Quasi-isostatic Forging Research Articles

On the thermal stability of ultrafine-grained Al stabilized by in-situ amorphous Al2O3 network

Bulk Al materials with average grain sizes of 0.47 and 2.4µm, were fabricated by quasi-isostatic forging consolidation of two types of Al powders with average particle sizes of 1.3 and 8.9μm, respectively. By utilizing the native amorphous Al2O3 (am-Al2O3) film on the Al powders surfaces, a continuous, ∼7nm thick, am-Al2O3 network was formed in situ in the Al specimens. Systematic investigation of the changes to the am-Al2O3 network embedded in the Al matrix upon heating and annealing up to 600°C was performed by transmission electron microscopy (TEM). At the same time, the stability of the Al grain structure was studied by transmission Kikuchi diffraction (TKD), electron back-scatter diffraction (EBSD), and TEM. The am-Al2O3 network remained stable after annealing at 400°C for 24h. In-situ TEM studies revealed that at temperatures ≥450°C, phase transformation of the am-Al2O3 network to crystalline γ-Al2O3 particles occurred. After annealing at 600°C for 24h the transformation was completed, whereby only nanometric γ-Al2O3 particles with an average size of 28nm resided on the high angle grain boundaries of Al. Due to the pinning effect of γ-Al2O3, the Al grain and subgrain structures remained unchanged during annealing up to 600°C for 24h. The effect of the am-Al2O3→γ-Al2O3 transformation on the mechanical properties of ultrafine- and fine-grained Al is discussed from the standpoint of the underlying mechanisms.

Mechanical behavior of microstructure engineered multi-length-scale titanium over a wide range of strain rates

In this work, we studied the mechanical behavior of commercial purity Ti powder consolidates with an engineered microstructure containing multiple-length-scale features, over a wide range of loading rates. The microstructural length scales were engineered by mixing powders of different sizes, followed by either hot quasi-isostatic forging (QIF) or spark plasma sintering (SPS). We used electron backscatter diffraction and transmission electron microscopy to examine the microstructure of the Ti materials. A bimodal grain size distribution has been achieved for the majority of the QIFed samples, while those consolidated via SPS exhibit a near-equiaxed morphology. All samples synthesized with powders milled in liquid argon show considerable uniform plastic deformation under quasi-static compression, with no failure, and their strength values are considerably high when compared to those of commercial purity Ti. Moreover, the materials consolidated from milled powders exhibit adiabatic shear banding under high rate uniaxial compression via the Kolsky bar technique. Samples prepared from a preselected proportion of powders milled in liquid nitrogen showed quasi-static strength as high as 2000MPa, and dynamic peak stress as high as 2700MPa, comparable to the strength of high-strength steels. However, these super-strong Ti samples are brittle under both quasi-static and dynamic compression. The strengthening of these Ti materials with an engineered microstructure is primarily attributed to the presence of interstitials. Twins were observed in nanometer-sized grains in the strongest and brittle samples, along with evidence of the TiN phase, which was attributed to exposure to a high level of nitrogen introduced during milling in liquid nitrogen. The high rate behavior can be rationalized on the basis of an adiabatic shear band model that takes into account strain and strain rate hardening.

The Influence of Processing on Microstructural Development, Tensile Response and Fracture Behavior of Aluminum Alloy 5083

In this paper, the specific influence of quasi-isostatic forging and rolling of cryomilled powder on microstructural development and resultant tensile deformation and fracture behavior of aluminum alloy 5083 is highlighted and comparison made with the coarse grained counterpart. The specific influence and contribution of strain hardening to enhancing strength of the ultra-fine grain microstructure of the aluminum alloy is presented and discussed. It is shown that the capability of the ultra fine grain microstructure to recover strength through the mechanism of work hardening is quite similar to the conventionally processed counterpart. The influence and role of intrinsic microstructural features in governing tensile deformation and fracture behavior is elaborated upon. The viable microscopic mechanisms governing final fracture behavior is discussed in light of the competing and mutually interactive influences of nature of loading, intrinsic microstructural effects, and deformation kinetics. Key Words: aluminum alloy 5083, processing, microstructure, tensile properties, fracture

Effect of grain size on strain rate sensitivity of cryomilled Al–Mg alloy

Al–Mg alloy powder was cryomilled to achieve a nanocrystalline (NC) structure having an average grain size of 50 nm with high thermal stability, and then consolidated by quasi-isostatic forging. The consolidation resulted in a bulk material with ultrafine grains of about 250 nm, and the material exhibited enhanced strength compared to conventionally processed Al–Mg alloy. The hardness of as-cryomilled powder, the forged ultrafine-grained (UFG) material, and the conventional coarse-grained (CG) alloy were measured by nanoindentation using various loading rates, and the results were compared with strain rate sensitivity (SRS) from uniaxial compression tests. Negative SRS was observed in the cryomilled NC powder and the forged UFG material, while the conventional alloy was relatively insensitive to strain rate. The dependence on loading rate was stronger in the NC powders than in the UFG material.

