N. Venkata Reddy

Slicing procedures in layered manufacturing: a review

Layered manufacturing (LM) or rapid prototyping is a process in which a part is produced using layer‐by‐layer addition of the material. In LM, slicing of the CAD model of a part to be produced is one of the important steps. Slicing of CAD model with a very small slice thickness leads to large build time. At the same time if large slice thickness is chosen, the surface finish is very bad due to staircasing. These two contradicting issues namely reduction in build time and better surface quality have been a major concern in laminated manufacturing. This contradiction has led to the development of number of slicing procedures. The present paper reviews various slicing approaches developed for tessellated as well as actual CAD models.

Optimal part deposition orientation in FDM by using a multicriteria genetic algorithm

In rapid prototyping processes, the deposition orientation of the part is very important as it affects part surface quality, production time and the requirement for support structure and hence cost. Depositing the part with thinner slices results in a larger build time. At the same time, if a large slice thickness is chosen, the surface finish is very poor due to stair-stepping. These are two contradicting issues and are tackled by using adaptive slicing. In adaptive slicing, slice thickness is calculated based on local geometry of the computer-aided design model and rapid proto-typing machine specifications. Even though adaptive slicing controls part surface quality by compromising on build time for a deposition orientation, an optimum orientation can further reduce build time and enhance part surface quality. In the present work, an attempt has been made to determine the optimal part deposition orientations by considering two objective functions at a time, namely average part surface roughness (average part surface quality) and build time. The two objectives are minimized simultaneously using a multicriteria genetic algorithm.

Die design for axisymmetric extrusion

Abstract Determination of total extrusion power and die pressure distribution is very important for die design. In this work, an upper-bound model with strain hardening is proposed, the prediction of the extrusion power of which is as accurate as that determined by the finite-element method (FEM) and is in excellent agreement with published experimental results. The upper-bound model, when combined with the slab method, also predicts the die pressure distribution, which again is in reasonable agreement with FEM results. Further, the computational time taken by the combined upper-bound/slab method is significantly less than that for FEM. The proposed combined upper-bound/slab method is applied to compare eight different die shapes, namely, stream-lined (third and fourth-order polynomial, cosine and modified Blazynskis CRHS), elliptical, hyperbolic, conical and Blazynskis CRHS. Based on the consideration of total extrusion power (under optimal conditions), it is concluded that third- and fourth-order polynomial dies and the cosine die are the best amongst the profiles considered. Parametric study is carried out for the third-order polynomial die to study the effects of reduction ratio, friction factor and strain-hardening on the optimal die length and die pressure distribution.

Formability and Surface Finish Studies in Single Point Incremental Forming

Incremental sheet metal forming (ISMF) has demonstrated its great potential to form complex three-dimensional parts without using a component specific tooling. The die-less nature in incremental forming provides a competitive alternative for economically and effectively fabricating low-volume functional sheet parts. However, ISMF has limitations with respect to maximum formable wall angle, geometrical accuracy, and surface finish of the component. In the present work, an experimental study is carried out to study the effect of incremental sheet metal forming process variables on maximum formable angle and surface finish. Box―Behnken method is used to design the experiments for formability study and full factorial method is used for surface finish study. Analysis of experimental results indicates that formability in incremental forming decreases with increase in tool diameter. Formable angle first increases and then decreases with incremental depth and it is also observed that the variation in the formable angle is not significant in the range of incremental depths considered to produce good surface finishes during the present study. A simple analysis model is used to estimate the stress values during incremental sheet metal forming assuming that the deformation occurs predominantly under plane strain condition. A stress-based criterion is used along with the above mentioned analysis to predict the formability in ISMF and its predictions are in very good agreement with the experimental results. Surface roughness decreases with increase in tool diameter for all incremental depths. Surface roughness increases first with increase in incremental depth up to certain angle and then decreases. Surface roughness value decreases with increase in wall angle.

Ductile fracture criteria and its prediction in axisymmetric drawing

Ductile fracture occurs due to micro-void nucleation, growth and finally coalescence into micro-crack. The ductile fracture criteria (P.F. Thomason, Ductile Fracture of Metals, Pergamon, 1990; S. Dhar et al., A continuum damage mechanics model for void growth and micro-crack initiation, Engineering Fracture Mechanics 53 (1996) 917) developed based on the microscopic phenomena of void nucleation, growth and coalescence along with a simple criterion (N.V. Reddy et al., Central bursting and optimal die profile for axisymmetric extrusion, ASME Journal of Manufacturing Science and Engineering 118 (1996) 579) based on the concept of the hydrostatic stress component at a point in the deformation zone falling to zero and compressive elsewhere are used to predict the fracture initiation in drawing (i.e. central bursting). Even though the first two criteria are based on microscopic description, the material parameters required are available for a few steels only and their determination involves difficult metallurgical experimentation. The above criteria used along with the results of Eulerian Rigid-Plastic and Elasto-Plastic formulations are presented in this paper. Finite element formulations for obtaining the generalized strain distribution and for obtaining the damage distribution by using the critical damage criteria are also presented. The present study shows that predictions based on the simple criterion are in good agreement with the experimental as well as numerical results published earlier and are, in general, conservative. Further, comparison of the predictions of the three criteria shows that the hydrostatic stress criterion is highly conservative and hence safe for die design.

