Experimental Investigation of Laser Welding Process in Overlap Joint Configuration

This paper presents an experimental investigation of laser overlap welding of low carbon galvanized steel. Based on a structured experimental design using the Taguchi method, the investigation is focused on the evaluation of various laser welding parameters effects on the welds quality. Welding experiments are conducted using a 3 kW Nd:YAG laser source. The selected laser welding parameters (laser power, welding speed, laser fiber diameter, gap between sheets and sheets thickness) are combined and used to evaluate the variation of three geometrical characteristics of the weld (penetration depth, bead width at the surface and bead width at the interface). Various improved statistical tools are used to analyze the effects of welding parameters on the variation of the weld quality and to identify the possible relationship between these parameters and the geometrical characteristics of the weld. The results reveal that the reached hardness values are similar for all the experimental tests and all welding parameters are relevant to the weld quality with a relative predominance of laser power and welding speed. The effect of the gap is relatively limited. The investigation results reveal also that there are many options to consider for building an efficient welds quality prediction model. Results achieved using an artificial neural network based simplified model provide an indication of the prediction model performances.


Introduction
Laser welding is more and more gaining place against resistance spot welding, which is considered as the most popular joining process in the automotive in-Journal of Materials Science and Chemical Engineering dustry for several decades. This transition is due to many advantages of laser welding such as low heat input, high energy density, small heat affected zone, fast welding and deep penetration as well as esthetic weld seams. Sheets with different alloys, shapes, thicknesses or material properties can be welded using laser. However, to improve the corrosion resistance of the vehicle parts in automotive industry, various coatings alternative can be considered. Among these techniques, zinc surface coating is the most popular [1]. Due to the low boiling temperature of the zinc (1180 K) compared to the fusion temperature of steel (1808 K), the laser welding process of galvanized steel in the overlap configuration exhibits instabilities. This is caused by the premature vaporization of the zinc coating at the sheets interface generating a high pressures ranging from 50 to 100 bars at temperatures varying from 1800 to 2000 K [2]. The pressurized vapors disturb the welding process by ejecting the molten metal outside the melt pool, and zinc vapors can be trapped in the weld after solidification, as blowers and spatters [3].
Various studies are conducted for understanding the chaotic behavior of the zinc during laser welding process. Fabro et al. [2] reported that zinc vapors flow first into the keyhole and then expand rapidly in the volume of the molten metal, creating a jet of gas that disrupts the molten flow. A study of the dynamics of the liquid zinc flow between the overlapped sheets during laser welding process, suggests that the zinc moves away from the fusion zone when metal is liquid and moves back to the weld pool after solidification [4]. Norman et al. described three modes of defects evolution during welding and presented the main causes of various defects types [5] [6].
Many approaches are proposed to overcome the zinc related problems and improve the weld joint quality. Providing a gap between the sheets allows a lateral escape of zinc vapors without affecting the weld pool. Therefore, the selection of optimal gap can lead to defect free welds [7]. In contrast, an inappropriate gap reduces the weld quality. In fact, a very small gap is not sufficient to release the vapor, while a large one does not allow the fusion of the two parts to be welded together [8]. A study of the laser overlap welding process behavior of galvanized steels reported that a gap ranging from 0.04 to 0.15 produces high strength and homogenous welds [9]. To produce acceptable welds, Akhter et al. [10] proposed a simplified model illustrated by equation below to estimate the size of the required gap from the volume of the zinc vapor to be exhausted.
However, the difficulty to maintain a constant gap along the weld line remains unresolved.
where, g is the gap, k is material constant, t zn is the zinc coating thickness and t p is sheet thickness and v is the welding speed.
Several other methods, using additional elements that can interact with zinc before its evaporation, such as copper or aluminum have been tested [11]. Although these elements contributed to the stability of the laser welding process, tionship between the variation in weld bead width, measured at the top surface of the lap welded joint, and the mechanical properties of the weld bead, as well as the effects of the gap and other welding parameters on this variation [12]. This study reported that a wide width variation reflects a poor quality of the weld. Zhao et al. conducted an experimental study in order to evaluate the effects of laser welding parameters on the weld bead geometry in the overlap configuration of thin-gauge galvanized steels by using response surface methodology [13]. It was demonstrated that an optimal combination of these parameters increases the aspect ratio of the weld joint by 30%. Wei et al. reported that the increase in laser power makes it possible to switch from the conduction welding mode to the keyhole mode [14]. Consequently, the keyhole mode can be considered as a degasing channel, but the deals lies in the stability of the keyhole during the welding process. Elongating the keyhole in order to facilitate the zinc vapor escape can be achieved by defocusing the laser beam, by tilting it, or by using multiple laser spots [15]. Fabro reported that an elongated keyhole improves weld quality with CO 2 laser beam but not with Nd:YAG laser source [2]. A fast frequency modulation of laser power allows partial reduction of zinc-related defects during lap welding of galvanized steels [15]. Using an optimum speed-power combination, Pieters and Richardson reveal that defect free welds can be achieved in overlap configuration without gap or special manipulation technique. This is possible only with full penetration mode [16]. Based on these remarks, it is obvious that a good quality welds during overlap laser welding of galvanized steels depends on the adjustment of the laser parameters and the size of the gap between the sheets. A structured experimental design combined to improved statistical analysis tools can provide a deep understanding of the effects of laser parameters, welding conditions and their interactions on the variation of the geometrical and mechanical characteristics of the welded joints and can conduct to efficient and robust model for predicting the welds quality. This paper presents an experimental investigation of overlap laser welding of zinc coated low carbon steel. Based on a structured experimental design, the investigation is focused on the evaluation of the effects of various laser welding parameters and conditions on the variation of the geometrical and mechanical characteristics of the weld quality.

