S. Jana

A technical review of the laser drilling of aerospace materials

Abstract Drilling cooling holes in aerospace materials with a neodymium-doped: yttrium aluminum garnet (Nd:YAG) laser is an established technique used in the aerospace industry. However, there are new challenges as the industry begins to use materials that can witstand higher temperatures and pressures. In this paper, the mechanisms of some laser-drilling processes, the types of laser used, the quality characteristics of a laser-drilled hole, the effects of drilling parameters, and the advantages and limitations of the laser hole-drilling operation are reviewed.

THE EFFECT OF METAL TYPE AND MULTI-LAYERING ON FRICTION SURFACING

Abstract Friction surfacing was attempted with a stainless steel, aluminium and brass consumables on a mild-steel substrate in the open air and in flowing nitrogen. Stainless steel formed a strongly-bonded thick layer but friction surfacing with both aluminium and brass was not possible. Both brass and aluminium consumables failed to form a heated layer in contact with the mild steel, high thermal conductivity of either metal being the probable cause of failure to perform friction surfacing. Nitrogen ventilation caused cooling of the heated layer between the stainless steel and the mild-steel substrate and lowered the quality of the coating. Multi-layer surfacing by stainless steel consumables was successful, with at least three strongly-bonded layers possible, provided that the surfaces are free of oily contaminants.

Friction surfacing of metal coatings on steel and aluminum substrate

Friction surfacing of (i) tool steel, inconel, aluminum and titanium rods onto mild steel substrates and (ii) stainless steel, mild steel and inconel onto aluminum substrates was investigated. It was found that tool steel and inconel were efficiently deposited onto steel to form a dense strong coating while aluminum was only deposited at high contact pressures. Titanium could not be deposited under the tested conditions. Stainless steel, mild steel and inconel could be deposited onto aluminum substrates to form dense coatings, however only SS displayed a lack of intermetallic compound formation with Al. Melting of Al by frictional heat was observed and this led to the formation of brittle and therefore undesirable intermetallic compounds at the interface of the coating. X-ray microscope investigations of the coatings confirmed that the structure of the coating is a continuous series of discrete layers inclined at a shallow angle to the phase of the substrate. Fissures were found between these layers close to the outer surface of the coating, which implies that the uppermost layers should be removed to obtain the best quality of coating. Microscopic observations revealed that there was little mixing between the coating and the steel substrate but some surface roughening and sub-surface deformation of the steel substrate by friction surfacing. The Al-alloy substrate showed similar types of surface alteration but to a far greater extent. Careful control of process parameters such as substrate, speed, load and consumable was necessary for coating quality.

Optimization of laser deep-hole drilling of Inconel 718 using the Taguchi method

Abstract The use of a Nd:YAG laser to drill deep cooling holes in aerospace components is a stringent operation. Various characteristics pertaining to the input beam, focussing lens, and assist gas have to be optimized in order to produce a hole economically and of sufficiently good quality. This paper reports the use of the Taguchi technique of experimental design in optimizing the process parameters for drilling deep-holes in nickel-based superalloy, Inconel 718. The thickness of the material is 25.0 mm. Oxygen is the assist gas andthe focal length of the focussing lens is 300 mm. The effects of five process parameters — pulse energy, pulse duration, pulse shape, focal position, and assist gas pressure — have been explored. The various parameters are assigned to an L18 orthogonal array. The primary response under study is the drilling time. It is predicted that a minimum drilling time of 31.51 s is needed to drill a hole with a pulse energy of 30.0 J, a pulse duration of 1.8 ms, a “treble” pulse shape, a focal position of 0.0 mm into the material, and an oxygen pressure of 0.35 MPa. Confirmatory experiments have produced results that lay within the 95% confidence interval.

