Partially Melted Zone in Aluminum Welds — Liquation Mechanism and Directional Solidification
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WELDING RESEARCH SUPPLEMENT TO THE WELDING JOURNAL, MAY 2000 Sponsored by the American Welding Society and the Welding Research Council Partially Melted Zone in Aluminum Welds — Liquation Mechanism and Directional Solidification Liquation is initiated eutectically and intensified by melting above the eutectic temperature, and the resultant liquid solidifies upward and toward the weld regard- less of its position relative to the weld BY C. HUANG AND S. KOU ABSTRACT. Aluminum Alloy 2219 was Introduction Unlike steels or nickel-based superal- welded by gas metal arc welding and the loys, little, if any, has been reported about microstructure was examined in the par- Aluminum alloys tend to be suscepti- the mechanism of welding-induced GB tially melted zone (PMZ), which is a nar- ble to liquation along GBs during weld- liquation in wrought aluminum alloys or row region immediately outside the fu- ing in a very narrow region immediately about the GB microstructure after liqua- sion zone. Extensive liquation was outside the fusion zone called the par- tion. As aluminum alloys are gaining observed at three different locations: at tially melted zone (PMZ) (Ref. 1). Grain popularity (e.g., in the auto industry) it is large θ (Al2Cu) particles, along grain boundary liquation in aluminum welds essential to better understand the weld- boundaries (GBs) and at numerous iso- can have a serious consequence — it can ing of them. lated points within grains. Liquation was make the PMZ susceptible to hot crack- initiated at the eutectic temperature TE, ing (intergranular) during welding or Experimental Procedure by the eutectic reaction α + θ→LE and ductility loss after welding. Liquated GBs intensified by further melting, above TE, are obviously weak and can be torn by The workpiece was Alloy 2219, a of the α matrix surrounding the eutectic tensile stresses induced during welding. high-strength aluminum alloy often used liquid (LE). The microstructure of the li- Most studies on PMZ liquation in alu- for aerospace applications. The actual quated-and-solidified GB material is in- minum welds focused on the susceptibil- composition of the workpiece was Al- triguing. First, the material consisted of a ity to hot cracking during welding (Refs. 6.33%Cu-0.34%Mn-0.13%Fe-0.12%Zr- new GB of mostly thin, divorced eutectic 2–7). However, even if hot cracking is 0.07%V-0.06%Si-0.04%Ti-0.02%Zn by and a eutectic-free strip of α immediately avoided during welding, the PMZ can weight. It was selected because it is es- next to it. Second, within an individual still be susceptible to ductility loss after sentially a binary alloy of Al-6.3wt-%Cu grain, the strip was along the top and the welding, as observed in tensile testing of and its microstructure is, therefore, fairly side facing the weld. Third, with respect the resultant welds (Refs. 8–10). easy to understand. The dimensions of to the weld, the strip was always behind the workpiece were 20 cm by 10 cm by the new GB. These three characteristics 6.4 mm. It was welded in the as-received point to an important phenomenon, that condition of T851. T8 stands for solution is, solidification of the liquated GB is di- heat treating, cold working and followed rectional — upward and toward the by artificially aging, and T51 stands for weld, as a result of the temperature gra- KEY WORDS stress relieving by stretching (Ref. 11). dients across the PMZ. A thin, brittle eu- Two bead-on-plate welds were made tectic GB and a soft ductile α strip side by Aluminum Alloys in the same workpiece by gas metal arc side are expected to be much weaker Grain Boundaries welding (GMAW), one perpendicular to than a normal GB before welding. Gas Metal Arc Welding the rolling direction and the other paral- Liquation lel. The welding parameters were 6.35 Eutectic mm/s (15 in./min) welding speed, 25.5 RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT C. HUANG and S. KOU are with the Depart- Partially Melted Zone (PMZ) V arc voltage, 190 A average current and ment of Materials Science and Engineering, argon shielding. The filler metal was an University of Wisconsin, Madison, Wis. Alloy 2319 wire of 1.2-mm diameter. Its actual composition was Al-6.3%Cu- WELDING RESEARCH SUPPLEMENT | 113-s rolling direction Fig. 2 — Partially melted zone in a GMA weld in 2219 aluminum alloy. Fig. 1 — Al-Cu phase diagram (Ref. 12). eutectic also decomposes into θ particles and α, but this occurs much more rapidly in view of the smaller size and hence 0.3%Mn-0.18%Zr-0.15%Ti-0.15%Fe- Base Metal shorter distance required for diffusion. 