A Hot-Cracking Mitigation Technique for Welding High-Strength Aluminum Alloy
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A Hot-Cracking Mitigation Technique for Welding High-Strength Aluminum Alloy The mechanical strain associated with hot cracking is significantly reduced with the introduction of a heat sink trailing a gas tungsten arc welding torch BY Y. P. YANG, P. DONG, J. ZHANG AND X. TIAN ABSTRACT. A hot-cracking mitigation results confirmed the effectiveness of the ture range BTR and to increase its ductil- technique for gas tungsten arc welding trailing heat sink technique. With a ity, one documented method is to intro- (GTAW) of high-strength aluminum alloy proper implementation of the trailing duce alloy elements, such as Ti, Zr, V and 2024 is presented. The proposed welding heat sink method, hot cracking can be B in the aluminum alloy electrode. In technique incorporates a trailing heat completely eliminated in welding alu- doing so, the grain structure of the weld sink (an intense cooling source) with re- minum alloy 2024 (AA 2024). metal can be refined, resulting in an im- spect to the welding torch. The develop- proved ductility and resistance to hot ment of the mitigation technique was Introduction cracking (Ref. 1). A variation of this ap- based on both detailed welding process proach is to control the solidification mi- simulation using advanced finite element Hot cracking has been a subject of in- crostructure of the weld metal by using techniques and systematic laboratory ex- tensive studies over the last few decades. special solidification techniques. For in- periments. Hot cracking occurs during the solidifi- stance, magnetic arc oscillation and The finite element methods were used cation process due to a combination of electromagnetic stirring have been used to investigate the detailed thermo metallurgical behavior on cooling and to refine weld microstructure and to mechanical behavior of the weld metal the surrounding thermomechanical con- change solidification orientation (Refs. that undergoes the brittle temperature ditions. In general, two basic approaches 2–6). The alternating columnar grains re- range (BTR) during welding. As ex- are usually taken: 1) improving the weld sulting from transverse arc oscillations at pected, a tensile deformation zone and heat-affected zone (HAZ) material a low frequency can be effective in re- within the material BTR region was iden- ductility and 2) improving the thermo- ducing solidification cracking. tified behind the weld pool under con- mechanical conditions during welding. However, it has been realized that for ventional GTA welding process condi- Historically, the majority of the research some aluminum alloys, such as 2024, the tions for the aluminum alloy studied. To work has been focused on the former, aforementioned techniques may not offer mitigate hot cracking, the tensile zone i.e., improving the weld and HAZ mate- satisfactory results, especially for elimi- behind the weld pool must be eliminated rial ductility. nating liquation cracking. Consequently, or reduced to a satisfactory level if the To reduce the weld material tempera- thermomechanical-based techniques weld metal hot ductility cannot be further have received an increasing attention improved. With detailed computational over the recent years. Zacharia (Ref. 7) modeling, it was found that by the intro- studied the relationship between the dy- duction of a trailing heat sink at some dis- namic stress distribution and the ob- tance behind the welding arc, the tensile served cracking behavior in a Sigmajig KEY WORDS strain rate with respect to temperature in test specimen, particularly near the trail- the zone encompassing the BTR region ing edge of the weld pool. Feng (Ref. 8) Weld Hot Cracking can be significantly reduced. A series of analyzed thermal and mechanical condi- Aluminum Alloy 2024 parametric studies were also conducted tions associated with weld metal solidifi- High-Strength Aluminum to derive optimal process parameters for cation cracking. As a better understand- Trailing Heat Sink the trailing heat sink. The experimental ing of the thermomechanical conditions Welding Process Model associated with a hot crack was estab- Finite Element Modeling lished, Karlsson (Ref. 9) proposed a local Y. P. YANG, P. DONG and J. ZHANG are with Gas Tungsten Arc Welding heating approach based on the charac- RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT the Center for Welded Structures Research, GTAW teristics of welding-induced stress distri- Battelle, Columbus, Ohio. X. TIAN is with bution in a large plate. It was postulated Harbin Institute of Technology, Harbin, P.R. that the local heating of the base metal on China. WELDING RESEARCH SUPPLEMENT | 9-s each side of the weld could be attributed to the fact the transverse and produce additional thermal longitudinal strain fields were altered by stresses to neutralize tensile the use of local heating. For instance, the stresses in the weld zone. The results obtained by Wu (Ref. 14) indicated local heating techniques