D. R. F. West

Metal-ceramic functionally gradient material produced by laser processing

A technique of injection with a powder feed of a mixture of metal + ceramic which combines the processes of laser alloying, cladding and injection, has been applied to study the feasibility of using a continuous wave CO2 laser to produce a functionally gradient material. A 2 kW CO2 laser has been used to produce, on a nickel alloy substrate, single alloy/clad tracks and three totally overlapping clad tracks using powder mixtures of Al-10 wt% SiC, Al-30 wt% SiC and Al-50 wt% SiC, respectively. The variation of composition and structure with position in the processed material has been investigated with reference to the effect of processing traverse speed and the powder feed rate.

The constitution of the Ni−Al−Ru system

The constitution of the Ni-Al-Ru system has been investigated in the range 0 to ∼50 at% Al. Isothermal sections at 1523 and 1273 K have been determined using microstructural observations, electron probe microanalysis and X-ray diffraction. The phases present were: nickel-based solid solution (γ); γ′ (based on Ni3Al); solid solutions based on NiAl and RuAl, respectively (designated β1 and β2), and ruthenium-based solid solution (Ru). The maximum solubility of Ru in γ′ was ∼5 at%. β1, and β2 show extensive range of solubilities, namely up to ∼20at% Ru in β1 and up to ∼25 to 35 at% Ni in β2. Three-phase equilibrium between γ, β2 and (Ru) existed at 1523 and 1273 K. Also at 1523 K, three-phase equilibria existed between γ, γ′ and γ1 and γ,β1 and γ2, while at 1273 K, the equilibria were between γ′, γ1,γ2 and γ, γ′, β2 indicating the occurrence of a reaction γ+β1, → γ′ +β2 at a temperature between 1523 and 1273 K. Liquidus features have been deduced from data on as-solidified structures. Lattice parameter data and hardnesses are also reported.

Functionally gradient titanium-aluminide composites produced by laser cladding

The laser surface cladding of Ti-Al/TiB2 composites was investigated as a means of producing a functionally gradient material on a commercially pure Ti substrate. Single and double layers were produced. The processing parameters were: 1.7 kW laser power, 3 mm beam diameter, 3–22 mms−1 traverse speeds, 2.2–6.2 g min−1 powder flow rates. The results showed that functionally gradient Ti-Al/TiB2 systems ∼ 2 mm thick, with a progressive increase in the Al content from 0 to 35 wt% Al (50 at % Al) could be produced by vertically overlapping two clad layers using powder mixtures of selected compositions. Microstructural and compositional characterization were performed on single-layer and two-layer clads. Preliminary wear testing carried out on a laser surface Ti-Al/TiB2 single-clad layer and on commercial-purity titanium demonstrated the beneficial effect of laser cladding on the wear resistance.

PULSED LASER PROCESSING OF THERMAL BARRIER COATINGS

Results are reported of the effects of surface melting (sealing) produced by a 1 kW laser in pulsed mode on the structure of plasma-sprayed 8wt% yttria partially stabilized zirconia (YPSZ); pulse lengths in the range of 1 to 90msec were used. Smooth surfaces were produced with shallow cracks at values of laser energy 5 to 40 J. Comparison of the data is made with results obtained by sealing using continuous wave CO2 laser processing.

Characterization of plasma-sprayed layers of fully yttria-stabilized zirconia modified by laser sealing

Abstract The effect of laser surface melting of plasma-sprayed layers of 20wt.% yttria-stabilized zirconia was investigated. The plasma-sprayed material consisted of approximately 90 mol% cubic (c) phase and approximately 10 mol% tetragonal (t) phase; on laser sealing at relatively high specific energies the proportion of the c phase increased. No t′ phase was observed either in plasma-sprayed or sealed layers. The microstructure of the sealed layers was of cellular or dendritic morphology depending on the processing parameters. Cracks formed during sealing penetrated through the sealed thickness and, in some cases, even through the unsealed region (plasma-sprayed zone) down to the bond layer.

Fibre/matrix interactions in magnesium-based composites containing alumina fibres

An investigation has been made of composites with magnesium-based matrices CPMg, AZ61 and AZ91 reinforced with Safimax low-density (LD), standard-density (SD) and RF Saffil alumina fibres, using either a squeeze or a gas-pressure casting route. Detailed investigations of structural features have been made using SEM, TEM and EDX analysis. The overall extent of reaction between matrix and fibre was affected by the volume fraction of fibres and (locally) by the formation of metal channels between fibre bundles. Fibre microstructure and porosity are the key features which significantly influence the extent of chemical interaction. LD (Safimax) alumina fibres were fully reacted and cannot be employed to produce liquid-metal infiltrated composites, unless a method to stabilize or protect the fibres can be found. In the case of SD Safimax fibres, the metal/ceramic interaction produced a considerable penetration of magnesium into the fibres. However, there was negligible chemical reaction in composites containing RF Saffil alumina fibres.

Microstructures of titanium-aluminides produced by laser surface alloying

The microstructures of Ti-Al layers (from 43–80 at %Al) produced by laser surface alloying of titanium substrate with a powder feed technique have been investigated. The laser processing parameters were; 1.8 kW laser power, 3 mm beam diameter, 7 mm s−1 traverse speed, and values of powder flow rates of aluminium ranging from 0.07–0.11 g s−1. The microstructures were dendrites of α2 and interdendritic regions of α2+γ in the Ti-43 at %Al alloy; dendrites of either α2 or α2+γ with interdendritic γ in the Ti-50 at %Al alloy; dendrites of α2+γ with interdendritic γ in the Ti-55 at %Al alloy; single phase γ in the Ti-60 at %Al alloy and TiAl3 dendrites and Al solid solution in the interdendritic regions in the Ti-80 at %Al alloy. The microstructures were fine and comparable to those produced by other methods of rapid solidification processing. The microstructures of the Ti-50 and Ti-55 at %Al alloys were in agreement with the existence of the peritectic reactions:L + β → α andL + α →γ, in the Ti-Al system.

Thermal barrier coatings produced by laser cladding

A 2kW C02 laser has been used to clad a mild steel substrate with two different ceramic coatings, namely yttria partially stabilized zirconia (8 wt% YPSZ) or a mixture of YPSZ and pure alumina powder. A range of laser processing parameters has been investigated. Results have been obtained showing the possibility of using the laser beam for producing a clad layer of thermal barrier coating with different topography depending on the processing conditions.

Pulsed laser sealing of plasma-sprayed layers of 8 wt % yttria stabilized zirconia

Results are reported of the laser surface sealing of plasma-sprayed layers of 8 wt% yttria partially stabilized zirconia (YPSZ) using pulsed treatments with powers of 0.4 and 1 kW. The structural features of the processed material were examined for a range of laser processing parameters including preheating, processing temperature and power density. By controlling the processing parameters it was possible to produce pulsed laser sealed layers of similar, or even better, quality than those produced by a continuous wave laser.

Stability of ZrO2-Y2O3 t' phase formed during laser sealing

Abstract Results are reported concerning the effect of thermal treatments in the temperature range 1050–1500°C for 1–1000 h on the stability of the t′ phase present in laser sealed layers of ZrO2–8·5 wt-% Y2O3. Aging for up to 1000 h at 1050°C is not sufficient to cause significant transformation of the t′ phase. At temperatures >1300°C, the t′ phase is less stable and transforms to mixtures of the t and c phases in proportions which are a function of both the time and temperature of aging. An improvement in the fracture toughness of the sealed layer was found to occur when substantial proportions of t phase were produced by annealing.MST/1529