Rajiv Shivpuri

Modeling microstructural development during the forging of Waspaloy

A model for predicting the evolution of microstructure in Waspaloy during thermomechanical proc-essing was developed in terms of dynamic recrystallization (DRX), metadynamic recrystallization, and grain growth phenomena. Three sets of experiments were conducted to develop the model: (1) preheating tests to model grain growth prior to hot deformation; (2) compression tests in a Gleeble testing machine with different deformation and cooling conditions to model DRX, metadynamic recrystallization, and short time grain growth during the post deformation dwell period and cooling; and (3) pancake and closed die forging tests conducted in a manufacturing environment to verify and refine the model. The microstructural model was combined with finite element modeling (FEM) to predict microstructure development during forging of Waspaloy. Model predictions showed good agreement with microstructures obtained in actual isothermal and hammer forgings carried out at a forging shop.

A new approach to optimal design of multi-stage metal forming processes with micro genetic algorithms

Abstract This paper describes a new method for design optimization of process variables in multi-stage metal forming processes. The selected forming processes are multi-pass cold wire drawing, multi-pass cold drawing of a tubular profile and cold forging of an automotive outer race preform. An adaptive micro genetic algorithm (μGA) scheme has been implemented for minimizing a wide variety of objective-cost functions relevant to the respective processes. The chosen design variables are die geometry, area reduction ratios and the total number of forming stages. Significant improvements in the simulated product quality and reduction in the number of passes has been observed as a result of the micro genetic algorithms-based optimization process.

A Cobalt Diffusion Based Model for Predicting Crater Wear of Carbide Tools in Machining Titanium Alloys

In machining titanium alloys with cemented carbide cutting tools, crater wear is the predominant wear mechanism influencing tool life and productivity. An analytical wear model that relates crater wear rate to thermally driven cobalt diffusion from cutting tool into the titanium chip is proposed in this paper. This cobalt diffusion is a function of cobalt mole fraction, diffusion coeficient, interface temperature and chip velocity. The wear analysis includes theoretical modeling of the transport-diffusion process, and obtaining tool‐chip interface conditions by a nonisothermal visco-plastic finite element method (FEM) model of the cutting process. Comparison of predicted crater wear rate with experimental results from published literature and from high speed turning with WC/Co inserts shows good agreement for different cutting speeds and feed rate. It is seen that wear rates are independent of cutting time. @DOI: 10.1115/1.1839192#

Microstructure-Mechanics Interactions in Modeling Chip Segmentation during Titanium Machining

Abstract Chip segmentation in machining of titanium alloys is strongly influenced by the microstructural state of the material. A numerical model is presented that incorporates material changes into the phenomenological behavior of the chip. It is calibrated by comparing results with experimental measurements at different cutting speeds and feeds. It predicts that at lower cutting speeds fracture propagates in the α-β phase towards the cutting tool face resulting in a discontinuous chip. At higher cutting speeds, temperature in the secondary shear zone reaches β transus increasing material ductility; the fracture propagates towards the outer surface resulting in a continuous but segmented chip.

A multilayer coating architecture to reduce heat checking of die surfaces

The primary reason for thermal fatigue cracking in die casting is the thermal load and its cycling during the operation. Conventional single and multiplayer coatings have been found to at best equal the thermal fatigue lives of hot working steels used for die casting dies. This paper presents a new approach to fatigue resistance that uses a three-layer coating architecture to reduce the heat transfer and chemical diffusion to the die steel substrate. In this, the outer layer consists of a thermal barrier coating of rare-earth oxide, the middle layer a TiAlN diffusion barrier coating and the inner layer a thin adhesive Ti layer. Thermal cycling tests in liquid aluminum up to 4000 cycles using this architecture and conventional commercial multi-layer PVD coatings confirm that the new coating significantly improves the thermal fatigue resistance of the substrate. � 2002 Elsevier Science B.V. All rights reserved.

