Dual-beam laser welding of AZ31B magnesium alloy in zero-gap lap joint configuration
Introduction
One of the largest concerns for the automotive industry is the CO2 emission targets mandated by governments around the globe. One reaction to those targets is a renewed focus on mass savings. Magnesium is one of the most ideal choices for mass savings since it is the lightest structural metal and has the best strength-to-weight ratio among commercial metals. Magnesium is also recyclable [1]. Magnesium alloys are best employed for optimum mass savings in parts with a high rotational speed that can reduce inertial forces [2] and are currently used in the automotive, aerospace, and defense industries [1]. The joining of magnesium alloys can be done by a variety of processes such as laser welding [3], arc welding [4], laser-arc hybrid welding [5], friction stir welding, [6] and electron beam welding [7]. However laser welding has attracted more attention due to its beneficial contribution to weld properties. A low heat input, high depth-to-width ratio, a narrow fusion zone, good mechanical properties, and highly automatable properties are among the advantages of laser welding [1].
The welding of a magnesium alloy contains issues such as porosity and cracking that require different techniques and remedies to mitigate these types of defects [1]. The main defect in the welding of a magnesium alloy is pore formation in the weld bead that can be caused by a variety of reasons such as hydrogen pores [8], coating on the alloy surface [1], [9], pre-existing pores as in die cast material [2], [10], and collapse of an unstable keyhole [11]. Different techniques can be employed to mitigate the pore formation during laser welding of a magnesium alloy. Process parameter optimization [10], [12], preheating [9], dual laser beam [13], interlayering [1] and laser-GTA hybrid welding with cold wire [14] are among the techniques that are reported to be successful. Marya and Edwards [12] evaluated the influence of different laser welding process parameters such as welding speed, laser power, and focal distance on the quality of the weld. They found that selecting optimum values for process parameters such as welding speed and beam intensity results in welds with a good quality. Mikucki and Shearouse [8] revealed that the hydrogen content in liquid magnesium increases pore formation in solidified ingot. They recommended keeping the amount of hydrogen as low as possible. Harooni et al. [9] used a plasma arc preheating source prior to laser welding and reported that this process mitigated pores effectively. They declared that by using a preheating source, magnesium hydroxide decomposed into magnesium and water vapor prior to laser welding allowing for the vapor to escape through the keyhole and thereby achieving a high quality weld.
A dual-beam laser technique has been used to mitigate pore formation in different alloys such as aluminum [15], galvanized steel [16], dissimilar welding of magnesium–aluminum [13] and magnesium–steel [17]. Miyashita et al. [13] used a dual-beam laser to weld dissimilar magnesium and aluminum alloys in a lap-joint configuration. They reported that the use of a dual-beam laser decreased the formation of brittle intermetallics at the interface of the joint resulting in an increase in the failure load. They also numerically modeled the flow of the molten pool and revealed that the flow is stronger in the case of dual-beam laser welding. Laser welding of an aluminum alloy with a dual-beam laser was studied by Deutsch et al. [15] showing that the dual-beam laser could effectively mitigate a wavy surface and undercut of the weld bead. However, they reported that the melting ratio was decreased regarding the loss of energy in the lagging laser beam. Iqbal et al. [16] investigated the effect of a dual-beam laser on the weld quality in laser welding of galvanized steel. They revealed that the leading beam can cut a slot, and the second beam welds and fills the slot. Zinc vapor at the faying surface of overlapped sheets was vented out through the slot. By employing this method, they achieved better weld quality than by use of a single beam. Laukant et al. [18] performed dual-beam laser welding of aluminum to zinc coated steel. They reported that by using this technique, they could combine laser welding with laser brazing in order to control the thickness of intermetallic, resulting in higher tensile strength of the joint. Xie [19] performed a dual-beam laser welding on aluminum and steel separately. He reported that beneficial results were achieved for each of these two alloys, such as decreasing hardness, porosity, and irregularity. He also reported that the plasma plume in the single beam was unstable; however, in dual-beam laser, the instability was mitigated. Haboudou et al. [20] studied the effect of a dual-beam laser heat source on porosity in laser welding of two dissimilar aluminum alloys. They found that by changing the distance between two laser spots, the pore formation can be significantly decreased.
The effect of oxide layer on the weld quality of AZ31B magnesium alloy in zero-gap lap joint configuration was studied by Harooni et al. [9]. It was found that by using a plasma-arc preheating source prior to laser welding, pore formation that was similar to research done by Li and Liu [14] was effectively mitigated in the weld bead as a result of the decomposition of the magnesium hydroxide. In the current study, a laser beam and a plasma arc as a preheating source are replaced with a dual-beam laser heat source resulting in a heat source of higher efficiency. The lead laser beam preheats the sample, and the lag laser beam welds the two overlapped metal sheets. To the best of our knowledge, no research has been done on using a dual-beam laser to mitigate porosity caused by oxides on the surface. This research is significant since cleaning of the surface prior to laser welding is a time-consuming and costly procedure for industrial application. In the current study, a high quality weld was achieved by using a dual-beam laser heat source with an optimized beam ratio. In order to evaluate the quality of weld for different parameters, a cross-sectional study of welds was carried out by using an optical microscope. The mechanical properties of the welds were evaluated by tensile and hardness tests. In order to evaluate the dynamic behavior of the weld pool during the laser welding process, a high speed CCD camera with a green laser as an illumination source was used to record the molten pool and keyhole images in real time. In a separate experiment, the plasma plume was captured by a high speed CCD camera in order to evaluate the stability of the keyhole during the laser welding process.
Section snippets
Experimental procedure
In order to join AZ31B magnesium sheets in a lap joint configuration, a fiber laser with a maximum power of 4 kW was used. A laser beam focused on the top surface of the two overlapped metal sheets was used to weld the samples. The AZ31B magnesium alloy sheets with a thickness of 1.5 mm were cut by an abrasive waterjet cutting machine to the desired coupon size. The nominal chemical composition of this alloy is listed in Table 1. A 4-point clamping fixture was fixed over the two overlapped AZ31B
Effect of dual-beam on the surface quality of the weld bead
The top surface quality of the weld beads of single-beam and dual-beam laser welding was evaluated using an optical profilometer in order to detect surface roughness on top of the weld bead. A length of 15 mm of the weld bead was scanned. Fig. 6 presents the profiles of the weld bead obtained during the laser welding process with two beam configurations. It is shown that the surface quality of the weld bead with dual-beam laser is smoother than the top surface of the weld bead obtained by a
Acknowledgement
This work was funded by the NSF Grant No. IIP-1034652. The authors acknowledge the mutual collaboration between Southern Methodist University–Research Center for Advanced Manufacturing (RCAM) and General Motors Company Research Center to develop research on laser welding of light-weight alloys. Also, authors acknowledge Dr. Fanrong Kong and Andrew Socha for their assistance in performing the experiments.
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