Nanosecond-laser Texturing of Aerospace Aluminum-alloy Surface and Its Bonding Properties
Owing to the development of aerospace technology,conventional aluminum-alloy surface-treatment technology can no longer satisfy the demands of industrial production for environmental protection and strength consistency.In this regard,nanosecond-laser texturing is an ideal surface-treatment technology that offers the advantages of high processing consistency,zero environmental pollution,and no damage to the processing substrate.However,studies regarding the nanosecond-laser texturing of LY12CZ aluminum alloy to enhance bonding performance are few,and the law of process parameters on micropore formation remains unclear.Therefore,the effects of process parameters on the micro-morphology of aluminum alloy are investigated and the relationship between the bonding-performance-enhancement mechanism and micro-morphology is determined.Specifically,a YLPN-2-20x500-300-5M nanosecond pulse laser is used to laser process a 25 mm × 15 mm aluminum-alloy sample.The single-factor-analysis method is adopted to investigate the effects of laser energy(E),laser pulse width(τ),and laser frequency(f)on the morphology of micropores.Laser texturing is performed under a hole spacing of 100 μm,a marking speed of 40 mm/s,and a jump speed of 5 mm/s.E,τ,and f are varied separately to observe their effects on the surface morphology of micropores under a metallographic microscope.A three-dimensional profilometer is used to inspect the processed samples,and the diameter and depth of the micropores are recorded.The analysis software SensoVIEW is used to obtain the average results and construct a graph showing the effects of E,τ,and f on the hole diameter and depth.The formation rules and mechanisms of the micropores are analyzed based on observation results.The tensile shear strength of the sample is measured in accordance with GB/T7124-2008.The tensile shear strength of adhesives(rigid material to rigid material)is determined,and the average results are obtained.The optimal processing parameters are selected after comparing the shear strength,and the relationship between the micro-morphology and shear strength is analyzed.The result shows that E is the main factor affecting the surface microstructure of aluminum alloys.As E increases,the diameter and hole depth increase gradually.When τ is 20-120 ns or the pulse frequency is increased to 10-90 kHz,the hole diameter and depth increase.When τ exceeds 120 ns and increases,the hole depth decreases,whereas the hole diameter increases and then decreases gradually.When f increases above 70 kHz,the hole depth and diameter increase.Meanwhile,when lower E and τ values or higher f values are selected,the aluminum alloy in the laser-processing area flows from the edge to the center of the molten pool.This causes a buildup of molten metal,thus forming in a bulge in the center of the pool.After the aluminum alloy solidifies,a Mexican-hat shape with a convex center and concave edges is formed on the aluminum-alloy specimen.Its hole depth and diameter are small,thus rendering the adhesive's bonding effect unsatisfactory.Consequently,the bonding performance of the aluminum alloy is subpar.When the appropriate E,τ,and f are selected,the aluminum alloy in the laser-processing area forms a crater-like microstructure owing to Marangoni convection.The micropores exhibit large diameters,large depths,small micropore spacings,and a volcano-like microstructure around the micropores,thus facilitating the penetration of the adhesive and resulting in a"pinning"effect.Consequently,the shear strength of the aluminum-alloy bonding increases to 16.77 Mpa,which satisfies industrial production requirements(10 Mpa).The recommended process parameters are as follows:laser pulse energy,1.8 mJ;pulse frequency,70 kHz;pulse width,240 ns;ablation times,5;and micropore spacing,100 μm.The results of this study serve as useful reference for the high-quality nanosecond laser texturing of LY12CZ aluminum-alloy surfaces.
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