Alan Lawley

Analysis of the spray deposition process

Abstract Net or near net shape products can be manufactured by technologies involving solidification processing, metal forming, paniculate processing, and droplet consolidation. One example of droplet consolidation is spray deposition in the Osprey tm mode. In this process, a stream of liquid metal is atomized by an inert gas to form a spray of molten droplets; these are accelerated towards a substrate where they impinge and consolidate. An integral model for the Osprey tm spray deposition process has been developed using established theoretical principles. Mathematical models describe the interconnected processes of droplet-gas interactions in flight and subsequent droplet consolidation on the substrate. The models predict droplet velocity and temperature as a function of flight distance, the extent of droplet solidification on arrival at the substrate, and temperature distribution in the consolidated material during deposition. This approach demonstrates the utility of modeling studies in order to establish quantitative guidelines for optimization of the process in terms of the evolution of microstructure in droplet consolidation.

Spray casting : an integral model for process understanding and control

Abstract This paper discusses the scientific and technological aspects of spray casting. An integral model is presented which encompasses and addresses each unit stage of the overall process, namely atomization, spray, consolidation, shape, pre-form solidification and microstructure. The atomization model uses an empirical relationship to correlate the droplet size distribution with spray-forming process parameters. The spray model quantities the condition of the spray upon impact with the substrate; the condition of the spray is represented in terms of the fraction of liquid in the spray and the proportion of solid, mushy and liquid droplets. The sticking efficiency is obtained from the consolidation model and was found to be a critical parameter which governs the yield, shape and microstructure of sprayed deposits. The shape model dynamically predicts the evolution of pre-form shape for different combinations of substrate and spray motion. The model for pre-form solidification uses a two-dimensional heat transfer analysis to compute temperature-liquid fraction profiles in the perform. The microstructure model predicts the final grain or cell size in sprayed deposits as a function of the local solidification time. Individual models are interlinked to identify and asses the effect of critical process parameters on the integrity of spray-cast product.

Processing Effects in Spray Casting of Steel Strip

Spray cast strip of AISI 1026 and M2 has been produced by the Osprey™ process under controlled conditions of deposition. Droplet flight distance was varied over the range 325 to 475 mm and strip was spray cast onto either planar or roller substrates of copper and steel. Substrate surface speed was in the range of 0.02 to 1 m/s, which produced strip of 0.025 to 0.0007 m thickness, respectively, with a width of 0.1 m. Surface condition, microstructure, and extent of porosity in the strip were characterized as a function of distance from top and bottom surfaces. The microstructure of the strip is comprised of three regions —a ‘chill zone’ at the bottom surface consisting of fine grains of ferrite and pearlite with numerous pores; a middle region containing equiaxed or columnar grains, Widmanstätten plates, and fine pores; and a top region made up of equiaxed grains comprising Widmanstätten plates and a few pores. Process variables of primary importance with respect to microstructural integrity and surface condition of the strip are substrate velocity, the surface condition of the substrate, flight distance, and the uniformity of droplet flux in the spray cone. Flight distance determines the amount of cooling of the droplets by the atomizing gas and, therefore, the average temperature of the spray incident on the substrate. Microstructure is determined by convective cooling of the spray, and, to a lesser extent, by the substrate velocity and temperature. The processing conditions required to spray cast strip with a homogeneous microstructure and uniform thickness/surface condition have been established.

SPRAY CASTING OF STEEL STRIP : PROCESS ANALYSIS

Near-net shape manufacturing (NNSM) of thin steel sections by spray casting eliminates casting as a separate step with attendant improved microstructures and properties and significant energy savings. The process involves atomization of a stream of liquid metal and deposition of droplets in the generated spray on a moving substrate at mass flow rates of 0.25 to 2.5 kg/s. In this paper, NNSM of steel strip by the Osprey spray casting process is investigated by combining numerical simulation and experiments. Critical input parameters for the computation are quantified utilizing existing state-of-the-art mathematical models and specific experiments. Numerical computation of the consolidation of the spray at the substrate during manufacture of thin sections is conducted using bothcontinuum anddiscrete event (“splat solidification”) approaches to predict: (1) variation of strip thickness in the transverse dimension and (2) isotherms and cooling rates across the strip thickness. Predicted geometries of the strip simulated by the continuum model are in good agreement with measurements. Predicted isotherms in narrow strip by the continuum approach are in reasonable agreement with thermocouple measurements for intermediate thicknesses (2 to 5 mm), and the observed microstructure is consistent with predicted cooling rates. The discrete event model predicts significantly higher cooling rates than the continuum model in the basal portion of the strip. This is consistent with the observed grain size in thin strip (<l-mm thick) and in the basal portion of thick strip. Beyond a threshold thickness, however, the discrete event model confirms the formation and persistence of a partially liquid layer at the growing surface of the deposit with an attendant decrease in the cooling rate. The influence of critical parameters on “splat solidification” is analyzed and assessed.

