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Laser Welding of High-Strength Galvanized Steels in a Gap-Free Lap Joint Configuration Under Different Shielding Conditions

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Laser Welding of High-Strength Galvanized Steels in a Gap-Free Lap Joint Configuration Under Different Shielding Conditions Yang Supplement December 2010:Layout 1 12/13/10 4:32 PM Page 8 Laser Welding of High-Strength Galvanized Steels in a Gap-Free Lap Joint Configuration under Different Shielding Conditions By designing the specific shielding conditions, completely defect-free lap joints of the galvanized steels in a lap joint configuration are achieved by a single laser beam without pre- and postweld processing BY S. YANG, B. CARLSON, AND R. KOVACEVIC veloped at the interface of the two metal ABSTRACT sheets. The highly pressurized zinc vapor expels the liquid metal out of the molten It is a great challenge to laser weld zinc-coated steels in a gap-free lap joint con- pool and produces weld defects such as figuration due to the formation of highly pressurized zinc vapor. In this study, dif- spatter and porosity during the laser weld- WELDING RESEARCH ferent shielding conditions were designed to mitigate the highly pressurized zinc ing process. These defects significantly de- vapor. Argon, helium, the mixture of argon and carbon dioxide, and the mixture of grade the mechanical properties of the argon and oxygen were selected as the shielding gases to study the effects of shield- weld joints. ing conditions on weld quality. The introduction of a side shielding gas not only blew In the past several decades, many ef- away the laser-induced plasma but also suppressed the instability of the molten pool forts have been made to suppress the ef- caused by the highly pressurized zinc vapor. Under the optimal setting of shielding fect of the highly pressurized zinc vapor on gas conditions, a stable keyhole was consistently formed that provided a channel to the weld quality. The American Welding vent out the zinc vapor. Under this welding condition, the laser welding process was Society requires complete removal of the very stable. Consequently, a completely defect-free lap joint was achieved in a gap- zinc coating layer at the interface of two free lap joint configuration. Experimental results demonstrated that this newly de- metal sheets along the weld interface prior veloped laser welding procedure was robust and cost effective, does not require pre- to laser welding (Refs. 1, 2). Currently, set- or postweld processing and can be directly applied in the industrial conditions. A ting a small gap between the two metal high-speed CCD camera, assisted with a green laser as the illumination source, was sheets is a common way for industries to used to monitor the behavior of the molten pool and the keyhole dynamics in real join the galvanized steels in a lap joint con- time. Energy-dispersive X-ray spectroscopy (EDS) experiments were carried out to figuration (Ref. 3). Mazumder et al. (Refs. analyze the chemical compositions in the welds. Furthermore, tensile shear tests and 4–6) developed a technique of alloying the microhardness measurements were conducted to evaluate the mechanical properties zinc with the copper before the steel is of the welds. melted. The melting point of the copper- zinc compound is 1083°C (between the melting temperature of steel and the boil- Introduction high-powered lasers to joining galvanized ing temperature of zinc). However, the steels. However, it is difficult to achieve a solubility of copper into the steel could In order to reduce fuel consumption, high-quality weld of galvanized steels in a lead to additional problems such as hot enhance passenger safety, and improve lap joint configuration with using a single cracking and corrosion (Ref. 7). Re- corrosion resistance, different grades of laser beam because of the presence of designing the lap joint to allow the zinc high-strength galvanized steels are in- highly pressurized zinc vapor. The boiling vapor to be evacuated, prior to the molten creasingly used in the automotive indus- point of zinc is 906°C, which is lower than pool reaching the interface of the two try. In the past, the high-strength galva- the melting point of steels (over 1500°C). metal sheets, has been explored in order nized steels used in the automotive During laser welding of galvanized steels to mitigate the effect of the zinc vapor industry were commonly joined with re- in a gap-free lap joint configuration, the (Refs. 8–11). In addition, Pennington et al. sistance spot welding. Considering the highly pressurized zinc vapor is easily de- (Refs. 12, 13) proposed to deposit a nickel high speed, low heat input, deep penetra- coating along the weld interface after tion, and high flexibility of laser beam stripping off the zinc coating at the inter- welding, the automotive industry