Mathieu Brochu

Spark plasma sintering of prealloyed aluminium powders

Abstract The central objective of this research was to assess the effects of temperature and particle size on the spark plasma sintering (SPS) response of two prealloyed aluminium powders atomised from wrought alloys AA 2024 and AA 7075. A SPS temperature of 400°C was found to yield fully dense specimens of both alloys with hardness values that were comparable to the starting wrought ingot materials. Such samples also exhibited appreciably lower concentrations of residual oxygen and hydrogen when compared to those present in the raw powders. Degassing experiments completed through thermogravimetric analyser (TGA)–gas chromatography (GC)–mass spectrometry (MS) indicated that the release of CO2 and adsorbed/chemisorbed H2O were responsible for the enhanced purity of the SPS products. Particle size was also a factor of influence with the most favourable results for density and minimised O/H concentrations achieved with particles ≧180 μm in diameter.

Consolidation of cryomilled Al–Si using spark plasma sintering

Cryomilled eutectic aluminum–12% silicon powder was sintered using spark plasma sintering (SPS) to create bulk compacts. The cryomilling serves to break up and disperse the eutectic phase in the powder to create a well-distributed Si phase throughout the matrix and to modify the morphology of the Si phase from plate-like to spherical, whilst refining the aluminium grain size to the nanometric level. The effects of different sintering times and temperatures using SPS on the densification of the powder, the aluminium grain size evolution, the growth of the Si phase and the morphology change of the Si phase were investigated. The compacts were analysed using X-ray diffraction, scanning electron microscopy and optical microscopy. The initial stages of densification appear to be highly dependent on the yield strength of the powder. An estimate of the temperature gradient seen in the powder bed was made and calculated to be near 200 °C at the highest point. The Al and Si phase growth was investigated and it was observed that the Si coarsening rate is increased due to the increased volume of grain boundaries. As the Si coarsens, any pinning effect on the Al grains is lost, resulting in a highly unstable microstructure that coarsens rapidly.

Development of Metastable Solidification Structures Using the Electrospark Deposition Process

Electrospark deposition (ESD) was studied to investigate the possibility of forming non-equilibrium microstructures in Al alloy systems. The results have demonstrated that the level of supersaturation is system-dependent. The Al-10 wt% Mg system exhibited a complete supersaturation, while the Al-Si system had a supersaturation level reaching about 1wt%. The ESD process was also used to demonstrate the possibility of forming metastable intermetallics in the Al-Ni system, such as the Al9Ni2. When comparing the present results with the literature of rapid solidification of these alloys using other processes, the discrepancy between the extent of non-equilibrium microstructure obtained is believed to be associated with the formation and duration of the weld pool prior to solidification.

Supersolidus Liquid Phase Sintering Modeling of Inconel 718 Superalloy

Powder metallurgy of Inconel 718 superalloy is advantageous as a near-net shape process for complex parts to reduce the buy-to-fly ratio and machining cost. However, sintering Inconel 718 requires the assistance of supersolidus liquid formation to achieve near full density and involves the risk of distortion at high temperatures. The present work is focused on modeling the onset of sintering and distortion as a function of temperature, grain size, and part geometry for Inconel 718. Using experimental sintering results and data available in the literature, the supersolidus liquid phase sintering of Inconel 718 was modeled. The model was used to define a processing window where part distortion would be avoided.

Recyclability assessment of Al 7075 chips produced by cold comminution and consolidation using spark plasma sintering

A solid-state recycling route for processing aluminium alloy 7075 chips is proposed. This process involves pulverizing the chips using an industrial scale comminution line to produce a broad particle size distribution of granulated products and powders. Upon sieving, the −140 mesh fraction of the pulverised material was consolidated by spark plasma sintering into fully dense compacts, i.e. >99%. The properties of the pulverised powders and compacts were compared to commercially available aluminium alloy 7075 powder used in industrial press-sinter applications. The comminution line produced powder particles which possessed a 7·9 ± 0·6-nm thick native oxide layer with adhering exogenous contaminants, and aluminium oxide particles dispersed throughout the powder. The presence of the oxide layer and exogenous contaminants was found to be deleterious to the powder consolidation and mechanical properties. A flexural strength and strain and hardness of 94·7 ± 7·2 MPa, 2·1 ± 0·3% and 69·3 ± 5·2 HV were obtained, respectively.

