me 651 advanced casting technology

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Ni BASED SUPER ALLOYS AN INTRODUCTIONNickel-based alloys can be either solid solution or precipitation strengthened. Solid solution strengthened alloys, such as Hastelloy X, are used in applications requiring only modest strength. In the most demanding applications, such as hot sections of gas turbine engines, a precipitation strengthened alloy is required. Most nickel-based alloys contain 10-20% Cr, up to 8% Al and Ti, 5-10% Co, and small amounts of B, Zr, and C. Other common additions are Mo, W, Ta, Hf, and Nb.

GENERAL CHEMICAL COMPOSITIONCr & Al - Oxidation resistanceY - Help the oxide scale to cohere to the substrateC, Cr, Mo, W, Nb, Ta, Ti and Hf - Grain boundary strengthening elementsCo, Fe, Cr, Nb, Ta, Mo, W, V, Ti and A Solid solution strengtheners


MAJOR PHASES IN NICKEL SUPER ALLOYSGamma (): The continuous matrix is an face-centered-cubic (fcc) nickel-based austenitic phase that usually contains a high percentage of solid-solution elements such as Co, Cr, Mo, and W.Gamma Prime ('): The primary strengthening phase in nickel-based super alloys is called gamma prime ('). It is a coherently precipitating phase (i.e., the crystal planes of the precipitate are in registry with the gamma matrix) with an ordered L12 (fcc) crystal structure.Carbides: Carbon, added at levels of 0.05-0.2%, combines with reactive and refractory elements such as titanium, tantalum, and hafnium that begin to decompose and form lower carbides which tend to form on the grain boundaries. These common carbides all have an fcc crystal structure.Topologically Close-Packed Phases: The cell structure of these phases have close-packed atoms in layers separated by relatively large interatomic distances.

METHODS FOR MANUFACTURING Ni BASED SUPER ALLOYSConventional casting (CC)Mechanical alloyingDispersion oxide strengtheningPowder metallurgy methodDirectional solidification (DS)

DIRECTIONAL SOLIDIFICATIONIt is the process of controlled feeding of the molten metal into a temperature-controlled mould to produce a part that is free of hollow spots, called shrink defects. It is also used to refine the metal during the casting, because the impurities in the molten metal will continue to rise to the surface of the pool.The molten metal at the far end of the mould begins to cool and solidify before the rest of the mould does. As the metal on the bottom of the mould cools, this line of solidification moves steadily upward toward the molten metal feed.

WHY DS FOR SC Ni BASED SUPER ALLOYS?In the case of SC super alloys, alignment or elimination of the grain boundaries normal to the stress axis enhances high-temperature ductility by eliminating the grain boundary as the failure initiation site. This permits the microstructure to be refined with a solution heat treatment that increases alloy strength.The second reason is that the DS process provides a preferred low-modulus orientation parallel to the solidification direction. This results in a significant enhancement in thermal fatigue resistance, so important in high-temperature components.

DIRECTIONAL SOLIDIFICATION BY LMC PROCESSPRINCIPLEThe principle of the LMC process is in which the mould is withdrawn from the furnace directly into a bath of liquid-metal coolant. Heat extraction by convection from the mould surface is more efficient than radiation through a vacuum.

EXPERIMENTA columnar-grain variant of single-crystal REN N4 was used for this investigation. All castings were made using ingots from a single heat with a nominal composition of Ni-9.7 Cr-8.0 Co-6.0 W-4.7 Ta-4.2 Al-3.5 Ti-0.4 Nb-0.15 Hf-0.07C-0.008B (weight percent). The liquidus and solidus temperatures of the alloy were extracted from differential thermal analysis (DTA) heating curves.Thermocouples inserted in the casting directly measured thermal gradients during solidification. The LMC process exhibited higher gradients at all withdrawal rates. The higher thermal gradients resulted in a refined structure measurable by the finer dendrite-arm spacing.

