Challenges and solutions of reducing porosity formation during remote laser welding of die casting aluminium

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Mikhail Sokolov, Pasquale Franciosa, and Dariusz Ceglarek of the University of Warwick, describe how new laser technologies can overcome dissimilar material welding challenges

Increasing demand for car weight reduction with e-mobility has led to an increased rate of designs featuring dissimilar lightweight die casting alloys.

The structure weight of vehicles can be significantly reduced if individual components, such as engine blocks and transmission casings, can be made using such alloys.

However, welding dissimilar light alloys is a significant challenge in the automotive industry. 

The average aluminium content per vehicle currently amounts to nearly 179kg, with the total content for the whole car market (including electric vehicles) estimated to be 2,989 kilotonnes – most of which comprises aluminium castings1

Riveting methods are commonly used to join casting alloys. They generally require an elongation rate of 10 per cent, which is hard to achieve with standard alloys. Welding methods provide interesting alternatives. However, since casting alloys are prone to significant porosity formation, new welding methods are required to reduce and prevent porosity formation in the welds. 

Large-clustered porosity affects the mechanical properties of welded joints, such as tensile strength and fatigue. Cluster porosity is not a critical defect, but it does increase the chance of other defects occurring, such as corrosion and weld root porosity, which decrease bending strength. 

There are several theories that explain the formation of porosity in aluminium alloys, a number of which cite hydrogen originating from moisture in the air. Some hydrogen is initially present inside the alloy or on its surface. When entering solid solution during welding, this gas forms bubbles during cooling. More pores are formed by the keyhole instability caused by the vaporisation of low boiling point elements in the alloy, such as magnesium.

Porosity reduction methods 

The high flexibility of remote laser welding (RLW) systems enables the introduction of sophisticated welding patterns, fast beam e-positioning with superior processing speed, laser power modulation and beam shaping2. The combination of these features has successfully been proven to weld challenging materials, such as non-ferrous alloys and highly reflective materials like copper and wrought aluminium. However, the application of RWL to casting alloys has not yet been fully exploited.

We have therefore proven beam oscillation using an adjustable ring mode (ARM) laser to be an effective porosity reduction method during RLW (see figure 1)3. The beam oscillation provides keyhole stabilisation at acceptable welding speeds, enabling the release of the entrapped gases from the weld pool.

Figure 1: Beam oscillation using adjustable ring mode (ARM) lasers can be used to reduce porosity during aluminium welding.

The ARM laser provides a novel solution of dual-beam welding, as the core beam helps to initiate faster and sufficient keyhole formation, while the ring-shaped beam provides effective temperature distribution in and around the molten pool, avoiding rapid cooling and hence stabilising the welding process (see figure 2). A stable keyhole leads to a stable surface with less spattering, therefore reducing the porosity. 

Figure 2: Illustration of laser beam oscillation method with an adjustable ring mode (ARM) laser. Welding parameters: Pc,i – laser power of core beam, modulated transversally to the welding direction on three points: on the upper part (A), reference point (B) and lower part (C); Pr,i – laser power of the ring beam, not modulated; f – oscillation frequency; Ay – oscillation amplitude; Oy – is measured from the reference point, and defines the position in the y direction of the laser beam the zero-position of Ay; Az – focal point position offset; and Sx – laser welding speed was constant at 4m/min.

Experiments and results 

Our experiments were conducted using aluminium die casting alloy AlSi7Mg (on top, 2mm thickness) and aluminium extrusion AA6063 (bottom, 3mm thickness). These dissimilar materials were welded using a continuous-wave, multi-mode Coherent fibre laser (HighLight FL-ARM 10000) in a fillet lap joint welding set-up. All experiments were performed without shielding gas or filler wire. Samples were wiped with acetone before welding to remove surface contaminants.

After welding, the samples were cut at three places: 7mm from the weld start, middle of the weld and 7mm from the weld end. Once the samples were extracted and polished, the cross sections were photographed using a Nikon Eclipse LV150VN microscope in black-and-white mode. The pores were recognised on the cross-section photographs using an automatic pore recognition code developed in Matlab. The key weld indicator was defined as porosity area (Ap) that indicates the percentage of the weld pool area covered with porosity.