has KEYWORDS face of two metal sheets. The nickel has a shown significant interest in applying melting point of 1453°C, which is higher Galvanized Steels than the boiling point of zinc. By replacing Gap-Free Lap Joint the zinc coating with a nickel coating in the S. YANG ([email protected]) is senior re- weld area, the laser welding process be- searcher, GM China Science Lab, Shanghai, P.R. Configuration Laser Beam Welding comes stable and accompanied by an as- China. B. CARLSON is lap group manager, Gen- sociated corrosion protection. Unfortu- eral Motors R&D, Warren, Mich., and R. KO- Keyhole VACEVIC is professor, Mechanical Engineering, Shielding Conditions nately, this method will impose additional Southern Methodist University, Dallas, Tex. cost and reduce productivity. Pulsed laser (Ref. 14), dual laser beam or two lasers (Refs. 15–20), and hybrid laser welding 8-s JANUARY 2011, VOL. 90 Yang Supplement December 2010:Layout 1 12/13/10 4:32 PM Page 9 A B Fig. 1 — A — Experimental setup; B — schematic representation of lap joint configuration. (Refs. 21–24) were also used to weld gal- A vanized steels in a gap-free lap joint con- figuration. Gualini et al. (Ref. 18) modi- fied the dual beam to join the galvanized steel sheets in a gap-free lap joint config- uration where the first beam cut a slot to provide an exit path for the zinc vapor and B the second beam was applied to join the metal sheets. However, experimental re- sults demonstrated that spatter and poros- ity were still present in the lap joints. The laser-arc hybrid welding technique was C also used by Kim et al. (Ref. 24) to join SGCD1 galvanized low-strength steel with a 1.0-mm thickness in a gap-free lap joint configuration. It was revealed that the for- mation of porosity was the main concern D when using hybrid laser-arc welding of gal- vanized steels. In addition, they showed that process instability was the main cause of the generation of spatter and porosity in the welds. Spatter significantly dam- Fig. 2 — Lap joints obtained in Experiments 1 and 2: A — Top view of the lap joint obtained in Experi- aged the torch electrode and the porosity ment 1; B — bottom view of the lap joint obtained in Experiment 1; C — top view of the lap joint obtained in Experiment 2; D — bottom view of the lap joint obtained in Experiment 2. lowers the mechanical properties of the welds. Additionally, Gu et al. (Ref. 25) WELDING RESEARCH also introduced the arc into the laser weld- ing process where two heat sources share vacevic (Ref. 28) proposed a new welding this welding procedure in a highly auto- the common molten pool. They claimed procedure, which is a combination of a mated welding application. The automo- that the arc enlarges the molten pool pro- fiber laser with a gas tungsten arc welding tive industry continues to search for a new viding more space for the zinc vapor to es- (GTAW) torch used to preheat the top laser welding procedure to weld high- cape. However, spatter and porosity were surface of the galvanized metal sheets. strength galvanized steels in a gap-free lap still observed in the welds. Recently, a The GTAW preheating process burns the joint configuration with a single laser method was proposed and patented by Li zinc coating at the top surface of the metal beam without the pre- and/or postweld et al. (Refs. 26, 27) in which a thin alu- sheet, which helps in generating a thin film processing requirements. Until now, there minum foil layer was placed along the of the metal oxides (Ref. 28). The heated is no reference in the open literature on weld interface at the interface of two gal- surface with the thin film of metal oxides using a single laser beam to successfully vanized steel sheets to form an Al-Zn alloy will drastically improve the absorption of join galvanized steels in a gap-free lap con- during the laser welding process. They laser beam energy into the welded mate- figuration without the pre- and/or post- claimed that the level of the zinc vapor rial. Furthermore, the zinc coating at the processing requirements. Therefore, it is pressure was decreased through the for- interface of the two metal sheets is trans- important to develop an efficient and ro- mation of the Al-Zn alloy. In order to formed into zinc oxides, which has a bust laser welding technique to satisfy the achieve high-quality lap joints, the two higher melting point (above 1900°C) than demand from the automotive industry. metal sheets should be tightly clamped. Li that of steel (over 1500°C) resulting in less The main objective of this work was to re- et al. (Ref. 27) claimed that if a gap existed vapor generation and a more stable weld- spond to this demand and develop a cost- at the interface of two metal sheets, weld ing process. A completely defect-free, effective and easy-to-automate laser weld- defects would be produced in the welds.
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