Creep and microstructural stability of dispersion strengthened AlFeVSiEr alloy

Abstract A planar flow cast AlFeSiV alloy modified with 0.75 wt.% Er was evaluated in terms of creep response and elevated temperature stability. Creep tests were conducted at temperature and stresses ranging from 291 to 426 °C and 71.86 to 186.4 MPa respectively. In addition, isothermal coarsening was assessed at temperatures of 375, 475 and 525 °C for times up to 456 h. Steady state creep rates, at 375 °C were in the range 1.7 × 10 −10 to 1.2 × 10 −5 s −1 . Creep properties were analyzed in terms of Arrheninius-type equations and the creep response was found to be controlled by the self diffusion of aluminium. Coarsening was monitored by hardness measurements and dispersoid size and was found to be insignificant at 375 °C. At 475 and 525 °C the average precipitate size increased from 0.087 μm to 0.163 μm and 0.195 μm respectively after 456 h; this resulted in softening of the alloy. The measured coarsening rates were in good agreement with the rates predicted by the Lifshitz-Slyozov-Wagner (LSW) theory.

Microstructure and properties of spray cast CuZr alloys

CuZr alloys were produced by spray casting in the Osprey mode with zirconium levels in the range 0.1–0.8 wt.%. Microstructures were characterized and mechanical properties and electrical conductivity monitored in the spray cast condition, and following thermo-mechanical processing. In the spray cast condition, the alloys exhibit a fine equiaxed grain structure with no macro segregation. Two phases are present at the grain boundaries, one of which was identified as Cu5Zr. This phase is stable during solution treatment and does not contribute to precipitation strengthening on aging. Strengthening during aging is attributed to the formation of zirconium-rich clusters in the copper. The grain boundary network of Cu5Zr enhances resistance to grain growth during recrystallization. Attractive combinations of tensile strength and electrical conductivity were achieved by thermo-mechanical processing of the spray cast Cu0.4 wt.% Zr alloy, for example 525 MPa/87% international annealed copper standard (IACS). This combination is superior to strength-electrical conductivity combinations in ingot metallurgy or powder-processed CuZr.

Work of fracture in aluminum metal-matrix composites

The effect of isothermal exposure and thermal cycling on the toughness of B/Al (1100), B/Al (6061), and A12O3/A1 composites has been investigated. In B/Al (1100), isothermal exposure at 773 K for 45 × 104 s (125 hours) reduced toughness, measured by the work of fracture, from 76 kJm-2 to 10 kJm-2, and a similar reduction occurred after equivalent thermal cycling. The corresponding reduction in toughness after isothermal exposure in B/Al (6061) was from 44.5 kJm-2 to 8 kJm-2; however, the effect of thermal cycling was less detrimental. In the FP-A12O3/A1 composite, the work of fracture was insensitive to both forms of thermal treatment. Changes in the toughness of the B/Al composites have been correlated with and analyzed in terms of modifications to matrix, fiber, and interface properties, in particular, matrix softening, interface reaction products, and fiber notch sensitivity.

Process control, modeling and applications of spray casting

Spray casting involves sequential atomization and droplet consolidation at deposition rates above 0.25 kg/s, and at least eight independent process parameters must be optimized to achieve the desired preform shape, microstructure and yield. Because effective utilization of spray casting requires control of the preform shape with metallurgical integrity, there is a compelling need to quantify the influence of process parameters on shape, microstructure and overall yield. Coupling this knowledge base with appropriate sensor and control technology establishes a means for process control.

Mechanical behavior of powder metallurgy AlFeNi alloys

Abstract The tensile and creep response of three P/M processed AlFeNi dispersion-strengthened alloys has been evaluated at temperatures up to 400°C; dispersoid (FeNiAl 9 ) volume fractions were 0.19, 0.25 and 0.32. Ambient temperature strength increases with increasing volume fraction of dispersoid and decreases with dispersoid size for a given volume fraction. Strengthening at room temperature in these alloys is primarily due to the fine grains stabilized by the dispersoids. The observed yield strength is explained by the Hall-Petch relationship with a grain size exponent larger than 0.5 and a significantly higher friction stress than that of pure Al. The yield strength of the alloys decreases with increasing temperature. Below about 250 °C it is still determined by the grain size, but above 250 °C, the deformation mechanism changes and dispersoid volume fraction and size do not have a significant effect on yield strength. Fracture mode also exhibits a change at about 250 °C; below this temperature tensile failure occurs by void initiation around the grain boundary dispersoids and subsequent coalescence leading to fracture while above 250 °C the fracture surface exhibits coarse features. Creep data are independent of dispersoid volume fraction in the temperature range 250–400 °C. Sherbys structure-invariant dislocation model and Arzts interface reaction-controlled diffusional creep model have been found to be consistent with the experimental creep data.

Powder metallurgy T15 tool steel: Part I. Characterization of powder and hot isostatically pressed material

The microstructure and constitution of T15 tool steel processed from gas-atomized powder have been characterized. From the atomized powder, four particle size ranges (≤840, 250 to 840, 44 to 100, and ≤44 Μm) were consolidated to full density by hot isostatic pressing (“hipping”) at 1130 ‡C or 1195 ‡C. Both atomized powder and consolidated material were examined by means of optical and electron microscopy, X-ray diffraction, chemical analysis, and micro-hardness. A segregated structure exists in the gas-atomized powder, independent of particle size; MC and M2C carbides are present, primarily at cell boundaries. The matrix of the powders is a mix of martensite and retained austenite. Weight fraction and overall composition of the carbides are insensitive to particle size, but the proportion of MC carbides increases with decreasing particle size. After consolidation, MC, M6C, and M23C6 carbides are present in a ferrite matrix. The carbide size distribution is skewed to larger carbide sizes at the higher consolidation temperature, independent of the prior particle size fraction, but there is no significant change in carbide volume fraction. For a given consolidation temperature, the size distribution of the MC and M6C carbides is broader for the coarser particle size fractions.