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Papers by Pradyumna Rallapalli

The dimensional and geometrical accuracy of wax patterns used in the investment casting process h... more The dimensional and geometrical accuracy of wax patterns used in the investment casting process has a direct effect on the quality of the final casting. Wax patterns are produced by injecting wax in the die cavity. The quality of wax patterns depends on the injection parameters. The objective of the paper is to analyze the influence of various injection parameters on the dimensional deviations of the component. Predicting the shrinkage and warpage associated with the wax injection process helps in avoiding costly and time consuming iterations involved in die design and development. Simulation of wax injection in the die cavity provides insight in the flow process in terms of weld lines, air trap, drop in temperature and pressure during filling, packing and cooling stages of the process. The stresses developed in the component is estimated which determines the warpage in the component.

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Gas Turbines in Aerospace applications operate at extreme conditions of temperatures and stresses... more Gas Turbines in Aerospace applications operate at extreme conditions of temperatures and stresses. The Blade/Vane components of these turbines have sculpted, twisted and hollow aerofoil geometry made using Nickel based super alloys through the investment casting process. Hollow wax patterns produced with use of ceramic cores are the first input to the investment casting process. The quantification and prediction of dimensional deviations due to shrinkage & warpage, which are dependent on injection parameters, shape of part and gating system adopted, is mandatory to arrive at acceptable tooling and castings. The insight of flow pattern in the die cavity imparts knowledge of possible weld lines, air-trap, jetting etc. The gating system may be designed, based on simulation results, to minimise such defect. The effects of injection molding parameters such as holding time, packing pressure and melt temperature on shrinkage and warpage of green part using moldex3D software are studied in this paper.

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Gas Turbines used in fighter aircraft depend on starter units for initial starting as well as mid... more Gas Turbines used in fighter aircraft depend on starter units for initial starting as well as midair
restarting. These units in turn work on gas turbine principles having two distinct parts viz., Gas
Generator Turbine (GGT) stage and Free Power Turbine (FPT) stage. Generally, both these stages
consist of one set of Rotor and Stator castings having a number of highly twisted blades integrally cast
on a central hub. Investment casting is the standard method adopted for manufacture of these castings
which are made of nickel based super alloys, to meet stringent dimensional, metallurgical and
mechanical property specifications.
The tooling for the castings usually consists of a wax pattern mould/die (segmental die),
which yields a single aerofoil pattern segment. Multiple injections with investment casting wax are
carried out on this die and individual segments are then assembled using a dedicated fixture, using
highly skilled manpower. These assembled wax patterns are subjected to CMM inspection to
dimensionally qualify various critical parameters including blade geometry, individual blade
orientation, inter-blade spacing, hub/shroud diameters, etc. This method suffers from disadvantages of
high cycle time, high rate of rejection and costly CMM inspection, but tolerable during development
stage of these critical castings. For a production environment, however, an integral die wherein total
wax pattern assembly is achieved in a single injection would be desirable. Such a tool/die would be
very complex, having as many inserts as number of blades in the rotor, with added complication of
retracting these inserts simultaneously on a unique path without breaking/damaging the wax pattern.
Issues in developing such a die include component modeling, identification of retraction path, insert
design, insert manufacture, dimensional qualification of insert, assembly of die, etc., the list being
endless.
The paper presents such an effort at DMRL where CAD/CAM, CNC machining, CMM
inspection and Rapid Prototyping technologies have been synergized for successful development of an
integral tooling for rotor castings.

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Estimation of the remaining service life of critical components in defence hardware is emerging a... more Estimation of the remaining service life of critical components in defence
hardware is emerging as an important research field, attracting the attention of researchers,
designers and manufacturing agencies as a tool for ensuring the reliability of mission &
safety critical systems. This paper brings out the salient aspects of the life prediction
technology being evolved at DMRL, briefly outlining the numerical simulation and
experimental measurement techniques. Various philosophies applied to the design and failure
analysis of critical systems are discussed, with particular emphasis on methods of analysis for
failure prediction and non-destructive evaluation.

