Andrew R. Kennedy

Laser cladding of aerospace materials

Abstract In recent years, the aerospace industry has devoted a large amount of resources to the research and development of new repair technologies for gas turbine components. Traditionally their main repair tool is tungsten inert gas (TIG) welding but a new non-traditional process is emerging called laser cladding. This paper shows that by using this process, protective coating materials can be clad onto aerospace component substrates. We have shown that it has the potential to form pore-free and crack-free coatings. Two cladding materials, in the form of fine alloy powder, were clad onto five different substrate materials. The microstructure, hardness, cracking, porosity and dilution levels were recorded in each case and compared to TIG welded samples. Some of these results are discussed here.

The effect of TiH2 heat treatment on gas release and foaming in Al–TiH2 preforms

TiH2 powders were heat treated and compacted with pure Al powder to make foamable precursors. Differential scanning calorimetry showed that hydrogen is normally released at approximately 495 C and that heat treating in air delays hydrogen evolution to higher temperatures. Heat treatment did not, however, prevent gas being released before the melting of the pure Al precursor. 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

The microstructure and mechanical properties of TiC and TiB2-reinforced cast metal matrix composites

TiC and TiB2 particles have been spontaneously incorporated into commercial purity aluminum melts through the use of a K-Al-F-based liquid flux that removes the oxide layer from the surface of the melt. The combination of spontaneous particle entry and close crystal structure matching in the Al-TiB2 and Al-TiC systems, results in low particle-solid interfacial energies and the generation of good spatial distributions of the reinforcing phase in the solidified composite castings. The reinforcement distribution is largely insensitive to the cooling rate of the melt and the majority of the particles are located within the grains. Modulus increases after TiC and TiB2 particle additions are greater than those for Al2O3 and SiC. It is thought that interfacial bonding is enhanced in the TiC and TiB2 systems due to wetting of the reinforcement by the liquid and particle engulfment into the solid phase. TiC-reinforced composites exhibit higher stiffnesses and ductilities than TiB2-reinforced composites. This has been attributed to stronger interfacial bonding in the Al-TiC system, due to the increased tendency for nucleation of solid on the particle surfaces.

The microstructure and mechanical properties of Al-Si-B4C metal matrix composites

B4C particles have been added to molten Al- 7wt% Si- 0.3 wt% Mg alloys, at levels of 5 and 10 wt%, using a propriatory K-Al-Ti-F flux. The resulting composites were examined metallographically and mechanically tested in the as-manufactured condition and after heat treatment for 48 hours at 500°C and 700°C. During incorporation into the melt, a complex Ti-B-C reaction layer was formed on the particle surfaces. The reaction layer remained intact during heat treatment and the stable, protective nature of this layer gave rise to a significantly reduced rate of particle degradation compared to other Al-B4C composites. Significant increases in stiffness were observed; modulus increases per volume percent of particles added were similar to those for the Al-TiC system where strong interfacial bonding occurs. Improved adhesion between the solidified matrix and the B4C reinforcement was encouraged by the enhanced metallic character of the reaction layer. Solid state reaction at 500°C produced little change in mechanical properties. Heat treatment and reaction at 700°C resulted in an increase in the volume fraction of stiff, brittle reinforcing phases, leading to an increase in stiffness and a decrease in ductility.

Carbide stoichiometry in TiCx and Cu–TiCx produced by self-propagating high-temperature synthesis

Abstract TiC x and Cu–TiC x have been formed by self-propagating high-temperature synthesis (SHS) from elemental powder mixtures with a range of C/Ti ratios. When no copper was present, the carbide stoichiometry closely followed that of the starting powders. In the presence of copper, formation of copper–titanium intermetallics and solid solutions resulted in a different carbide stoichiometry.