Fatigue Crack Growth Behavior of 2099-T83 Extrusions in two Different Environments

Aluminum-lithium alloy 2099-T83 is an advanced material with superior mechanical properties, as compared to traditional alloys used in structural applications, and has been selected for use in the latest generation of airplanes. While this alloy exhibits improved fatigue crack growth (FCG) performance over non-Li alloys, it is of interest to simulate the impact of fluctuating loads under variable temperature during airplane service, particularly in terms of the potential effects of material processing history. In the present paper, the FCG behavior in an Integrally Stiffened Panel (ISP) has been investigated both at room temperature and at 243 K. It has been shown that the resistance to crack growth in a cold environment was higher than in ambient laboratory air. Results of this investigation are discussed from the microfractographic point of view, with regard to the variation of the local extrusion aspect ratio, a parameter which correlates with both the crystallographic texture and the grain structure.

Bulk nanostructure and amorphous metallic components using the electrospark welding process

Purpose – The purpose of this paper is to outline the feasibility of using the electrospark welding (ESW) process to free‐form metallic components with nanostructured or amorphous microstructures.Design/methodology/approach – ESW was used to deposit amorphous and nanostructure coatings for high‐wear resistance applications. The ESW process was also used to freeform three‐dimensional objects via multiple deposition passes. The near‐net shape capability is interesting as it significantly reduces the post‐processing operations.Findings – This paper demonstrates that it is possible and economically feasible to produce components possessing metastable structures, i.e. nano or amorphous, using the ESW process.Practical implications – The ESW process possesses the ability to manufacture advanced materials and can pattern surfaces to provide appropriate functionality with respect to the service environment.Originality/value – This paper represents a summary of the capabilities of ESW to fabricate advanced materia...

Determination of E2 for nitride ceramics using FE-SEM and the Duane-Hunt limit procedure.

This work demonstrates the possibility of using the Duane-Hunt limit of the bremsstrahlung to determine E2 values of Si3N4 and AlN ceramics. The E(DHL) versus E0 graph demonstrates that for conductive materials, the experimental curve is parallel to the theoretical (E(DHL) = E0), but both curves cross in the case of insulators. The intersection points (E2 value), are 3.01 keV for Si3N4 and 2.67 keV for AlN. Imaging of ceramic grain structure at high magnification was performed to demonstrate the validity of the calculated E2 values.

Erratum to: Microstructure Evolution and Rapid Solidification Behavior of Blended Nickel-Based Superalloy Powders Fabricated by Laser Powder Deposition

Laser powder deposition was performed on a substrate of Inconel 738 using blended powders of Mar M247 and Amdry DF3 with a ratio of 4:1 for repairing purposes. In the as-deposited condition, continuous secondary phases composed of γ-Ni3B eutectics and discrete (Cr, W)B borides were observed in inter-dendritic regions, and time-dependent nucleation simulation results confirmed that (Cr, W)B was the primary secondary phase formed during rapid solidification. Supersaturated solid solution of B was detected in the γ solid solution dendritic cores. The Kurz–Giovanola–Trivedi model was performed to predict the interfacial morphology and correlate the solidification front velocity (SFV) with dendrite tip radius. It was observed from high-resolution scanning electron microscopy that the dendrite tip radius of the upper region was in the range of 15 to 30 nm, which yielded a SFV of approx 30 cm/s. The continuous growth model for solute trapping behavior developed by Aziz and Kaplan was used to determine that the effective partition coefficient of B was approximately 0.025. Finally, the feasibility of the modeling results were rationalized with the Clyne–Kurz segregation simulation of B, where Clyne–Kurz prediction using a partition coefficient of 0.025 was in good agreement with the electron probe microanalysis results.

The effects of applied current on one-dimensional interdiffusion between copper and nickel in spark plasma sintering

Spark plasma sintering (SPS) is a powder metallurgy technique that employs the use of fast sintering kinetics to produce final consolidated components in a matter of minutes. In order to use blended powders in SPS to obtain fully alloyed parts, diffusion during sintering must be understood. An investigation into the effects of current on the diffusion of copper and nickel was performed using SPS. Bulk specimens were used to generate diffusion couples in SPS in alternating orientations with respect to the direction of the current. Control samples were produced using a horizontal insertion vacuum furnace. Experiments were performed at temperatures between 850 °C and 1000 °C for 3 h. Concentration profiles were obtained by the use of both energy-dispersive spectroscopy and a Monte Carlo simulated correction curve. Diffusion coefficients and activation energies were calculated for samples produced by SPS and annealing without current. It was shown that, at temperatures near 0.9 Tm, the application of current ...