PROCESSMoulds with a maximum height of 300 mm were supported on a 150-mm-diameter chill plate and heated using resistance elements. The liquid-metal bath container held approximately 500 kg of tin and was temperature controlled by a stirrer and a recirculating thermal oil system.The coolant temperature was set to 250 C for all LMC casting experiments. A dynamic baffle consisting of alumina bead ranging in diameter from 0.6 to 2.5 mm floated on top of the tin bath to thermally isolate the cooling bath from the mould heater. The baffle thickness was approximately 10 mm.When solidifying a liquid-metal-cooled casting, a stainless steel chill plate cooled by the liquid-metal bath was employed rather than the water-cooled copper chill plate. The thermal conductivity of stainless steel is significantly lower than copper (14 W/m/K vs 401 W/m/K at 273 K), since copper dissolves in liquid tin at moderate temperatures, an alternate chill-plate material was required.

CONTThe bottom section of the casting was designed to be representative of a blade-tip cross section from a large IGT, which is commonly cast tip down. The middle portion dimensions represented the air foil cross section of a large IGT blade. The topmost portion provided an intermediate cross section while adding constraint to contraction in the middle portion. A pocket area was included in the bottom portion of the castings, because recessed areas may serve as sites for defect nucleation.A plug of the same alloy being cast, which was approximately 8-mm thick, was placed in the bottom of each mould, which was open to the chill plate. Cobalt aluminates, which are used as grain nucleants, were bonded to thetop surface of the plugs using an alumina cement to increase the initial columnar-grain density for all castings.

CONTThe liquid-metal-cooled castings were made using withdrawal rates of 5.1, 6.8, and 8.5 mm/min to determine the capability and benefits of faster withdrawal rates with the LMC process.Each casting section was instrumented with a thermocouple located in the side of each section at the midpoint along the casting withdrawal axis through thermocouple holes.Before casting, the chamber door was closed, a vacuum of at least 10^4 mbar was achieved, and the mould was preheated to a temperature of 1500 C. After solidification, the castings were cleaned and lightly polished to remove mould material. The castings were subsequently macro etched to reveal their grain structure and surface casting defects using solution of 80 Ml HCl:2 mL HNO3:11 mL H2O:16 g FeCl3.The PDAS was measured using micrographs from surfaces normal to the growth direction.

(a) An example casting with three cross sections shown in macro etched condition to reveal grain structure and (b) corresponding investment mould

Variation of grain orientation with withdrawal rate and corresponding solid/liquid interface shape and location diagram

LMC 5.1 mm/minLMC 6.8 mm/min

LMC 8.5 mm/min

CAST MICROSTRUCTUREThe PDAS is a very important measure of microstructure that is often used to predict thermal gradient for nickel-based super alloys.In these experiments, the castings processed by LMC had improved thermal gradients and a much smaller PDAS than corresponding Bridgman casting sections, even at the same withdrawal rate. The dendritic microstructures were qualitatively similar in appearance in all sections and at all withdrawal rates and cooling methods, but varied substantially in microstructural scale.Slower withdrawal rates and thicker cross sections generally resulted in a larger PDAS.Because of the complex heat transfer processes in LMC process, it is very difficult to predict the withdrawal rate for maximizing the gradients along axial growth direction for a given casting configuration.

Variation in PDAS and dendrite morphology with section thickness for an LMC casting withdrawn at 5.1 mm/min, viewed normal to the growth direction:


(a)51mm (b)38mm

FRECKLE DEFECTSThey are vertically oriented chains of small equi-axed grains wit a composition closer to the alloy eutectic in the solidifying material.This occurrence can be reduced by controlling the composition of alloy, casting geometry and casting conditions.These were observed on the corners of the bottom section of the casting.No freckles were visible in or around the recessed areas within the thick sections of any castings.

MISALIGNED GRAINSIt was classified as a single grain being greater than 15 degree off-axis with respect to the withdrawal direction or as two grains converging or diverging at an angle of greater than 20 degree.

PLANAR DISCONTINUITYA planar discontinuity was defined as a grain boundary transverse to the withdrawal direction, indicating interrupted grain growth.


CONCLUSION1. The LMC process produces thermal gradients more than twice those of the conventional process and the capability for withdrawal rates at least 2 to 3 times faster than the conventional process for the configuration investigated, with cooling rates up to 7.5 times greater than the conventional process.

2. The solid/liquid interface influences interface shape and the number of misaligned grains and is dependent on casting parameters, including withdrawal rate and heating and cooling efficiency.

3. The LMC process is able to maintain a favourable interface location and shape at relatively high withdrawal ra


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