Papers by Author: Mamidala Ramulu

Abstract: A mechanistic model capable of predicting end milling cutting forces in brittle porous media is described. A coefficient which is capable of reproducing the random shape and variation in cutting forces due to porosity is proposed. In addition, a method of experimental determination of cutting force coefficients is outlined. The proposed model is based on the hypothesis that the random shape and variation in cutting forces of brittle porous media coincide with the shape and variation of pore size and distribution in the media. The developed coefficient and model is compared to end milling tests conducted in CB1100, a porous machinable alumina based ceramic manufactured by UMECO. High correlation between predicted and measured cutting forces is shown. Experiments show that the model is capable of accurate prediction of variation in individual cutting tooth force profile shape and overall magnitude over the entire range of machining conditions tested. The benefit of the model lies in its ability to greatly reduce the number of cutting tests required when investigating cutting forces in novel brittle porous materials.

Abstract: This paper includes a review of the manufacturing technologies related to the combination of friction stir welding (FSW) and superplastic forming (SPF) to fabricate large monolithic titanium structures for aircraft. The particular focus is on the post FSW residual stress measured on the weld nugget after thermal treatments and SPF forming.

Abstract: Heat treatment processes for standard fusion welding techniques in titanium are well established, but the optimal heat treatment for Friction Stir Welded titanium has not been evaluated. In this study, 6 mm thickness titanium 6Al-4V butt welds were subjected to heat treatments ranging from 700 to 900 C. Results of the metallographic analysis for each heat treatment condition will be presented in addition to microhardness, tensile and fatigue properties. It was found that increased heat treatment temperatures lead to lower hardness and tensile strengths, higher elongation to failure and improved high cycle fatigue performance. Furthermore, fracture toughness and crack growth tests were performed for welds subjected to a standard post-weld stress relief. The fracture toughness was lower than the parent material, but crack growth rates in the weld were similar to that of the base metal.

Abstract: Friction Stir Welding of Ti-6Al-4V was performed on 5 mm thickness plate in order to assess the affect of welding conditions on the resulting microstructure and superplastic forming behavior of the joints. A variety of welding conditions were tested and all welds were subsequently Superplastically formed. It was found that the weld parameters do influence the microstructure and degree of superplastic performance of the joints. Spindle speed was found to have the most dominant affect on the resulting microstructure and superplastic forming behavior. Low spindle speed welds lead to fine grained microstructures and highly superplastic welds, relative to the base material, while high spindle speed welds larger grained microstructures and less superplastic welds.

Abstract: In friction stir welding (FSW), the semi-circular shaped FSW pin tool feed marks that are left behind varied in depth and shape which are detrimental not only in fatigue performance but also in further processing such as superplastic forming (SPF). Experimental investigation was conducted to determine the effects of changes to the FSW process parameters on the surface roughness of the weld of fine grain 2 mm thick titanium alloy, Ti- 6Al-4V. In addition to optimizing the surface texture of the welds, the superplastic performance of the weld nugget was made to be equal to the superplasticity of the parent metal by altering the spindle speed and feed rate used during FSW to identify the quality in terms of cold weld or hot weld. FSW process conditions of spindle speed of 500 RPM and a feed rate of 150 mm/min was found to produce a uniform deformation in both weld and parent metal when the joint was superplastc formed.

Abstract: Ultra high-pressure waterjets (WJ) have gained consideration as a viable alternative to conventional material removal and cleaning methods such as chemical milling, grit blasting, and grinding. Waterjets have often been considered for the stripping of unwanted coatings, but they also exhibit an inherent ability for the controlled milling of metallic materials. The waterjet material removal process poses a secondary benefit – the material erosion occurs due to the repetitive impact of droplets within the waterjet stream, which also serve to induce compressive residual stresses in the shallow sub-surface layers of the work piece. An experimental study was conducted to explore the removal of the alpha case layer on Superplastically Formed conventional and fine grain Ti-6Al-4V specimens. The resulting surface roughness parameters and completeness of alpha layer removal were characterized. Based on the experimental results, waterjet removal processes may serve as an alternate method for the post-processing of Superplastically Formed titanium alloys.

Abstract: Fatigue crack propagation life of weld toe crack through residual stress field was estimated using Elber's crack closure concept. Propagation of weld toe crack is heavily influenced by residual stresses caused by the welding process, so it is essential to take into account the effect of residual stresses on the propagation life of a weld toe crack. Fatigue cracks at transverse and longitudinal weld toe was studied, these two cases represent the typical weld joints in ship structures. Numerical and experimental studies are performed for both cases. Residual stresses near the welding area were estimated through a nonlinear thermo-elasto-plastic finite element method and the residual stress intensity factor with Glinka's weight function method. Effective stress intensity factor was calculated using the Newman-Forman-de Koning-Henriksen equation, which is based on the Dugdale strip yield model in estimating the crack closure level, U, at different stress ratios. Calculated crack propagation life coincided well with experimental results.

Abstract: Thirty three SENB and nine TPB specimens, which were machined from an 8.1mm thick 7075-T7351 aluminum stock plate, were fatigued in the as-received or shot peened conditions. The SENB specimens were then fractured by overload and were recorded. The crack growth rate of the tunneling crack profiles were measured by fractography at various life cycles. SN curves of the as received and shot peened TPB specimens were also constructed. Peened surface roughness, subsurface microstructure and micro-hardness profiles were examined. X-ray diffraction technique was used to measure the residual stresses in selected shot peened SENB and TPB specimens. The fatigue life under a severe shot peening intensity (Almen scale 0.016A) of the SENB specimens was slightly larger than that of the as received SENB specimens at low stress ratio of R = 0.1 but the difference was found to be smaller at R = 0.8. The SN curves of the as received and severely shot peened TPB specimens were nearly identical. Grain distortion due to peening was observed to a depth of 410µm and was correlated with the measured residual compressive stresses. The compressive residual stress had little effect on the crack growth rates in shot peened SENB and TPB specimens.