were the tensile strain rate within the brittle also used by Sekiguchi and temperature range in the weld zone was Mayake (Refs. 10, 11) to over- reduced with local heating. However, the come end-cracking problems resulting large HAZ and coarser grain mi- in welding high-strength steel. crostructure as a result of local heating The end cracks were success- have been of concern over its applicabil- fully prevented by preheating ity in practice. It was also worth noting the panel to 120°C (248°F), local mechanical rolling could also be maintaining at a specified used to improve the thermomechanical inter-pass temperature of conditions in welding of some simple 200°C (392°F), and postweld specimens, as recently illustrated by Liu heating of 700°C (1292°F) with and Tian (Ref. 15). a local heating torch. Hernan- In the following, detailed finite ele- dez and North (Refs. 12, 13) ment results were presented to charac- used a pair of oxyacetylene terize the detailed evolution of instanta- heating torches to heat the base neous stress and strain behavior metal on either side to prevent associated with hot cracking. A novel Fig. 1— Thermomechanical conditions associated with solidification cracking of H30 welding technique with a trailing heat hot cracking (Ref. 15). aluminum alloy sheets. sink is then proposed to mitigate hot The mechanisms associated cracking during welding of high-strength with the local heating methods aluminum alloys. Parametric studies in preventing hot cracking can were conducted to investigate optimal A B Fig. 2 — Illustration of the effects of a trailing heat sink on isotherms. A — Conventional welding; B — welding with a trailing heat sink. A B RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT Fig. 3 — Welding conditions and specimen configuration. A — Conventional GTA welding; B — welding with a trailing heat sink. 10-s | JANUARY 2000 parameters for the trailing sink. Experi- mental studies were also carried out to validate the proposed welding technique on high-strength aluminum alloy 2024 panel specimens. Analysis Procedures Thermomechanical Conditions for Hot Cracking The necessary condition for hot crack- ing is the presence of tensile strains in the region that undergoes the brittle temper- ature range. Note in this study, mechani- cal strains were used as a measure of the driving force for hot cracking instead of stresses since at the BTR region the tran- sient stress level is usually low due to sig- nificantly reduced material yield strength at high temperature. As depicted in Fig. 1 (Ref. 15), if the tensile strain rate (with re- spect to temperature) exerting on the BTR region becomes smaller than “the critical Fig. 4 — Finite element model. strain rate for temperature drop (CST), as depicted by the tangent line (Curve B) to the ductility curve, or dε < CST dT (1) or in theory, hot cracking can be avoided, as depicted by line C. Decomposing dε/dT, one obtains the following: dεε∂∂/ t = dT ∂∂Tt/ (2) The first term ∂ε/∂t in Equation 2 becomes the standard expression of strain rate de- termined by the thermomechanical re- sponse during welding. The second term ∂T/∂t represents cooling rate typically controlled by the heat flow characteris- tics of the workpiece under considera- tion. For a given material, both the strain rate and the cooling rate can be altered by either modifying welding parameters or introducing local heating/cooling Fig. 5 — Temperature dependence of thermal physical properties for aluminum alloy 2024. mechanisms, or applying mechanical means. For instance, by introducing a trailing heat sink (Fig. 2), an auxiliary compression zone can be generated to temperature) should be obvious. As- moment in time becomes, within the trailing BTR region between suming a simple additive relationship be- RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT the heating source and the heat sink. The tween welding-induced strain (εw) and ε = εw – εc (3) effectiveness of the additional compres- the strain (εc) generated by the heat sink sive strains on the strain rate (with respect source, the total mechanical strain at any and Equation 1 takes the form of WELDING RESEARCH SUPPLEMENT | 11-s AB Fig. 6 — Temperature dependence of mechanical properties for AA 2024. A — Elastic modulus E, thermal expansion α and Poisson’s ratio ν; B — Yield stress σs and hardening modulus H. line were clamped out-of-plane) were imposed at clamp tightly by a hydraulic positions (Fig. 4) to simulate the clamp- keyboard jig to en- ing conditions during welding. sure uniformity of pressure in order to Moving Arc and Trailing Heat Sink simulate severe re- straint conditions in A moving heat source model was used production situa- as an ABAQUS user-subroutine tions, under which (*DFLUX) in the form of an equivalent hot cracking was typ- body heat flux generation. The energy- ically observed with density distribution of the moving arc the conventional was assumed as a Gaussian distribution GTA process. (Ref. 19) expressed by The commercial 2 22 3ηEI ()xvty− ++ z finite element pack- q = 2 exp 2 Fig.