0.10%V-0.10%Si, which is higher in Zr The presence of the large θ particles is an and Ti than Alloy 2219. The wire feed A scanning electron micrograph of indication that the much smaller eutectic speed was 13.5 cm/s (320 in./min). the base metal is shown in Fig. 3A. Large particles along GBs and within grains After welding, the microstructure near particles are present both within grains have already decomposed into θ and α. the weld was examined by optical mi- and at GBs. Electron probe microanaly- croscopy and by scanning electron mi- sis (EPMA-WDS) indicates the Al/Cu Liquation at Large θ Particles croscopy with a secondary electron weight ratio (e.g., 53/46) of these parti- image. Several etching solutions, includ- cles is close to that of about 53/47 for θ A scanning electron micrograph of ing Keller’s, were tried, and the solution (Al2Cu) — Fig. 1. As an approximation, the PMZ is shown in Fig. 3B. The two of 0.5 vol-% HF in water was found most these particles will be considered as the large particles within the grains do not satisfactory. θ phase even though they may contain look like the large θ particles within the very small amounts of other elements as grains of the base metal — Fig. 3A. well. As already mentioned, Alloy 2219 Rather, their composite-like structure in- Results and Discussion will be considered as a binary alloy of Al- dicates they are eutectic. This suggests 6.3% Cu as an approximation. Figure 3A that in the PMZ the large θ particles Overview of Partially Melted Zone also shows several small θ particles within grains react with the surrounding within grains. α matrix to become liquid, which upon For convenience of discussion, the The small particles along the GBs are solidification forms large eutectic parti- aluminum-rich portion of the Al-Cu believed to be the θ phase also, although cles within grains. In other words, liqua- phase diagram (Ref. 12) is shown in Fig. they are too small to be analyzed by tion occurs at large θ particles in the PMZ 1. The big gap between the solidus line EPMA. It is not clear why the GBs are not by the eutectic reaction α + θ→LE, and the liquidus line indicates the Cu fully loaded with these small particles. where LE is eutectic liquid. content of the α phase (Al-rich solid) is The GBs do not look much different Another scanning electron micro- much lower than that of the liquid. Since without etching. graph of the PMZ is shown in Fig. 3C. the 6.3% Cu content is about 20 times The eutectic liquid during the termi- Large eutectic particles are present both higher, or more than the content of any nal stage of solidification in ingot casting within grains and at GBs, just like the other alloying element, Alloy 2219 can solidifies and forms large and small eu- large θ particles before welding — Fig. be considered as a binary alloy of Al- tectic particles, along GBs and within 3A. This again suggests that in the PMZ 6.3% Cu as an approximation. grains. The solution heat-treating tem- the large θ particles react eutectically Figure 2 is an optical micrograph perature for Alloy 2219 is 535°C (Ref. with the surrounding α matrix to become showing an overview of the PMZ. The 13). From the phase diagram (Fig. 1), the liquid and form large eutectic particles PMZ includes the region in which the base metal is expected to consist of a θ upon solidification. GBs appear lighter in color. According to matrix plus additional undissolved θ Constitutional liquation was first dis- the Al-Cu phase diagram (Fig. 1), the li- (Al2Cu) particles (Ref. 14). During solu- covered by Pepe and Savage (Refs. 15, quation zone is in the narrow region im- tion heat treating of the ingot, the large 16) in Maraging steel and later observed mediately outside the fusion zone, eutectic islands decompose into large θ in Ni-based superalloys (Refs. 17–22) as where the maximum temperature expe- particles and α, which is connected to well. This constitutional liquation and rienced during welding ranges from the and hence indistinguishable from the α the liquation in the 2219 aluminum RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT liquidus temperature of about 642°C on matrix. During rolling of the ingot into welds are both initiated at the eutectic the fusion zone side (right) to the eutec- plates or sheets, some of the large θ par- temperature. They, however, differ from tic temperature of 548°C on the base ticles are displaced or even fractured. each other significantly in the following metal side (left). Like the large eutectic particles, the GB way. As pointed out by Pepe and Savage 114-s | MAY 2000 (Refs.