Probabilistic design of aluminum sheet drawing for reduced risk of wrinkling and fracture

Often, sheet drawing processes are designed to provide the geometry of the final part, and then the process parameters such as blank dimensions, blank holder forces (BHFs), press strokes and interface friction are designed and controlled to provide the greatest drawability (largest depth of draw without violating the wrinkling and thinning constraints). The exclusion of inherent process variations in this design can often lead to process designs that are unreliable and uncontrollable. In this paper, a general multi-criteria design approach is presented to quantify the uncertainties and to incorporate them into the response surface method (RSM) based model so as to conduct probabilistic optimization. A surrogate RSM model of the process mechanics is generated using FEM-based high-fidelity models and design of experiments (DOEs), and a simple linear weighted approach is used to formulate the objective function or the quality index (QI). To demonstrate this approach, deep drawing of an aluminum Hishida part is analyzed. With the predetermined blank shape, tooling design and fixed drawing depth, a probabilistic design (PD) is successfully carried out to find the optimal combination of BHF and friction coefficient under variation of material properties. The results show that with the probabilistic approach, the QI improved by 42% over the traditional deterministic design (DD). It also shows that by further reducing the variation of friction coefficient to 2%, the QI will improve further to 98.97%.

An evaluation of metallic coatings for erosive wear resistance in die casting applications

An accelerated testing procedure with a multi-pin die has been established for the study of the erosive wear of die materials and surface treatments in die casting applications, under production conditions. This test procedure utilizes the wear of core pins as a surrogate measure of die erosive wear, and is used to evaluate the efficacy of coatings and die surface treatments in reducing erosive wear in dies under actual production conditions. Metallic coatings of W, Mo, and Pt deposited on the test pins using the plasma source ion implantation technique (PSII) technology in the ion assisted deposition mode, were then evaluated for erosion resistance using this developed test procedure. The results of this evaluation on the effectiveness of these metallic coatings are presented here along with possible reasons for such coating behavior.

Friction stir welding of tailored blanks : Investigation on process feasibility

Tailor welded blanks (TWBs) are conventionally produced by laser or traditional welding processes. In either case, the joints are created by solid-liquid-solid phase transformations that result in undesirable microstructures and tensile residual stresses detrimental to joint performance. This study investigates feasibility of an alternate joining process, friction stir welding (FSW). The joining of AA7075-T6 blanks of different thickness is investigated through FE analyses and controlled experiments. It is found that for a successful joint, the welding parameters have to be carefully designed so that the resulting metal flow and the temperature history during FSW are consistent for the two thicknesses.

Material Flow in FSW of AA7075-T6 Butt Joints: Continuous Dynamic Recrystallization Phenomena

In the paper the continuous dynamic recrystallization (CDRX) phenomena occurring in the FSW of AA7075-T6 butt joints is investigated at the varying of the most relevant technological and geometrical parameters. In particular, both experiments and numerical simulations obtained utilizing a 3D Lagrangian implicit, coupled, rigid-viscoplastic model have been developed on FSW butt joints. The resulting microstructure at the core of the weldings is correlated to the material flow occurring during the FSW process.

Characterization of large area filtered arc deposition technology: part II — coating properties and applications

Abstract A dual-filtered cathodic arc deposition process was used to synthesize a variety of hard coatings on polished substrates, using large-area filtered-arc deposition (LAFAD) technology. The surface morphology showed that the coatings were free of macro defects or inclusions, and there was no degradation of the initial surface finish. Mechanical properties of the coatings deposited were measured by a nanoindentation technique. A duplex (heat treatment+deposition) process was used to deposit a unique multilayer hard coating on H-13 steel core pins used in aluminum die-casting application. Extensive characterization of the coated pins showed that the coating improved the erosion/corrosion resistance, as well as thermal cracking resistance, of the steel by nearly one order of magnitude over commercially used PVD coatings.