Ken Watkins

Advances in laser forming

Laser forming is fast emerging as a viable means for the shaping of metallic components, as a means of rapid prototyping and of adjusting and aligning. Relevant industry sectors include aerospace, automotive, and microelectronics. In contrast with conventional forming techniques this method requires no mechanical contact and hence offers many of the advantages of process flexibility associated with other laser manufacturing techniques such as laser cutting and marking. This article reviews the mechanisms involved and the main models proposed in laser forming and presents an overview of the main applications currently developed. An assessment is made of the potential for future development of this new technology.

Microstructure and corrosion properties of laser surface processed aluminium alloys: a review

Abstract Laser surface treatment techniques, including laser surface melting (LSM) and laser surface alloying (LSA), have been the subject of considerable interest as a means of enhancing the corrosion performance of aluminium and its alloys. Microstructural modification together with the incorporation of non-equilibrium concentrations of alloying elements resulting from relatively rapid rates of cooling compared with conventional surface treatment techniques provide the basis for property enhancement. This paper considers microstructural evolution in a range of laser surface treated aluminium alloys including LSM of Al–Cu, Al–Si, Al–Zn, Al–Fe and Al–transitional element alloy systems; LSA of Al–Ni, Al–Cr and Al–Mo. Where reported, corrosion and other property determination in these systems is discussed. It is shown that surface alloys with unique microstructural and compositional characteristics have been produced by these techniques and that in many cases promising improvements in hardness and critical pitting potential compared with conventional alloys have been reported.

The influences of particle number on hot spots in strongly coupled metal nanoparticles chain.

In understanding of the hot spot phenomenon in single-molecule surface enhanced Raman scattering (SM-SERS), the electromagnetic field within the gaps of dimers (i.e., two particle systems) has attracted much interest as it provides significant field amplification over single isolated nanoparticles. In addition to the existing understanding of the dimer systems, we show in this paper that field enhancement within the gaps of a particle chain could maximize at a particle number N>2, due to the near-field coupled plasmon resonance of the chain. This particle number effect was theoretically observed for the gold (Au) nanoparticles chain but not for the silver (Ag) chain. We attribute the reason to the different behaviors of the dissipative damping of gold and silver in the visible wavelength range. The reported effect can be utilized to design effective gold substrate for SM-SERS applications.

Laser alloying of aluminium alloys with chromium

Abstract The microstructure and corrosion resistance of laser-alloyed aluminium and ANSI 7175 aluminium alloy with chromium were investigated. Surface layers alloyed with chromium contain relatively large amounts of intermetallic compounds dispersed in a matrix of α-Al. The intermetallic compound particles present needle-like morphologies, organized in a dense network or distributed radially. Al 7 Cr, Al 11 Cr 2 and α-Al phases have been identified by X-ray diffraction. The alloyed layers may contain cracks, pores, inclusions and undissolved chromium particles, depending on the chromium concentration and the particle size. However, homogeneous layers were produced by a two-step process, consisting of laser alloying followed by remelting. The second treatment eliminates porosity and refines the structure. The hardness attains a Vickers hardness of 155 HV in chromium-alloyed aluminium and exceeds 300 HV in chromium-alloyed 7175. The corrosion behaviour of the above alloys was assessed using anodic polarization techniques. Laser alloying of aluminium and 7175 with chromium improves the pitting corrosion resistance of the alloys. The effect depends on the chromium content of the alloyed layers and is more significant in 7175 alloy.

Removal of small particles on silicon wafer by laser-induced airborne plasma shock waves

It has been found that effective removal of small particles from silicon wafer surfaces can be achieved by laser-induced airborne plasma shock waves initiated above the surface. This process has demonstrated successfully the removal of 1 μm sized tungsten particles from the surface. Tungsten is known to be one of the most difficult particles to remove using conventional laser cleaning techniques employing direct laser irradiation onto the surface. The area cleaned by the shock waves was over ten times larger than that achieved by conventional laser cleaning. This provides an advantage in speed for cleaning large areas. The cleaning efficiency was strongly dependent on the gap distance between the laser focus (where laser-induced sparking takes place) and the surface. The particle removal began to occur with a gap of 6 mm and the removal efficiency increased significantly with decreasing the gap resulting in a complete removal at 2 mm. The basic idea behind this new cleaning technique and how to apply it f...

Localized corrosion of laser surface melted 2024-T351 aluminium alloy

Abstract Laser surface melting of 2024-T351 aluminium alloy with a CO 2 laser operating at 2 kW with a spot size of 1.5 mm and a substrate traverse rate of 20 mm s −1 produced a relatively thin surface-melted layer with a refined microstructure and a modified distribution of the alloying elements. The laser treatment changed both the anodic polarization behaviour and the form of localized corrosion in deaerated 3% NaCl solution. Immersion tests in the same solution under the condition of natural aeration showed that for the as-received alloy both intergranular and pitting corrosion occurred with pits distributed mainly along the rolling direction while for the laser surface melted material only pitting corrosion was present with pits distributed uniformly. This difference in corrosion behaviour as a result of laser surface melting is attributed to changes in the distribution and composition of the second-phase particles present in the alloy.