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Modern turbine engines for aerospace applications need precision investment cast components such ... more Modern turbine engines for aerospace applications need precision investment cast components such as integrally cast rotors and
stators, for their hot end turbine stages. Traditionally, these turbines are used as starter engines. In recent times, such engines are
also used for strategic missile applications. The rotor/stator castings consist of a central hub (shrouded in some designs) over
which a number of aerofoil shaped blades are located. Since these components cannot be machined, investment casting is the
only available route for manufacture and hence stringent dimensional aerospace quality has to be in-built in the casting process
itself. In the process of investment casting, pattern generation by injection of wax into dedicated dies/moulds is the first critical
step. Traditional approach deals in producing individual blades with hub/shroud features through wax injection and assembly of
a set of such injected patterns onto a dedicated and precisely manufactured fixture to wax-weld and generate an integral wax
pattern, a process known as the ‘segmental approach’. It is possible to design a single-injection die with retractable metallic
inserts in the case of untwisted blades of stator patterns without the shroud. Such an approach is also possible for twisted blades
of rotors with highly complex design of inter-blade inserts and retraction mechanisms. DMRL has for long established methods
and procedures for the above to successfully supply precision castings for various defence related projects. In recent times, urea
based soluble insert approach has also been successfully applied to overcome the need to design and manufacture a precision
assembly fixture, leading to substantial reduction in component development times. Present paper deals in length various
approaches tried and established at DMRL to generate precision wax patterns for aerospace quality turbine rotors and stators. In
addition to this, the importance of simulation in solving issues related to wax injection is also touched upon.

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Ceramic cores are used as sacrificial inserts to generate intricate and complex hollow shapes in ... more Ceramic cores are used as sacrificial inserts to generate intricate and complex hollow shapes in components made of advanced
materials for aero-engine and aerospace applications. Development of tooling to precisely and accurately shape the ceramic core using
Ceramic Injection Moulding (CIM) method is of primary concern. To meet stringent dimensional requirements, the tool geometry has to be
precisely compensated for shrinkage and distortion phenomenon observed in the entire process of the component development. In advanced
aero-engine applications, the hot end components of the turbine stage consisting of blade/vane castings have extremely complex internal
geometry generated using the ceramic cores. This application of investment casting technology has been well established in DMRL. In a
recent effort, tooling for ceramic cores have been successfully developed to produce complex shaped rotors for space application using
powder metallurgy techniques. Present paper attempts to highlight issues related to tooling for ceramic cores for development of a near-net
shaped, Ni-based Superalloy Dual Property HIP Rotors.

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In the process of investment casting of precision blade/vane components for aero-engine applicati... more In the process of investment casting of precision
blade/vane components for aero-engine applications,
generation of a precison wax pattern is the first critical step. A
significant part of the dimensional deviations observed in the
casting can be attributed to the wax pattern. During the
process of generation of wax pattern through injection
moulding, two types of dimensional deviations occur, namely
shrinkage and warpage. Shrinkage deviations are predictable
to a an extent as they are predominantly an outcome of the
injection temperature, which is controllable. Warpage on the
other hand is difficult to predict as it depends on many other
factors of injection molding process. However, control of
distortion is critical in order to conform to the strict
dimensional tolerances of aerospace standards imposed on the
turbine blade/cane castings. Simulation based studies are
emerging as the preferred route for prediction of extent and
location of warpage in precision wax patterns. In the present
study, Moldex3D based simulation package has been utilized
to anlyze the effect of injection temperature, packing pressure,
cooling time and flow rate on the distortion behavior of a
stepped pattern. Two types of waxes with different rheological
properties have also been included to gauge the effect of
change in wax on the warpage. A full factorial design of
experiments has been configured and results of the analysis of
variance is presented.

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