Fig. 1 — Variation in thermal gradient G, solidification growth rate R, and corresponding grain substructure along solidification front of GTA weld pool (top-sectional view). mentioned TiB2 and Al3Ti particles are known to need a low undercooling (< 1 K) to get activated in comparison to other particles, which is a further explanation for their effectiveness (Refs. 24, 28, 30, 31). Several analytical approaches were developed that describe the influence of the chemical composition on the final grain size of castings (Refs. 32–36). These approaches were applied elsewhere to weld metal microstructure (Ref. 37). Besides the presence of effective nucleant particles and alloying elements, the third control variable regarding the weld microstructure are the solidification conditions. In arc welds, these conditions are controlled, in addition to weld geometry, by the welding parameters arc current, arc voltage, and welding speed. These parameters control the solidification parameters in the weld metal, where • Thermal G (in K mm–1) gradient (local) • Solidification R (in mm s–1) growth rate are particularly important. It is of note that these solidification parameters and corresponding microstructures vary widely within the weld metal. Figure 1 shows the weld pool boundary of an arc weld (seen from above) where the welding direction is to the left. At the weld interface, the weld pool is in direct contact with the “cold” base metal, which causes high heat extraction and thus high G values. At the centerline, the just-solidified material extracts less heat, resulting in a minimum in G. As a consequence, the weld pool shape can vary from circular or elliptical (at low welding speed v, as shown in Fig. 1) to teardrop shaped (high v). Also, Fig. 1 illustrates that the grain substructure usually grows nearly parallel to the maximum temperature gradient that is perpendicular to the advancing weld pool boundary (Ref. 38). If one assumes that the dendrite solidification velocity corresponds to the solidification growth rate R due to competitive growth (Ref. 38), Rcan be approximated for the weld pool surface with Equation 1 where α is the angle between the directions of welding speed v and R at a particular point at the solid-liquid interface (Fig. 1). Thus, it becomes clear that R is zero at the weld interface and maximum (R = v) at the centerline. R = v•cos (α) (1) The large variation in both G and R along the pool boundary is often expressed as G/R (Refs. 39, 40) and has a significant influence on nucleation and grain growth. It was suggested that the extent of constitutional undercooling is inversely proportional to G/R0.5 (Ref. 39). Thus, high G/R values can be related to little constitutional undercooling ahead of the solid-liquid interface (Ref. 41) that favors planar or cellular growth (Ref. 42). Low G/R values, however, result in a large zone of constitutional undercooling (Ref. 41), which allows columnar dendritic, dendritic, or (at low G/R values) equiaxed dendritic structure to form (Ref. 42). As a consequence, one usually finds, dependent upon alloy content and welding conditions, two main grain morphologies: Columnar grains (with columnar dendritic or dendritic substructure) next to the weld interface and equiaxed grains (with equiaxed dendritic substructure) at the centerline — Fig. 1. This columnar to equiaxed transition (CET) is often observed in aluminum weld metal (Refs. 37, 43, 44). Large, columnar grains provoke anisotropic mechanical properties of the weld and facilitate the propagation of solidification cracks (Ref. 38). Consequently, it is of interest to know how to limit columnar grain growth and facilitate equiaxed grain growth, dependent upon nucleant particles, alloy composition, and welding conditions. This paper presents results from gas tungsten arc welding (GTAW) three aluminum alloys where welding speed and grain refiner additions were varied to investigate their influence on microstructure and nucleant particles. The second part of this study (Ref. 45) deals with an extensive thermal analysis that reveals the thermal conditions in the weld metal dependent upon welding speed. This data is used at the end in an analytical model to predict critical conditions for the prevention of columnar grain growth. Experimental Materials and Welding Conditions In this study, the wrought base metals used were Alloy 1050A (Al 99.5, temper H14), Alloy 6082 (Al Si1MgMn, temper T6), known for applications in the automotive industry or plant construction, and Alloy 5083 (Al Mg4.5Mn0.7, temper H111), FEBRUARY 2014, VOL. 93 54-s WELDING RESEARCH Fig. 2 — GTAW setup showing weld coupon and cast insert (dimensions in mm). Table 1 — Chemical Composition of Three Base Metals and the Grain Refiner (Al Ti5B1) as Measured by Optical Emission Spectrometer Chemical Composition in wt-% Alloy Si Fe Cu Mn Mg Cr Ni Zn Ti B V Zr Al 1050A 0.09 0.24 0.01 0.004 0.001 0.001 0.004 0.01 0.008 0.0003 0.01 0.001 Bal. (Al 99.5) 6082 0.86 0.42 0.09 0.43 0.75 0.06 0.01 0.07 0.032 0.0001 0.01 0.003 Bal. (Al Si1MgMn) 5083 0.25 0.40 0.07 0.58 4.57 0.09 0.01 0.07 0.027 0.002 0.006 0.002 Bal. (Al Mg4.5Mn0.7) Al Ti5B1 0.06 0.11 — — — — — — 4.98 0.99 0.02 — Bal.
Welding Journal | February 2014
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