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WELDING RESEARCH -S211WELDING JOURNAL ABSTRACT. The mechanisms by which li- quation is initiated in the partially melted zone

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Content	WELDING RESEARCH -S211WELDING JOURNAL ABSTRACT. The mechanisms by which li- quation is initiated in the partially melted zone of wrought, multicomponent alu- minum alloys during welding were studied using three representative liquation- susceptible alloys 2024, 6061 and 7075 as examples. Three different liquation mech- anisms were identified. In Mechanism I, which is for alloys beyond the solid solu- bility limit, liquation-inducing particles are always present and liquation can occur at any heating rate. In Mechanism II, for alloys within the limit but with the parti- cles, liquation requires high heating rates. In Mechanism III, for alloys within the limit and without the particles, liquation occurs when the matrix starts to melt. These three mechanisms cover most, if not all, wrought aluminum alloys since an alloy is either beyond or within the limit. Ternary phase diagrams were found a use- ful approximation for checking if the al- loys were within or beyond the limit. Alloy 7075, which contained liquation-inducing CuMgAl2particles and Cu2FeAl7coatings on Fe-rich particles, was well within the limit and it liquated by Mechanism II. Alloy 6061 was also within the limit, but the mechanism depended on whether the solution heat treatment of the heat was thorough enough to dissolve liquation- inducing Si-rich particles. If so, it liquated by Mechanism III; if not, by Mechanism II. Alloy 2024, which contained liquation- inducing CuAl2particles, was near the limit and liquated by Mechanism I if the heat was beyond the limit and by Mecha- nism II if within. The liquation reactions caused by these particles or coatings were identified. Liquation-induced grain boundary segregation was severe, suggest- ing severe degradation of mechanical properties, as demonstrated in binary alloy 2219. Introduction The partially melted zone (PMZ) is a region immediately outside the weld metal where liquation can occur during welding and lead to hot cracking and degradation of mechanical properties (Ref. 1). Since the 1950s, liquation and li- quation-induced hot cracking in alu- minum alloys have been studied exten- sively, and alloys 2024, 6061, and 7075 are among the most frequently investigated materials (Refs. 2–12). Attention appears to have been focused much more on li- quation-induced hot cracking than liqua- tion itself. Optical micrographs of the PMZ showed dark-etching grain bound- aries (GBs) as evidence of GB liquation, but without further microstructural de- tails. Liquation mechanisms were often not discussed, the solidification of GB liq- uid was not studied, and the resultant GB segregation was not measured. Recently, Huang and Kou (Refs. 13–15) studied PMZ liquation in GMA welds of alloy 2219. This was a binary alloy of about Al-6.3 wt-% Cu, with an Al-rich αmatrix and θ(CuAl2) particles. Large θ particles were present in the grain interior and sometimes at the GBs, and small θ particles were observed along the GB. To help discussion later in this report, the most significant results are briefly de- scribed below. First, liquation was initiated at the eu- tectic temperature by the eutectic reaction between the αmatrix and the θparticles to form the eutectic liquid. Unlike the constitutional liquation mechanism pro- posed by Pepe and Savage for maraging steels (Refs. 16, 17), rapid heating is not required for liquation to occur. This is be- cause in alloy 2219, the θparticles are thermodynamically stable all the way to the eutectic temperature. The closer to the fusion boundary, the higher the peak temperature and the more adjacent αma- trix dissolved in the eutectic liquid, and it turned it into a hypoeutectic liquid. Second, GB liquid solidified with a pla- nar mode and with severe solute segrega- tion. Besides a dark-etching eutectic GB, a distinct light-etching αband, in fact, ex- isted along the GB. This microstructure clearly suggests the hypoeutectic GB liq- uid solidified with a planar solidification front, first as a soft and ductile solute- depleted αband and last as a hard, brittle solute-rich eutectic at the new GB. This was confirmed by both microsegregation measurements and microhardness testing after welding. Third, solidification of GB liquid was directional — upward and toward the weld because of the high-temperature gra- dients in the PMZ. Fourth, while the soft ductile αband yielded under tensile loading, the brittle GB eutectic fractured into pieces, thus ex- plaining the dramatic strength and ductil- ity losses of the PMZ in tensile testing after welding. The liquation of the large θ particles in the grain interior and the sub- sequent solidification of the liquid re- sulted in large eutectic particles and an α ring surrounding each particle. While the ductile αrings yielded under tensile load- ing, the large brittle eutectic particles frac- tured into pieces just like the GB eutectic. Work on the binary alloy 2219 has greatly improved fundamental under- standing of liquation and solidification in the PMZ of aluminum welds. However, most commercial aluminum alloys are Liquation Mechanisms in Multicomponent Aluminum Alloys during Welding BY C. HUANG AND S. KOU Three mechanisms cover most, if not all, wrought aluminum alloys and, for a given alloy and temper, the mechanism can vary from heat to heat KEY WORDS Aluminum Alloys Grain Boundaries Hot Cracking Liquation SolidificationC. HUANG and S. KOU are respectively, Grad- uate Student and Professor in the Department of Materials Science and Engineering, University of Wisconsin-Madison, Wis.
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