Parameter Identification
The experimental investigations are conducted using ASTM A635CS galvanized K. Oussaid et al. Journal of Materials Science and Chemical Engineering steel with A40 coating type. Three sheet thicknesses varying from 0.8 to 3.6 mm are selected for the experimentations to conform to the thickness range commonly used in the automotive industry. The sheet specimens having 1, 2 and 3 mm thickness are cut using hydraulic shear at the size of 30 × 50 mm. The sheets are then superimposed two by two to perform the laser overlap welding. Table 1 illustrates the chemical compositions of the used sheets provided by the steel manufacturer. Note that the very small variations in chemical composition are neglected.
Laser power (P), welding speed (S), laser spot diameter (D) and Gap (G) are the considered parameters in this experimental investigation. The upper and lower limits of these parameters are set using some results from the conducted preliminary tests and others relevant information extracted from the related literature.

Experimental Setup
The experimental investigation is carried out using a welding laser cell composed of a FANUC M-710iC six-axis robot, directing a laser beam coming from a HIGHYAG BIMO laser head powered by an IPG YLS-3000-ST2 fiber laser source. The laser power is transferred through an optical fiber with a diameter of 200 µm. The maximum power that can be emitted by the Nd:YAG laser source is 3 KW with a wavelength of 1070 nm. The laser head is equipped with a variable-zoom collimator and a fixed focusing lens. The collimator adjustment provide, circular focal spots with a diameter ranging from 340 to 520 μm, for a focal length of 300 mm. Figure 1 shows the used laser welding setup for the experimentations.  After welding, the samples are processed following a standard metallography procedure: 1) cutting assemblies perpendicular to the weld to obtain the desired cross section, 2) specimens preparation for microscopic observation including grinding polishing, etching and finally 3) microscopic observation using a Clemex MMT Type A microscope. The microscope is equipped with contour identification programme permitting the evaluation of the weld geometrical attribute in the cross section.
As defined in Figure 2, the measured weld dimensions are the depth of penetration (DOP), bead width at the surface (WS) and bead width at the interface (WI). Each measurement was taken three times and then averaged to constitute the database used for the statistical analysis. Vickers micro-hardness testing is also conducted using a load of 500 g and a dwell time of 15 s. The base material measurements are taken far from the fusion zone.