The incorporation of ceramic particles in molten aluminium and the relationship to contact angle data

Abstract A number of ceramic particles have been added to molten aluminium using a K–Al–F flux-assisted casting process. It was impossible to incorporate oxides and covalently bonded ceramics using this method but several transition metal carbides, borides and nitrides, which have metal-like bonding, were readily incorporated. Incorporation events have been linked to both contact angle data in the literature and established trends in wetting behaviour. It was found that contact angle measurements made at temperatures similar to those experienced during the flux-assisted incorporation process, do not predict the successful incorporation of transition metal compounds.

Characterising particle-matrix interfacial bonding in particulate Al-TiC MMCs produced by different methods

Abstract By measuring the change in elastic modulus with increasing plastic strain, particle–matrix bonding in Al–TiC MMCs has been characterised. Data show that rates of damage accumulation are lowest, damage initiation stresses are highest and hence interfacial bonding is strongest, in cast composites. It is thought that this is due to the attainment of intimate contact between Al and TiC, through the use of a flux that dissolves surface oxides, and the nucleation of solid Al on the particle surfaces. Intimate metal–ceramic contact is not so easily achieved in composites manufactured from metal powders, owing to the presence of oxide films on their surfaces. Increasing the shear stresses present during solid state consolidation processes does, however, improve the particle–matrix interface strength due to more effective break-up of the oxide barrier.

Reaction in Al–TiC metal matrix composites

Reactions in Al–10 wt.% TiC metal matrix composites have been investigated by heating samples between 600 and 900°C for 48 h and holding at 700°C for periods up to 240 h. X-ray diffraction, scanning electron microscopy and image analysis have been used to identify the composition, morphology and quantities of the reaction phases present. A maximum reaction rate was observed at 700°C and at this temperature the reaction products formed were large Al3Ti precipitates in the bulk of the matrix and Al4C3 blocks at the particle–matrix interface. At 900°C, TiC appeared to be stable in Al. The reaction kinetics for TiC followed a parabolic rate indicating a diffusion-controlled process. The rate of TiC dissolution is very much less than that for SiC in pure Al.

Sliding wear behaviour of aluminium-based metal matrix composites produced by a novel liquid route

TiC-reinforced MMCs have been produced in a range of aluminium alloys using a novel casting technique which results in spontaneous incorporation of the particles into the melt and thus strong bonding between the particles and the matrix. The sliding wear behaviour of the extruded composites has been studied as a function of load and particle volume fraction and has been compared with a commercially available SiC-reinforced composite. In all cases, alloy reinforcement resulted in a reduction in wear rate and an increase in the load at which the transition from low rate wear to high rate wear occurred. In the low rate wear regime, the wear coefficients of all the alloys in both the reinforced and unreinforced states were similar, and since the TiC-reinforced A356 alloy was the hardest (due in part to the grain refining action of TiC), it exhibited the lowest wear rate (lower than that of the SiC-reinforced composite). Wear of the steel counterface depended on the mechanism of wear of the composite. An increase in load generally resulted in an increase in wear rate of both the composite pin and counterface, and the reasons for this are presented. Increasing the volume fraction of particles in a composite reduces its wear rate but generally increases the wear rate of the counterface. It is suggested that when both counterface and composite wear are considered, an optimum volume fraction of particles exists at which wear is lowest.

The wetting and spontaneous infiltration of ceramics by molten copper

Infiltration trials have been conducted by filling Cu tubes withceramic powders and melting them under argon. No externalforces were applied; successful infiltration of the ceramicrelied solely upon favourable metal-ceramic wetting conditions.Oxides and covalently bonded compounds could not bespontaneously infiltrated but transition metal compounds such asNbC, Cr3C2, WC, NbB2 and Cr2N were. It was impossible toinfiltrate any ceramics when oxygen was present in the system.Contact angle data in the literature were found to predict, withfair reliability, infiltration events in Cu-ceramic systems.The good correlation is thought to be due to the ease with whichthe oxide film can be prevented from forming on molten Cu duringboth sessile droplet experiments and infiltration processing.