The experiments were focused on investigating the effect of selected welding process parameters and analysing their influence on Ap. Three welding parameters were chosen:

  • Frequency (f) – frequency beam oscillation above 100Hz was used to facilitate the stirring effect that increases keyhole stability and degasification of the molten pool.
  • Power at ring beam (Pr) – to control the distribution of heat and cooling rate in and around the molten pool for keyhole stabilisation.
  • Focal offset (Az) – to control the distribution of the spatial energy input.

Constant welding parameters were chosen to ensure the minimum weld penetration depth of 1.2mm – 40 per cent of the bottom plate at zero gap between the plates.

See table 1 for experimental parameters, factors and levels:

The main effects of the chosen parameters and their interaction effects on weld quality were compared by analysing Ap. The results are shown in figure 3.

Figure 3: Contour plots of porosity area (Ap) and microsection illustrations at different levels of focal offset (Az) and ring laser power (Pr) for three frequency levels (f). Lowest porosity result of 1.6 per cent indicated by (A).

The analysis indicates that the influence of Az on Ap was statistically significant, Pr also influences Ap, possibly through the stabilisation of the keyhole. With one exception, overall, the interaction effects were not significant, indicating there were no combined effects of Pr and Az, Az and f. Hence, only the interaction effect of Pr and f was statistically significant on Ap, while f as a single factor had no influence on the porosity. Therefore, the joint effect of the dual beam welding and high frequency beam oscillation has a significant effect on Ap.

Conclusions and future work 

Weld porosity reduction has been demonstrated using a dual-laser beam and frequency beam oscillation. A reduction of porosity from 7 per cent to below 2 per cent (1.6 per cent average) of the weld area was achieved with oscillation frequency (f) = 200Hz, position offset (Az) = 0mm (on the surface of the top plate), ring laser power (Pr) = 2,000W with constant speed (Sx) 66.67mm/sec (4m/min) and with applied power modulation of the core laser power. 

Considerably more work will be needed to determine the distribution of porosity over the weld using computed tomography to assess porosity behaviour at higher frequency levels. Ongoing research indicates that increasing frequency leads to a nonlinear dynamic with porosity levels. Frequencies of 200Hz and 300Hz show overall reduction at all ring laser power levels, while at 400Hz we observe a significant rise in porosity, as shown in figure 4. 

Figure 4: Computed tomography scans: (A) frequency = 300Hz, (B) frequency = 400Hz.

A future area of exploration is the application of porosity reduction methods for laser- based 3D printed parts4. Multiple methods will be considered, such as varying beam and oscillation shape, and preheating to allow the pores to degas over a longer period5. Future work will also investigate other weld joint configurations and material sets with die casting aluminium and application of weld porosity reduction methods. 

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Dr Mikhail Sokolov is a research fellow at WMG, University of Warwick.

Dr Pasquale Franciosa is an associate professor at WMG, University of Warwick, and head of the laser welding applications laboratory at WMG.

Professor Dariusz Ceglarek is EPSRC Star Research Chair at University of
Warwick and a CIRP Fellow.

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References

[1] ‘European Aluminium,’ available: https://www.european-aluminium.eu

[2] T. Sun, P. Franciosa, C. Liu, F. Pierro and D. Ceglarek, ‘Effect of Micro Solidification Crack on Mechanical Performance of Remote Laser Welded AA6063 Fillet Lap Joint in Automotive Battery Tray Construction,’ Applied Sciences, vol. 11, no. 10, p. 4522, 2021.

[3] M. Sokolov, P.  Franciosa and D. Ceglarek, ‘Remote laser welding of die casting aluminum parts for automotive applications with beam oscillation and adjustable ring mode laser,’ in Lasers in Manufacturing Conference, 2021.

[4] B. Möller, K. Schnabel, R. Wagener, H. Kaufmann and T. Melz, ‘Fatigue assessment of additively manufactured AlSi10Mg laser beam welded to rolled EN AW-6082-T6 sheet metal,’ International Journal of Fatigue, vol. 140, no. 105805, 2020.

[5] C. Emmelmann and F. Beckmann, ‘Optimization of laser welding process for laser additive manufactured aluminum parts by means of beam oscillation and process-oriented component design,’ in Lasers in Manufacturing Conference, 2017.

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This study was partially supported by (1) APC UK projects Chamaeleon & ALIVE; (2) Innovate UK IDP15 project LIBERATE; (3) WMG HVM Catapult. We gratefully acknowledge the support of the X-Ray Computed Tomography (NXCT) at the University of Warwick under the EPSRC Project Number (EP/T02593X/1).

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24 September 2021