In welding, the depth of penetration, weld profile and the corresponding thermal cycle are the three basic outcomes that a user wishes to control flexibly. In laser welding applications, controlled application of power and energy density is the key to achieve predictable control of these characteristics. Creation of an analytical model is an important step towards understanding the underpinning science of laser metal interaction in controlling the depth, bead geometry and thereby temperature profile of a weld. The "power factor model", which correlates the power applied per unit length to the laser metal interaction time, has been originally developed and validated for mild steel, guides a user on the selection laser system parameters, to achieve specific weld profile. This study is performed to extend the power factor-interaction time model to aluminium alloys by understanding the underpinning laser aluminium interaction parameters in terms of power density, interaction time, specific point energy and their correlation with the weld bead profiles. Although the power factor and interaction time showed a rectangular hyperbolic relationship, as observed in low carbon steel, for a specific weld depth and profile, the absolute magnitude and the characteristic profile of the curve is different due to the intrinsic differences in physical and thermal properties of aluminium as compared to steel. It was shown that identical depth of penetration but different weld metal profile can be obtained for a specific beam diameter for a range of power and travel speed by keeping the energy input per unit length constant.

The digital control of the latest nanosecond pulsed wave (PW) fibre lasers allows very high flexibility in controlling the application of the total energy to a workpiece, which brings several advantages to the joining process. By choosing different pulse shapes in different spatial profiles, it is possible to apply low energy per pulse with high precision and accuracy resulting in lower heat input. Since the energy of each pulse is insufficient to generate melting, these lasers operate at very high pulse repetition frequencies near continuous wave (CW) regime. Nevertheless, the peak powers of PW lasers are much higher than CW. In this research, the effect of peak power, pulse energy, pulse width, pulse repetition frequency and duty cycle has been studied. The experimental work was conducted in bead on plate of austenitic stainless steel to investigate the effect of laser on the weld geometry, i.e. depth of penetration and width. An empirical model, previously established for CW mode, which enables the achievement of a particular penetration depth independent of the beam diameter, was redesigned and tested for PW mode. The “pulse power factor model” allows the laser user to select a weld profile that meets certain quality and productivity requirements independent of the laser system. It was shown that identical depth of penetration but different weld metal profile can be obtained for a specific beam diameter for a range of different system parameters by keeping a constant trade-off between pulse power factor and interaction time.

Selective Laser Melting (SLM) is an emerging Additive Manufacturing (AM) technique for the on-demand fabrication of metal parts. The mechanical properties of Selectively Laser Melted (SLMed) parts are sensitive to oxygen concentration within the SLM build chamber due to the formation of oxides, which may lead to various negative consequences. As such, this work explores the influence of SLM atmospheric Oxygen Content (OC) on the macroscopic mechanical properties of SLMed AlSi10Mg bulk material and Triply Periodic Minimal Surface (TPMS) lattices namely primitive, gyroid, and diamond. Standard quasi-static tensile and crushing tests were conducted to evaluate the bulk properties of AlSi10Mg and the compressive metrics of TPMS-lattices. Two oxygen concentrations of 100 ppm and 1000 were used during the SLM fabrication of the experimental specimens. The tensile test data revealed a small influence of the oxygen content on the bulk properties. The low oxygen concentration improved the elongation while slightly reduced the ultimate tensile strength and yield stress. Similarly, the influence of the oxygen content on the compressive responses of TPMS-lattices was generally limited and primarily depended on their geometrical configuration. This study elucidates the role of SLM atmospheric oxygen content on the macroscopic behaviour of SLMed AlSi10Mg parts.

A continuous wave (CW) and a nanosecond pulsed wave (PW) lasers were used to join 1-mm thick sheets of SS304L (SS) austenitic stainless-steel to AA5251 (Al) aluminium alloy in an overlap joint configuration. The weld shape (penetration depth and width), intermetallic compounds concentration, weld quality (cracking and porosity) and mechanical strength were correlated with the process energy and compared between each laser temporal mode. Successful CW joints were produced with the SS sheet on top of the Al, but the opposite configuration revealed to be impossible for the range of parameters tested. The PW joints were successful with the Al sheet on top of SS, but all the joints cracked at the interface when the opposite configuration was used. The mechanical tests showed that even though it is possible to achieve higher tensile shear load in CW welds due to the larger bonding area, the tensile shear strength revealed to be almost 5 × higher for PW welds at similar applied energy.

Laser welding is distinguished by low heat input, low distortion, high travel speeds and accuracy. Traditional high-power pulsed wave (PW) lasers are being replaced by high-frequency low-pulse energy fibre lasers. However, as these lasers operate at very high frequencies, near continuous wave (CW) operation, it is not clear the benefit of such frequencies in comparison to CW lasers for micro-welding. In this project, two lasers, one in high-frequency PW and another in CW are operated at the same conditions, including average power, average peak power, spot size and travel speed, and the differences in material response are investigated. It has been shown that frequency is one of the important parameters that affect the heat loss between individual pulses, referred to as inter-pulse losses. At low frequency, the PW laser provided lower melting efficiency and higher penetration efficiency than CW. On the other hand, at high frequency, the PW resulted in lower melting and penetration efficiency than CW. In addition, a new definition of interaction time has been proposed to capture conduction losses by travel speed and heat inter-pulse losses due to periodic lack of laser power. This allows a like-for-like comparison of CW and PW lasers and can be used to predict penetration depth with processing parameters.