Laser Welding of Aluminium Alloy 5083

There are two laser welding mechanisms, keyhole mode and conduction mode. Keyhole welding is widely used because it produces welds with high aspect ratios and narrow heat affected zones. However keyhole welding can be unstable, as the keyhole oscillates and closes intermittently. This intermittent closure causes porosity due to gas entrapment. Conduction welding, on the other hand, is more stable since vaporisation is minimal and hence there is no further absorption below the surface of the material.Conduction welds are usually produced using low-power focused laser beams. This results in shallow welds with a low aspect ratio. In this work, high-power CO2 and YAG lasers have been used to produce laser conduction welds on 2mm and 3mm gauge AA5083 respectively by means of defocused beams. Full penetration butt-welds of 2mm and 3mm gauge AA5083 using this process have been produced. It has been observed that in this regime the penetration depth increases initially up to a maximum and then decreases with increasing spot size (corresponding to increase in distance of focus above the workpiece). Results of comparison of tensile strength tests for keyhole and conduction welds are shown.This process offers an alternative method of welding aluminium alloys, which have a high thermal conductivity.There are two laser welding mechanisms, keyhole mode and conduction mode. Keyhole welding is widely used because it produces welds with high aspect ratios and narrow heat affected zones. However keyhole welding can be unstable, as the keyhole oscillates and closes intermittently. This intermittent closure causes porosity due to gas entrapment. Conduction welding, on the other hand, is more stable since vaporisation is minimal and hence there is no further absorption below the surface of the material.Conduction welds are usually produced using low-power focused laser beams. This results in shallow welds with a low aspect ratio. In this work, high-power CO2 and YAG lasers have been used to produce laser conduction welds on 2mm and 3mm gauge AA5083 respectively by means of defocused beams. Full penetration butt-welds of 2mm and 3mm gauge AA5083 using this process have been produced. It has been observed that in this regime the penetration depth increases initially up to a maximum and then decreases with incr...

Influence of the overlapped area on the corrosion behaviour of laser treated aluminium alloys

Abstract Laser surface treatments offer significant potential for improvement of materials properties such as corrosion performance and wear resistance. In large area applications, overlapping of individual tracks corresponding to the width of the laser beam is often required. This involves the heat treatment of a region adjacent to the previous track with microstructural changes such as precipitate coarsening and microsegregation at the overlap regions, which can lead to detrimental effects on the corrosion performance. This paper reports the characteristics and corrosion behaviour of the overlapped areas in the laser melting and alloying with Cr, W, Zr–Ni or Ti–Ni of 2014 aluminium alloy. It is shown that microsegregation occurs within the planar front zone of laser melted samples and microstructural coarsening occurs in the heat affected zones caused by the reheating effect at the laser track overlaps. Electrochemical testing has indicated that microsegregation within the overlapped areas leads to initiation of pitting corrosion in most of these cases.

Laser removal of oxides from a copper substrate using Q-switched Nd:YAG radiation at 1064 nm, 532 nm and 266 nm

The objective of the work reported is to study the effect of wavelength and the material removal mechanisms of a laser process for removing copper oxide from copper. The removal of copper oxide is necessary in electronics device fabrication in order to improve the surface wetability and so achieve a good quality solder joint. Such a laser cleaning process could be incorporated into an existing laser soldering process. The effect of Q-switched Nd:YAG radiation at 1064 nm, 532 nm and . 266 nm the first, second and fourth Nd:YAG harmonics on the oxidised surface of a copper alloy foil was studied with increasing fluence. The laser-treated surfaces were characterised using optical microscopy, scanning electron microscopy . . SEM , and static secondary ion mass spectroscopy SSIMS . Successful oxide removal was achieved at all wavelengths above certain thresholds that defined the lower end of the process operating window for single-pulse operation. The ablation mechanisms involved in the removal of copper oxide from copper at 1064 nm, 532 nm and 266 nm were found to combine both thermal and mechanical effects, the surface melting first occurring at fluences lower than those at which cleaning is initialised. q 1998 Elsevier Science B.V.

CO2 laser cutting of slate

Slate is a natural stone which has the characteristic that shows a well-developed defoliation plane, allowing to easily split it in plates parallel to that plane which are particularly used as tiles for roof building. At present, the manufacturing of slate is mostly manual, being noisy, powdery and unsafe for the worker. Thus, there is a need to introduce new processing methods in order to improve both the working conditions and the quality of the products made of slate. Following the previous work focused on the drilling and cutting of slate tiles using a Nd : YAG laser, we present in this paper the results of the work carried out to explore the possibilities to cut slate plates by using a CO2 laser. A 1.5 kW CO2 laser was used to perform different experiments in which, the influence of some processing parameters (average power, assist gas pressure) on the geometry and quality of the cut was studied. The results obtained show that the CO2 laser is a feasible tool for a successful cutting of slate. r 2001 Elsevier Science Ltd. All rights reserved.