tents (Alloy 6082 and particularly 5083), the undercooling provided by the alloying elements was sufficiently high to activate particles present for equiaxed nucleation. The above experimental results confirm this suggestion because the texture formation was most pronounced for Alloy 1050A and not present in Alloy 5083 welds, with Alloy 6082 in between. In addition, welding speed did not show any influence on the development of the observed crystallographic texture, even though a variation of welding speed came along with major changes in heat input and thus solidification conditions, as shown in the second part of this study (Ref. 45). Also, the weld pool shape changed significantly with increasing welding speed (Fig. 3). Instead, the chemical composition and corresponding promotion of constitutional undercooling seem to be the key factors regarding the texture formation. Decreasing alloy content (from Alloy 5083 to 6082 and to 1050A) and thus decreasing undercooling increased the tendency for epitaxial nucleation. Particle Size, Distribution, and Composition This study and other results (Refs. 9, 37, 55) clearly show that grain refiner additions to the weld metal can significantly decrease the weld metal mean grain size. After focusing on the influencing factors alloy composition and solidification conditions, it is of note to consider the potential nucleant particles in the weld metal. It is clear that Al Ti5B1 grain refiner additions introduce insoluble TiB2 particles and soluble Al3Ti particles to the weld pool. Some amount of such Al3Ti particles was expected to dissolve during welding and provide solute Ti and hence constitutional undercooling, dependent upon welding conditions. It is, however, not known how much Al3Ti was dissolved exactly since Al3Ti was found by a WDS analysis in 6082 weld metal (Fig. 7). These three WDS images show the titanium concentration and distribution from three different welds — low Al Ti5B1 additions and large mean grain size (Fig. 7A) to high Al Ti5B1 additions and low mean grain size — Fig. 7C. It is of interest that high grain refiner addition levels (needed to achieve a minimum grain size) produced large Ti-rich agglomerates with a thickness up to 15 μm — Fig. 7C. These particles were determined with WDS to be certainly Al3Ti (Ref. 37), although the Ti content of this weld was below the Ti concentration above which Al3Ti may form (0.15 wt-%) according to the equilibrium binary phase diagram of Al-Ti (Ref. 29). Al3Ti originates from the grain refiner and likely formed agglomerates at high grain refiner addition levels through collision upon entry to the weld pool. Al3Ti agglomerates of a similar size and shape are known from Al-Ti-B grain refiners (Ref. 10), and they were also observed in similar experiments with GTA weld metal that was inoculated by a Ti-bearing grain refiner (Ref. 56). Besides titanium, boron plays a key role in the grain refinement efficiency of Al Ti5B1 grain refiners (Refs. 19, 57). The WDS image in Fig. 8A shows both Ti and B distribution from the weld in Fig. 7C (high grain refiner content). The Ti-bearing particles in Fig. 8A are black, and the B-bearing particles are colored whereby the color scale indicates the B concentration. Figure 8A shows both Ti, and B- distribution. An important result from this analysis is that boron-rich particles were, in particular, found in the center of titanium rich particles. This observation supports the duplex nucleation theory, which suggests that TiB2 particles are covered with an Al3Ti layer that again nucleates α- aluminum (Refs. 20, 22, 23). Due to the TEM lamellae preparation technique, it is not known if these particles were in the center of Al grains or not. Further evidence for the duplex nucleation mechanism are the results from TEM analysis of 6082 weld metal, which revealed the size (about 1 m) and shape of two TiB2 particles — Fig. 8B and C. Interestingly, a thin Al3Ti layer was found on one of these two TiB2 particles — Fig. 8B. This suggests the B-rich particles from Fig. 8A to be TiB2 particles that are surrounded by an Al3Ti layer. The other TiB2 particle in Fig. 8C was covered partially by an intermetallic phase rich in Si and Fe, which is probably Al5FeSi or, due to the two-dimensional view, possibly Al8Fe2Si (Refs. 28, 58). These results are an important extension of former studies on aluminum GTAW that revealed Ti-rich particles (Ref. 48) and Al3Ti particles (Ref. 59) in the center of weld metal grains (Ref. 48). Furthermore, this study showed, on the basis of WDS and TEM analysis, that both TiB2 (Ref. 13) and Al3Ti (Ref. 14) are likely important particles for nucleation of aluminum grains in GTA weld metal. Moreover, the results suggest the duplex nucleation theory as main nucleation mechanism in aluminum weld metal that is refined with an Al Ti5B1 grain refiner. Conclusions The GTA bead-on-plate welding was accomplished with aluminum Alloys 1050A, 6082, and 5083. The influence of welding speed and grain refiner content on the weld metal grain morphology was investigated with the use of cast inserts that contained controlled amounts of commercial Al Ti5B1 grain refiner and were fused in the welding process. Increasing welding speeds from 2 to 11.5 mm s–1 revealed for the case of no grain refiner additions the following results: • Change of the weld pool shape from slightly elliptical to teardrop shaped. • Transition from predominantly columnar to predominantly equiaxed grain growth. • Increased tendency for equiaxed grain morphology with increasing alloy content. Increasing grain refiner contents facilitated predominantly equiaxed grain growth even at low addition levels (< 0.1 wt-% Ti). Furthermore, a crystallographic texture was observed in some welds, which was found to be caused by competitive growth during weld pool solidification. It was suggested that the corresponding nucleation mechanism is repeated epitaxial nucleation. The tendency for the formation of such a texture did not depend on the welding conditions but decreased strongly with increasing alloy content and grain refiner additions leading to the nucleation of equiaxed grains. The WDS and TEM analysis disclosed in Alloy 6082 weld metal TiB2 particles that were likely surrounded by Al3Ti. These results suggest the duplex nucleation theory for nucleation of aluminum grains in GTA weld metal that is refined with an Al Ti5B1 grain refiner. Furthermore, the following heterogeneous nucleation mechanisms are proposed: • For alloys with low alloy content — predominantly repeated epitaxial nucleation on existing grains. • For alloys with high alloy content and/or high grain refiner content — nucleation on Ti-bearing particles. Acknowledgments The authors are grateful to H. Hayen from Aljo Aluminum-Bau Jonuscheit GmbH, Germany, and P. Gudde from KBM Affilips B.V., Netherlands, for the kind donation of Alloy 5083 plates (Alijo) and grain refiner (KBM Affilips). They also would like to thank H. Strehlau (ICP-OES chemical analysis) and D. Köhler (casting of ingots) for their great support at BAM. In addition, the authors are thankful to the German Research Association on Welding and Allied Processes of the DVS for their support and the Program for Funding of Industrial Research and Technology (IGF) of the German Federal Ministry of Economics and Technology for funding the research project 16.242N. References 1. Schempp, P., Cross, C. E., Pittner, A., and Rethmeier, M. 2013. Influence of grain size on mechanical properties of aluminum GTA weld metal. Welding in the World 57(3): 293–304. 2. Arata, Y., Matsuda, F., Mukae, S., and Katoh, M. 1973. Effect of weld solidification mode on tensile properties of aluminum weld metal. Transactions of the JWRI 2: 55–61. FEBRUARY 2014, VOL. 93 58-s WELDING RESEARCH
Welding Journal | February 2014
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