A review of the Nd: YAG laser marking of plastic and ceramic IC packages

Abstract The marking of integrated circuit (IC) packages with neodymium-doped yttrium aluminium garnet (Nd: YAG) lasers has become a widely accepted technique in the microelectronics industry. This paper presents a literature survey and discussion of the various methods and mechanisms of Nd: YAG laser marking of plastic and ceramic IC packages. The effects of the material properties (e.g. the absorptivity and the melting point) and the marking parameters (e.g. power density, the focal position and the marking speed) on the mark legibility characteristics (e.g. the mark contrast and the mark width) are reviewed. Economic advantages and technological limitations of thr marking of IC packages with Nd: YAG lasers are discussed also.

Effect of heat input on the HAZ properties of two duplex stainless steels

Abstract Two duplex stainless steel, Avesta 2205 from Avesta AB, Sweden, and Cromweld 3Cr12 from Cromweld Steels Ltd, UK, were welded by shielded metal arc using arc energies of between 0.4 to 1.7 kJ/mm. The effects of varying heat inputs on the properties of the HAZ (heat affected zone), e.g., the hardness, the impact toughness, the tensile strength, the microstructure and the corrosion rate, are documented.

Study of the interfacial phenomena during friction surfacing of mild steel with tool steel and inconel

Friction surfacing was carried out with tool steel (AISI 01) and inconel 600 consumables on mild steel 1020 substrate in an argon atmosphere. Inconel bonded strongly with the substrate and there was evidence of interfacial compound formation between the substrate and coating. For tool steel coatings, a sharp boundary between the substrate and coating was observed by scanning electron microscopy. X-ray fluoroscopic imaging also revealed this boundary. Mechanical interlocking between the coating and the substrate appears to be insignificant so adhesion between the coatings and the substrate may be caused by solid-phase bonding. For friction surfacing of both tool steel and inconel, a nominal contact pressure as high as 21.8 MPa was required to obtain an adherent coating of uniform quality.

Study of the interfacial phenomena during friction surfacing of aluminium with steels

Friction surfacing was carried out with stainless steel 304 and mild steel 1020 consumables on to an aluminium 5083 substrate in an argon atmosphere. Mild steel bonded well with the substrate and there was evidence of interfacial compound formation whereas in the case of stainless steel consumable there was no evidence of mixing and the coating was found to have a rolled structure on the surface. No clear evidence of mechanical interlock was obtained for stainless steel on aluminium. In both cases a nominal contact pressure as high as 21.8 MPa was required to obtain a good coating. For the mild steel coating there was evidence of transfer of aluminium on to the coating and the matrix had a shear crack along the matrix/coating interface.

Pulsed laser processing of plasma sprayed thermal barrier coatings

Abstract The requirements for thermal barrier coatings on turbine blades in jet engines and gas turbines are stringent due to the extreme temperatures and constant thermal cycling that demand a material with excellent thermal shock resistant and thermal insulation properties. Plasma sprayed zirconia alloys have been found to fit this role with some degrees of success. However, the presence of surface pores and a network of inter-connecting micropores within the coatings accentuates the oxidation of the bond coat, Often leading to spallation of the ceramic coat. A pulsed mode Nd:YAG laser was used in the present study to process plasma sprayed zirconia alloy coatings. The results indicated the effective melting of the ceramic layer that yielded a shiny surface finish. Scanning electron microscopy observation of the fractured surfaces revealed distinct zones comprised of columnar grains, granular structures similar to that of sintered ceramics and the typical lamellar structure of plasma sprayed ceramic coatings. The crack area and depressions formed were quantified using an image and analyser. The results show that the average crack area in the treated surface increases with increasing laser energy density. However, the average area of depressions in the treated surface appears to be independent of the laser energy density input.

A review of the laser processing of aircraft components

Abstract Composite materials and some exotic alloys have been used in the fabrication of both structural and non-structural members of the air-frames and engines of aircrafts. Different machining processes, such as the use of traditional solid tools with modified cutters, ultrasonic cutting and drilling, water-jet cutting, abrasive water-jet cutting, electron beam welding, and various laser processes have been applied successfully to the processing of these materials. In this paper, the physical mechanisms, the advantages as well as the limitations, and examples of application of the laser processes that are used to machine aircraft components are reviewed.