THE LASER USER


ISSUE 115 MARCH 2025 LASER WELDING


additional thermal effects that may have caused brittleness in the weldment, contributing to reduced shear strength applied after welding. Further optimisation of peening parameters, such as pulse overlap and energy intensity, could enhance its efficacy.


Conclusion


Figure 2: A cross-section of the blue laser weld of copper-to-tinned steel (a) spot one (b) spot two and (c) spot three.


This feasibility study demonstrated that laser cleaning using a green laser significantly improves surface preparation by improving absorption of the incoming blue laser, and that blue laser welding can potentially obtain weldments of sufficient strength. However, higher laser power or smaller spot sizes would be required to achieve power densities that will produce a sufficient key-hole weld with minimal active-time to avoid entrapment of air and to minimise porosity.


Figure 3: Blue Laser welded cross-section in (a) and (b) the laser peened and blue laser welded cross-section.


making contact with the tinned steel substrate. The third spot illustrated maximum penetration with additional visible weld defects. Despite this, the strength of the welds was sufficient to meet the end user requirements. Cracking is evident in Figure 2(b), but it is believed to have rendered from cross-sectioning the weldments. The spot active time was 1 second for each spot. As the material heated up, the penetration depths increased, therefore, future trials were conducted by altering the laser spot timings for each of the spots.


Laser Shock Peening


Compressive stresses induced during laser shock peening were quantified as ranging from -50 MPa to -40 MPa in the transverse and longitudinal directions respectively. In comparison, the residual stress for the unpeened surface was –2 MPa in the longitudinal direction and +20MPa in transverse direction.


A number of grains counts/measurements were taken using the ASTM measurement technique [5]. The results showed that the ASTM number of average grains present on the as blue laser welded surface was 5.27 (G), whilst in comparison the blue laser welded surface of the copper that was laser shock peened was 7.22 (G). This meant that the number of grains present after laser shock peening had increased by 37%. In addition, the average hardness of the as blue laser welded 68HV whilst the hardness after peening the blue laser weld was 82HV.


This rendered an increase of 21% in hardness and aligns with the Hall-Petch relationship of hardness and grain size.


Shear Strength


Table 1 shows the shear strength data obtained from undertaking tests of the various welds under the aforementioned laser processing conditions. The highest shear strength was achieved in samples where laser cleaning preceded welding. The results underscore the importance of surface preparation in achieving strong metallurgical bonds. Laser peening, while beneficial for hardness and stress redistribution, introduced


Table 1: Results of shear strength test


Process Ultimate shear strength (MPa)


C-W


C-W-P W


W-P


318.33 221.84 208.63 164.88


Change (%)


Laser peening improved the top surface microstructures of the copper and some compressive residual stresses were introduced along with increase in hardness of 21% after laser shock processing. Shear strengths were sufficient for end-user requirements reaching above 220MPa with a Cleaning+Welding configuration and a Cleaning-Welding-Peening configuration. Future research should focus on optimising processing parameters, such as laser intensity and timing, to further improve joint performance and reliability.


Acknowledgments


This project was funded by Innovate UK and the Singapore Research Council under the project "Laser Treatment Tailoring of Microstructure on Dissimilar Metal Joints for Improved EV Performance", in collaboration with three other institutions from Singapore and the UK.


References


[1] David, R. (2004). CRC Handbook of Chemistry and Physics. 85th Edition, CRC Press.


[2] Nuburu. (2018). "Blue Laser Enables Copper Welding," International Copper Association. Available at: https://www.copper.org


[3] Hummel, M., et al. (2020). Journal of Advanced Joining Processes, 1, 100012.


[4] Auwal, S.T., et al. (2018). Int. J. Adv. Manuf. Technol., 96, 475-490.


70 % ↓ 65% ↓ 52% ↓


C= Cleaning; W = Welding; P = Peening


[5] ASTM E112-21. (2021). "Standard Test Methods for Determining Average Grain Size," ASTM International.


Contact: Pratik Shukla Pratik.Shukla@the-mtc.org www.the-mtc.org


Pratik Shukla is a Senior Research Engineer at the Manufacturing Technology Centre, UK.


SEE EE OBSERVATIONS P30 29 21


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