Cryomilled aluminum alloy and boron carbide nano-composite plate

The addition of ceramic particulate reinforcement via cryomilling can significantly increase the physical and mechanical properties of Al alloys. In the present study, boron carbide (B 4C) was cryomilled with Al 5083 to form a nano-grained metal matrix powder. This powder was blended with unmilled Al 5083 to increase ductility and was then consolidated into plates by three methods: (1) hot isostatic pressing (HIPping) followed by high strain rate forging (HSRF), (2) HIPping followed by two-step quasi-isostatic forging (QIF), and (3) three-step QIF. The effects of process method on microstructure and mechanical behavior for the final consolidated nano-composite plates were investigated.

Mechanical Behavior of Cryomilled CP-Ti Consolidated via Quasi-Isostatic Forging

Commercially pure (CP) Ti (Grade 2 with chemical composition 0.190 wt pct O, 0.0165 wt pct N, 0.0030 wt pct C, and 0.013 wt pct Fe) was cryomilled in liquid argon and liquid nitrogen for 8 hours. The influence of the milling environment on the chemistry, grain size, and grain-boundary structure of CP-Ti was studied by means of transmission electron microscopy (TEM), X-ray diffraction (XRD), and chemical analysis. The results show that the final average grain size obtained after 8 hours of cryomilling was ∼20 nm, for both liquid nitrogen and liquid argon cryomilling environments. Grains were observed to be heavily deformed and they did not reveal well-defined boundaries between them. Liquid nitrogen and liquid argon cryomilling environments led to differences in the final powder chemistry. Cryomilling in liquid nitrogen resulted in Ti powders with ∼2 wt pct nitrogen, which caused embrittlement that in turn affected the mechanical behavior of the consolidated materials. Cryomilling in liquid argon resulted in powders with slightly higher oxygen levels than those from liquid nitrogen experiments; this was attributed to the use of stearic acid (CH3(CH2)16COOH) as a process control agent (PCA). The cryomilled powders, in the form of various compositional blends from the argon and nitrogen milling experiments, were subsequently consolidated via quasi-isostatic (QI) forging, for mechanical behavior studies. The mechanical testing results showed that the QI-forged 85 pct as-received +15 pct liquid-argon-cryomilled powder blend exhibited ∼30 pct elongation to fracture, with a yield strength (YS) of 601 MPa and an ultimate tensile strength (UTS) of 711 MPa. In the case of 100 pct liquid-argon-cryomilled and QI-forged material, the YS, UTS, and elongation values were 947 and 995 MPa and 4.32 pct, respectively. The mechanical behavior was discussed in terms of the operative microstructure mechanisms. The enhanced ductility noted in the blended powders was discussed in terms of the presence of a bimodal microstructure.

A Ductile UFG Al Alloy via Cryomilling and Quasi-Isostatic Forging

Mechanical milling of Al alloy powder in liquid nitrogen leads to a large reduction in the scale of the microstructure and results in material with high thermal stability and strength. However, it is important to consolidate the powder and achieve bulk material with sufficient toughness and ductility for structural applications. In this investigation, hot isostatic pressing, followed by quasiisostatic forging and hot rolling, were performed to fabricate Al 5083 plate with a predominantly ultra-fine grained microstructure. Plate produced in this way possessed enhanced tensile strength and ductility, exceeding that of conventionally processed material.

Residual stresses and stress partitioning measurements by neutron diffraction in Al/Al–Cu–Fe composites

An unalloyed Al matrix was reinforced by spherical Al–Cu–Fe alloy particles, using either commercial purity (99.7%) or high purity (99.99%) powders (diameter <10 μm) and consolidated by either vacuum hot pressing or quasi-isostatic forging. Unusually pronounced strengthening in this simple metal matrix composite (MMC) system prompted the in situ study of residual stress effects in these samples to understand its possible role in the behavior. Residual stresses were measured in composites reinforced by 15, 20, and 30 vol.% reinforcement particles. Contrary to typical Al/SiC composites, the Al matrix exhibits compressive residual stresses. This unusual stress state may be promoted by a combination of effects including volume expansion of reinforcement particles caused by a phase transformation and the stiffness mismatch between the matrix and reinforcement phases. Matrix/reinforcement load transfer was also studied by neutron diffraction, using an in situ tensile frame. It was found that the load bearing stress ratio of reinforcement and matrix phases increases linearly with the reinforcement volume fraction. The composites produced from metal powders with a thinner oxide surface layer showed higher load transfer efficiency than the composites made from commercially atomized metal powders, as expected from the higher observed strength in the higher purity composites.