefined grain structure. A variation of torch speed led to a strong change of solidification parameters such as cooling rate that was measured in the weld metal and the corresponding solidification rate and thermal gradient. The ratio thermal gradient/growth rate (G/R) decreased from 50 K s/mm 2 (high torch speed) to 10 K s/mm 2 (low torch speed). However, the variation of torch speed did not change the tendency for solidification cracking. The microstructure of unrefined and completely refined weld metal was compared. The observed change in size and distribution of the interdendritic phases was related to the change in susceptibility to solidification cracking.

The optimization of HIP processing of aluminum cladding for LEU-10Mo monolithic fuel plates is discussed. Canned HIP process optimization focuses on reduction of materials usage and processing effort, while improving the final HIPed product. Small scale formed sheet-steel HIP cans have been modeled, designed, and produced that significantly reduce production costs and resources. A new canless HIP approach to fuelfoil production is being developed at LANL, avoiding the need for protective stainless steel canisters. This process involves hermetically sealing the outer perimeter of individual fuel foils using electron beam welding. Details of this process and discussion of the challenges encountered will be given. Grain growth across the aluminum clad-clad interface is desirable. It was characterized using electron backscatter diffraction as a function of process modifications including macroscopic grooving to enhance Al flow, changing the cleaning method, and adding cold work. The results of these initial studies will be presented.

The current study has investigated the influence of alternating current pulsing on the structure and mechanical properties of AZ31 magnesium alloy gas tungsten arc (GTA) weldments. Autogenous full penetration bead-on-plate GTA welds were made under a variety of conditions including variable polarity (VP), variable polarity mixed (VPM), alternating current (AC), and alternating current pulsing (ACPC). AC pulsing resulted in significant refinement of weld metal when compared with the unpulsed conditions. AC pulsing leads to relatively finer and more equiaxed grain structure in GTA welds. In contrast, VP, VPM, and AC welding resulted in predominantly columnar grain structures. The reason for this grain refinement may be attributed to the periodic variations in temperature gradient and solidification rate associated with pulsing as well as weld pool oscillation observed in the ACPC welds. The observed grain refinement was shown to result in an appreciable increase in fusion zone hardness, tensile strength, and ductility.

Refinement of the weld metal grain structure can improve the mechanical properties of the weld and decrease the susceptibility to solidification cracking of the weld metal. In this study, commercial Al Ti5B1 grain refiner was used to refine the microstructure of LB (laser beam) and GTA (gas tungsten arc) aluminum welds by inoculation. The grain refiner additions led to a significant decrease in the weld metal mean grain size whereby a transition from columnar to equiaxed grain structure (Columnar to Equiaxed Transition, CET) was observed. The development of both grain size and shape depended upon the base metal (Al alloys 1050A, 5083 and 6082) and upon the welding process. The GTA welding process allowed a more pronounced and a more efficient refinement than in LB welds. Furthermore, the influence of the solidification conditions on the CET was investigated through temperature measurements in the weld metal. The temperature profiles revealed a faster solidification of LB welds than in GTA welds. The results from the temperature measurements were also used to estimate (according to an existing model) the critical thermal gradient at which the CET occurs.

The refinement of the weld metal grain 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 Ti5B1 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.

Since the advent of doublecarbon-coated silicon carbide fiber in 1982, the turbine engine industry has shown significant interest in fiber-reinforced titaniumbased (Ti, Ti,AI, TiAI) metal matrix composites (MMCs). This promising material system, with its extraordinary specific properties, has been adopted by a new generation of turbine engine designers striving for revolutionary advancements in turbine engine technology within the next 10 years.