Preliminary Tests
Due to the anticipated difficulties related to the zinc vapors present at the inter-

Design of Experiments
Factorial designs are the simplest experimental designs to use. They provide the maximum data on the process to be studied. However, the number of necessary tests grows exponentially as soon as an additional factor or level are introduced to the design, making the experience more expensive and more time consuming.
However, fractional plans achieve fewer tests and allow a very good ratio between  Table 2 and the experimental design is presented in Table 3.
To Therefore, three based gap L 9 matrices are used.

Repeatability Tests
In order to establish a measurement quality reference, 8 repeatability tests are   Table 4 present a very good repeatability. The variations are less than 10%. These results ensure the measurement method validity and prepare for the experimentation phase with confidence.

Evaluation of the Laser Parameter Effects
Globally, the produced welds present acceptable visual characteristics, nevertheless some discontinuities of the welds and some projections of the metal observed in the case of certain samples representing experiments with 0.05 mm gap. Figure 3 presents typical welds achieved using P = 3000 W, S = 70 mm/s, D = 395 mm and G = 0.05 mm.
The experimental data are analyzed using three statistical tools: the graph of the average effect for each factor, the percent contribution of factors extracted   Table 5 and graphs of average effects in Figure 4 show that the laser power has positive effect on the variation of the weld characteristics. This effect is almost linear on DOP and WS and non-linear on WI. The contribution of the power in WS variation is 50% against 40% in WI variation and 14% for DOP. A nonlinear and significant negative effect of the welding speed on the variation of the three studied weld properties is observed. The speed contribution in DOP variation is 71%, against 37% for WS and 40% for WI. The effect of the laser spot diameter is not very important with negative effect on DOP variation and a contribution of 13%. The laser spot diameter contribution in WS variation is 12%. Its effect on WI variation is positive for small diameters and negative for large diameters with 13% of contribution.
The previous observations are confirmed by the correlations analysis between geometrical attributes of the weld and laser welding parameters presented in Table 6. A very significant correlation between the welding speed and the different characteristics of the weld is observed. The focal diameter presents a weak  correlation with weld dimensions with correlation coefficients less than 30%.
Laser power is strongly correlated with the WS. As expected, strong correlations are observed between different weld characteristics.

Evaluation of the Gap Effects
An L 27 orthogonal array using four three-level factors is formed from three L 9 Journal of Materials Science and Chemical Engineering blocks to allow the integration of the gap factor into an extended design.
ANOVA results in Table 7 and graphs of average effects in Figure 5 show a relatively limited effect of the gap on the variation of different weld geometrical characteristics. The maximum contribution of the gap is observed on DOP variation by 7.3%.

Simplified ANN Prediction Model for Weld Dimensions
The laser welding parameters that have important effects on weld quality variation are identified. Weld characteristics exhibit a complex and nonlinear relationship with specific parameters. To be able to implement an effective prediction      results suggest that power, speed, fiber diameter and gap are the largest contributors to the geometrical weld characteristics variation (DOP, WS and WI). Consequently, P, S, D and G are used as input to the predictive model. The modelling results demonstrate that the models can accurately predict the weld characteristics with an error less than 12%. The measured and predicted DOP, WS and WI are shown in Figure 10. These results suggest that the modeling approach can be effective for weld quality prediction. A more accurate definition of the weld quality attributes, an experiment covering more laser welding parameters Journal of Materials Science and Chemical Engineering and more factor levels for more training and validation data as well as an improvement of the modeling procedure can lead to more accurate and efficient models. This may probably lead to model improvement decreasing the modelling error to less than 5%.

Conclusion
This paper presents an experimental investigation of laser overlap welding of low carbon galvanized steel. The experimental work is focused on depth of penetration, bead width at the surface, and bead width at the interface and hardness using various laser welding parameters such as laser power, welding speed, laser fiber diameter, and gap between sheets and sheet thickness. There are 27 experimental tests taken as all factors known to have an influence on welds quality to conduct a systematic study using an efficient and structured experimental design. An error of less than 10% achieved in repeatability tests en- diameter. The gap between sheets has a very limited effect. Confirmed by a multiple correlation analysis between weld geometrical attributes and welding parameters, the results suggest that many options can be considered for building an efficient weld quality prediction model. An artificial neural network based simplified predictive model is given as an example to demonstrate the possible and promising performance of weld quality prediction that can be achieved.