A set-up model for tandem cold rolling mills

Abstract It is well known that experience is playing a vital role in the selection of operating parameters in tandem cold rolling mills. In the present work, an attempt is made to develop a set-up model for tandem cold rolling mill to maximize the throughput. In the present work, power consumed is calculated and compared for various reduction schedules obtained by distributing the strip thicknesses in arithmetic, geometric, harmonic and quadratic series. Simulations are carried out for 6-stand and 5-stand tandem cold rolling mills with different sets of operational data reported by Roberts [Flat Processing of Steels, Marcel Dekker, New York, 1995]. It is found that power consumed is minimum for the reduction schedule obtained by distributing strip thicknesses in harmonic series and is reasonably in good agreement with the values obtained using the operational data reported by Roberts. A one-dimensional model presented by Suryanarayana [A set-up model for tandem cold rolling mill, M.Tech. Thesis, IIT, Bombay, 1998] is used to calculate the power in the present work and its predictions are compared with available experimental and numerical results and are presented in Appendix A.

Central bursting and optimal die profile for axisymmetric extrusion

A simple criterion for predicting the initiation of central burst in extrusion is proposed. The geometric conditions to avoid central bursting are predicted by using the proposed criterion under various process conditions and material properties. The optimal die profiles which minimize the extrusion power are also obtained for various process conditions. The mixed (pressure-velocity) formulation is used along with the Householder method to solve the resulting equations. It is shown that the optimal die profiles satisfy the conditions for prevention of central burst. The predictions based on the proposed criterion are in good agreement with the experimental observations and are in conformity with the results published earlier.

Ductile fracture prediction in axisymmetric upsetting using continuum damage mechanics

Abstract Ductile fracture occurs due to micro-void nucleation, growth and finally coalescence into micro-crack. These micro-cracks grow in the presence of tensile stresses leading to a fracture. A ductile fracture criterion based on the microscopic phenomena of void nucleation, growth and coalescence is used to predict the micro-crack initiation in axisymmetric upsetting. The above criterion is explored along with the results of an axisymmetric large deformation elasto-plastic finite element code developed during the present work using updated Lagrangian formulation. A new incremental objective stress measure and the logarithmic strain measure are employed. The Newton–Raphson iterative technique is used to solve the non-linear incremental equations. In the present work, material is assumed as elasto-plastic, yielding according to von-Mises criterion and hardening according to a power law relationship. Interfacial friction is modeled using Coulomb friction law. A comprehensive parametric study is carried out to study the effect of two process parameters, namely friction and height-to-diameter ratio on the deformation level at which micro-crack initiate at three distinct locations of the deformation geometry namely, die–work interface, center of the workpiece and meridian surface. The micro-cracks first initiate along the die–work interface region nearer to the free surface, then at the center of the deformation zone and subsequently at the meridian surface. The possibility of formation of central cavity increases with height-to-diameter ratio. With increase in friction micro-cracks initiate at smaller deformation levels. Study of stress distributions reveals that the fracture at the meridian surface occurs at smaller deformation level when height-to-diameter ratio is equal to 1 rather than values other than unity.

On multistage deep drawing of axisymmetric components

A process analysis model to determine the limiting drawing ratio for the first draw as well as for redraws is presented considering the effects of normal anisotropy, co-efficient of friction, strain hardening and die arc radius. Predictions of the model presented are in good agreement with the available experimental results. The model can be used by the process planners to determine the minimum number of passes required to achieve the final component geometry. Using the present model, feasible bperation sequences which have the minimum number of stages through which the desired cup specification can be achieved is presented for two different cup geometries.

Virtual hybrid-FDM system to enhance surface finish

Many rapid prototyping systems which produce prototypes by layer-by-layer material deposition are now commercially available. The layer-by-layer deposition process leads to a stepped surface known as staircase. Staircase formation is a geometric constraint of the layered manufacturing, which can not be eliminated. The presence of staircase on the surface of a prototype detracts from the surface finish and hence restricts functionality of prototypes. It is realized that there is a need to make modifications in RP (rapid prototyping) systems so that prototypes with better surface finish can be produced without incurring high production costs. A virtual hybrid fused deposition modelling system (hybrid-FDM) is proposed in the present work that uses both layer-by-layer deposition and machining. In this system, CAD model is sliced adaptively using limited centre line average (Ra) value as a criterion (Pandey et al. 2003a). Hot cutter machining/ploughing (HCM) (Pandey et al. 2003b) is recommended to machine the build edges (staircase) of ABS material. Numerically controlled x−y traversing mechanism is proposed as an attachment to move hot cutters along the periphery of slices to machine build edges. In this paper, geometrical designs of cutters are proposed. A process planning system to decide the number of layers to be deposited and then machined in order to access intricate features of a part is implemented. The developed system simulates surface roughness, before and after hot cutter machining. An experimental study is carried out by machining the build edges of an axisymmetric FDM part on lathe machine to form a basis for a hybrid-FDM system.