Balakrishna Cherukuri

Scaling Up of Equal Channel Angular Pressing (ECAP) for the Production of Forging Stock

Over the past two decades equal channel angular processing (ECAP) and other severe plastic deformation (SPD) processes have been shown, in the laboratory scale, to produce material with promising properties for industrial applications. In particular, ultrafine grain (UFG) metals produced by ECAP process, for example, have been shown to exhibit higher strain rate sensitivity at lower temperatures and higher strain rates. These factors translate to improved hot formability. However, scale up of these processes to manufacture industrial size components has not been widely undertaken. In this study, billets of annealed AA6061 with 12.5 mm (0.5-in), 50 mm (2-in) and 100 mm (4-in) square cross section were ECAP processed. For the first time, these larger SPD billets were used as starting stock for subsequent hot forging. Several parts were forged on an industrial scale press with the UFG material, as well as conventional stock materials. These parts varied in complexity, as well as size in order to cover the variability in industrial components. This paper will present the effect of scaling up on the mechanical properties, microstructure, and the hot workability of the alloy from the laboratory scale (12.5 mm) to industrial scale (100 mm). Results show that both the forging temperature of the billets and the starting billet size can be substantially decreased compared to conventional forging practice. Therefore, the use of SPD materials, as forging stock, results in decreased energy usage and increased material yield. Results presented will include examples of forged parts, estimated energy savings associated with the use of SPDUFG stock, and properties after forging and subsequent heat treatment.

Properties of AA6061 Processed by Multi-Axial Compressions/Forging (MAC/F)

Multi-axial compressions/forgings (MAC/F) were conducted at room temperature to obtain severe plastic deformation (SPD) of AA6061 alloy. Microhardness measurements taken across the cross-section of each of the MAC/F processed samples indicated that the hardness distribution is nonuniform during initial compressions/forgings and becomes uniform with subsequent compressions. Microhardness and tensile testing showed that the hardness, yield strength, and elongation of the MAC/F processed material followed similar trends to that for AA6061 processed by equal channel angular pressing (ECAP). The MAC/F material exhibited high strain rate sensitivity and high percentage of elongation to failure in the temperature range of 300–350°C. This observation is in agreement with the high formability of SPD-processed AA6061 over the same temperature range.

Nucleation and Growth of α-Ti on TiB Precipitates in Ti–15Mo–2.6Nb–3Al–0.2Si–0.12B

The microstructure was investigated of a β-stabilized Ti–15Mo–2.6Nb–3Al–0.2Si–0.12B alloy at two different aging temperatures, 540°C/8 h and 660°C/8 h. In particular, the heterogeneous nucleation of α-Ti from TiB particles was studied at these aging temperatures. At the lower aging temperature, α-Ti precipitated as needle-like shapes on the TiB phase. In contrast, the higher aged sample exhibited globular α-Ti morphology around the TiB phase. This difference was rationalized in terms of the coarsening behavior of α-Ti around the TiB phase. Various orientation relationships were observed between these two samples. This difference is because of the precipitation of α-Ti on two different TiB planes. In addition, atom probe analysis confirmed the segregation of alpha and beta stabilizing elements to the respective phases. At the lower aging temperature, it was noted that silicon enriched the α-Ti/β-Ti interface when the α-Ti/β-Ti/TiB were all in contact. Upon α-Ti coarsening, silicon enrichment was observed at the α-Ti/TiB interface at the higher aging temperature.

Continuous Severe Plastic Deformation Processing of Aluminum Alloys

Metals with grain sizes smaller than 1-micrometer have received much attention in the past decade. These materials have been classified as ultra fine grain (UFG) materials (grain sizes in the range of 100 to 1000-nm) and nano-materials (grain size half the melting temperature on the absolute scale) and very low strain rates (< 0.0001/s). UFG metals have been shown to exhibit superplastic characteristics at lower temperature and higher strain rates, making this phenomenon more practical for manufacturing. This enables part unitization and forging more complex and net shape parts. Laboratory studies have shown that this is particularly true for UFG metals produced by SPD techniques. This combination of properties makes UFG metals produced by SPD very attractive as machining, forging or extrusion stock, both from the point of view of formability as well as energy and cost saving. However, prior to this work there had been no attempt to transfer these potential benefits observed in the laboratory scale to industrial shop floor. The primary reason for this was that the laboratory scale studies had been conducted to develop a scientific understanding of the processes that result in grain refinement during SPD. Samples that had been prepared in the laboratory scale were typically only about 10-mm diameter and 50-mm long (about 0.5-inch diameter and 2-inches long). The thrust of this project was three-fold: (i) to show that the ECAE/P process can be scaled up to produce long samples, i.e., a continuous severe plastic deformation (CSPD) process, (ii) show the process can be scaled up to produce large cross section samples that could be used as forging stock, and (iii) use the large cross-section samples to produce industrial size forgings and demonstrate the potential energy and cost savings that can be realized if SPD processed stock is adopted by the forging industry. Aluminum alloy AA-6061 was chosen to demonstrate the feasibility of the approach used. The CSPD process developed using the principles of chamber-less extrusion and drawing, and was demonstrated using rolling and wire drawing equipment that was available at Oak Ridge National Laboratory. In a parallel effort, ECAE/P dies were developed for producing 100-mm square cross section SPD billets for subsequent forging. This work was carried out at Intercontinental Manufacturing Co. (IMCO), Garland TX. Forging studies conducted with the ECAE/P billets showed that many of the potential benefits of using UFG material can be realized. In particular, the material yield can be increased, and the amount of material that is lost as scrap can be reduced by as much as 50%. Forging temperatures can also be reduced by over 150oC, resulting in energy savings in the operation of billet heating furnaces. Looking at only the energy required to make forgings from stock materials, estimated energy savings associated with reduced scrap and lower furnace operating temperatures were greater than 40% if ECAE/P stock material was used instead of conventionally extruded stock. Subsequent heat treatment of the forged materials to the T6 condition showed that the mechanical properties of parts made from the ECAE/P stock material were the same as of those made from conventional extruded stock material. Therefore, the energy and cost savings benefits can be realized by the use SPD processed material as forging stock without sacrificing properties in the final part.

Optimization of the Equal Channel Angular Pressing (ECAP) Process for Strain Homogeneity

Two-Dimensional finite element analysis was carried out to optimize the equal channel angular pressing process (ECAP) for strain homogeneity under frictionless and frictional conditions. The effect of outside corner angle (Ψ), inner radius (r) and shear friction (m) on the strain homogeneity was investigated. The strain homogeneity can be increased by correcting the outside corner to eliminate the corner gap between the sample and the die at the expense of average strain. Small inside radius and outside corner radius would provide large deformations without much loss in the strain homogeneity under frictionless conditions. The work piece deformation is by bending if the inner radius exceeds a critical value. No improvement in strain homogeneity was observed under frictional conditions.