Combined Laser and Mechanical Microdrilling of Nickel-Based Superalloy

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Abstract

Drilling is an industrial process in which holes are produced by removal of material. This process is relatively well established for macroscale machining. However, microscale mechanical drilling is a more challenging process, especially in parts made of difficult-to-cut materials such as nickel-based superalloys. Although laser drilling and electrical discharge machining (EDM) have been reported as alternatives, mechanical drilling continues to be widely used for industrial macroscale drilling. However, mechanical microdrilling suffers from premature drill breakage due to the fragile nature of the microdrill. Furthermore, mechanical-drilled holes are inherently associated with geometry and metallurgy defects such as burr and subsurface damage, respectively. In laser percussion drilling, the challenge is how to improve the quality of the hole by minimising taper, recast layer, and heat affected zone formation. In addition, drilling hollow parts such as airfoil blades without introducing damage to the back wall is a major challenge in laser drilling. In drilling, the accuracy of the process and the quality of the surface finish are of great importance for both the manufacturer and the customer. Hybrid machining has been identified as a promising process which combines the benefits of different machining processes especially when applied to machining of superalloys.This Thesis presents a novel method to microdrill an Inconel 718 alloy, at both normal and inclined angles to the surface, using laser followed by mechanical drilling (sequential drilling). The method was aimed at extending the twist drill life and improving the quality of the hole when compared with existing techniques. The effect of laser predrilled-hole geometry on the quality of the produced hole were studied and evaluated. Continuous wave (CW) fibre and pulsed Nd:YAG lasers were used to produce holes with different geometry (blind, positive and negative tapered holes) as a pilot hole for mechanical drilling. CW fibre and Nd:YAG laser microdrilling of Inconel 718 alloy were implemented and evaluated before conducting the sequential drilling process. Taguchi methods were employed to design the experiments and analyse the results to establish the optimum set of parameters that yields an acceptable level of the response target. The standard commercial statistical software package MINITAB was used to evaluate the results.Initial experiments on the use of CW fibre laser drilling showed a great improvement in the quality of the hole and drilling speed. Those encouraging results inspired more experimental work and further evaluation of microdrilling of an Inconel 718 alloy. This unprecedented work was aimed at establishing the optimum conditions of laser and process parameters for hole taper, recast layer, and machining time. The results proved that the CW fibre laser drilling mechanism could be considered as a keyhole laser welding before material breakthrough. Furthermore, the process gas must be used to push away the molten material through the hole exit. The results also showed that a near zero tapered hole with very small recast layer and free of micro-cracks could be achieved with air process gas. This would have huge economical and environmental impacts since air is cheap and also an abundant resource.In the case of Nd:YAG laser microdrilling, the results proved that using assisted gas in laser drilling would not always increase the drilling speed or improve the quality of the hole. It was also found that the quality of the holes produced by air process gas is sufficient to meet the requirements for mechanical finishing.The sequential laser mechanical technique reduced the width of cut compared to mechanical drilling and relieved the load on the drill point resulting in a decrease in the thermal and mechanical stresses on the cutting tool. When compared with pure mechanical microdrilling, mechanical finishing of near zero laser drilled hole resulted in 100-330% increase in the tool life, up to 75% reduction in burr height, and significant improvement in surface integrity. In addition, the sequential laser and mechanical drilling of laser blind holes would be an effective technique for decreasing burr size and avoiding the back-wall problem in laser drilling of hollow parts especially when the exit surface of the components to be drilled has a closed cavity or is hard to access. It was also found that a smaller predrilled hole provided stability to the twist drill at the entry stage. However, burr size at the exit side decreased when the size of the predrilled hole was increased. Therefore, the mechanical finishing of negative tapered hole technique was developed to maintain the stability of the drill, extend the drill life, improve the burr size and surface integrity. The burr size for the mechanical finishing of negatively tapered laser predrilled holes was measured to be 6 times smaller than that of pure mechanical drilling. Finally, the results proved that the new technique alleviated the indentation and secondary cutting edge action. This would enable manufacturers to grind drills to thicker web thickness, which in turn, will increase the drill strength.

Keyword(s)

laser, microdrilling, hybrid

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