Directional Solidification to Produce Columnar Grain and Single Crystal Structures

<div class="htmlview paragraph">A precision casting technique based on directional solidification, which imparts ductility and thermal shock resistance to high-temperature, creep-resistant, nickel-base superalloys has been developed at Pratt & Whitney Aircraft. The desired improvement in physical properties is achieved by controlling the grain solidification process so as to eliminate transverse grain boundaries. Furthermore, the feasibility of producing parts using this “directional solidification process” has been demonstrated in both experimental and production foundry facilities.</div> <div class="htmlview paragraph">To establish the worth of this new casting process, identically-shaped parts were fabricated from the same nickel-base superalloy, Mar M-200, following either the “controlled conventional process” or the “directional solidification process.” The parts were then subjected to identical laboratory physical tests and accelerated endurance tests in Pratt & Whitney Aircraft experimental engines. The results obtained from these tests are described.</div>

Nickel-based superalloys have served as the most competitive high temperature structural materials under highly stressed and aggressive operating conditions in a variety of applications for more than 60 years. The most demanding among all the applications has been the gas turbine aerofoil castings of modern aero-engines. These turbine parts operate in extremely aggressive environment of high velocity hot combustion gas-air mixture carrying highly corrosive ingredients at high pressure. Gas turbine aerofoil materials should therefore possess adequate resistance to creep, fatigue and aggressive environment. Materials design for such application therefore has been extremely challenging, particularly since the engine designers always aim at higher turbine entry temperature (TET) in order to achieve greater engine thrust and better fuel efficiency. In spite of enormous efforts made in the recent past towards developing ceramics and their composites, Ni-based superalloys continue to be most reliable blade and vane materials offering always the highest TET. This has been possible through better alloy design, improved blade cooling schemes, protective coatings and directional solidification (DS) of either columnar grains or single crystals (SC) along the most favorable 〈001〉 texture. During the last six decades, TET has gone up by about 500K. This article covers recent advances in cast Ni-based superalloys, including our own efforts in this direction. Extensive research at DMRL has led to the development of new generation Ni-based superalloys, designated as DMD-4 and DMS-4 for DS and SC processing, respectively. Simultaneously, expertise has been developed to cast DS and SC components for aero-engines. Technology has also been established for pilot scale production of these components.

Effect of growth rate on microstructure evolution in directionally solidified Ti–47Al alloy

Numerical simulation studies have been made on multi crystalline-Silicon (mc-Si) growth by directional solidification (DS) process for Photovoltaic (PV) application. Heat transfer plays an important role in the DS process as dislocation density and growth rate are controlled by temperature gradient of the DS furnace. The heat transfer in the DS furnace is controlled by movement of side wall insulation at different velocity upto 200 mm from the bottom insulation. The simulation results show that the thermal stress in the mc-si ingot for 0.2 mm/min velocity is minimum.

Microstructure and mechanical behavior of directionally solidified Fe35Ni15Mn25Al25

Numerical simulation of dendrite growth in Ni-based superalloy casting during directional solidification process

The service performance of single crystal blades depends on the crystal orientation. A grain selection method assisted by directional columnar grains is studied to control the crystal orientation of Ni-based single crystal superalloys. The samples were produced by the Bridgman technique at withdrawal rates of 100 μm/s. During directional solidification, the directional columnar grains are partially melted, and a number of stray grains are formed in the transition zone just above the melt-back interface. The grain selected by this method was one that grew epitaxially along the un-melted directional columnar grains. Finally, the mechanism of selection grain and application prospect of this grain selection method assisted by directional columnar grains is discussed.

<div class="htmlview paragraph">A gas turbine combustion system is an embodiment of all complexities that engineering equipment can have. The flow is three dimensional, swirling, turbulent, two phase and reacting. The design and development of combustors, until recent past, was an art than science. If one takes the route of development through experiments, it is quite time consuming and costly. Compared to the other two components viz., compressor and turbine, the combustion system is not yet completely amenable to mathematical analysis.</div> <div class="htmlview paragraph">A gas turbine combustor is both geometrically and fluid dynamically quite complex. The major challenge a combustion engineer faces is the space constraint. As the combustion chamber is sandwiched between compressor and turbine there is a limitation on the available space. The critical design aspect is in facing the aerodynamic challenges with minimum pressure drop. Accurate mathematical analysis of such a system is next to impossible. However, because of the advent of fast digital computers now a days it has become possible to model both geometrical and fluid dynamics aspects of a combustor using computers. The advances in modeling techniques have evolved over a period of time. Commercial codes are now available which has made the task comparatively easier.</div> <div class="htmlview paragraph">The present study is an attempt to predict numerically the entire flow field from compressor exit to combustor exit. The novelty of this study is in the modeling of all the holes amounting to 1700 in all, with appropriate grid distribution. The inclusion of conjugate heat transfer was a challenge. The appropriate governing equations are solved using RNG k- ε turbulence model for non-reacting flows whereas for reacting flows with heat transfer only k- ε is used. Eighteen million grid cells are employed to incorporate holes, fuel injection, combustion and conjugate heat transfer. The combustion is modeled using prePDF combustion model. It took nearly 2000 hrs of CPU time in a sun server (4 node machine) with the main memory of 128GB.</div> <div class="htmlview paragraph">Non-reacting flow analysis of the combustor reveals that size of the holes in dome and flare has little effect on the total pressure loss. However, increase in inlet velocity by 37% increases the total pressure loss by 86% across combustor. An Injection velocity of 75 m/s is found to give a good combustion characteristic inside the flame tube. Therefore conjugate heat transfer study is carried out with this injection velocity. Temperature distribution inside the flame tube and heat transfer through flare and the hot streaks in the liner wall are studied. Flare (without holes) is subjected to higher temperature (2480K) as it is found to be close to the primary combustion zone. It is also inferred from the study that the presence of holes protects the dome and flare from high temperature due to combustion. Holes in dome and flare are found to reduce the maximum temperature by 350K.</div> <div class="htmlview paragraph">The predicted results provide a complete picture of flow field as well as temperature distribution. Pressure drop and pattern factor have been evaluated. Even \cf4 cf4 tho\cf4 cf1 ugh this particular study has not been validated, the code has already been validated for a different geometry given in reference [<span class="xref">13</span>].\cf4 cf4 It\cf4 cf1 is gratifying to note that the present numerical procedure has predicted reasonable and plausible results.\cf4 cf4 It\cf4 cf1 \cf4 cf4 may be\cf4 cf1 concluded that CFD can \cf4 cf4 be \cf4 cf1 used with reasonable confidence, for the design and development of gas turbine combustion system. However, to enhance the confidence, validation must \cf4 cf4 be \cf4 cf1 attempted in future studies.</div>

<div class="htmlview paragraph">The development of gas turbine engines can benefit from the synergistic aspects gained by-partnership agreements. Development cost and risk are minimized by careful selection and application of modern technology. These conclusions are verified by examining the structure and progress of the following three major aircraft gas turbine development programs: <ul class="list disc"> <li class="list-item"><div class="htmlview paragraph">The T406-AD-400 turboshaft for the V-22 Osprey</div></li> <li class="list-item"><div class="htmlview paragraph">The T800-LHT-800 turboshaft engine for LHX</div></li> <li class="list-item"><div class="htmlview paragraph">The Model 578 propfan propulsion system for transport aircraft</div></li> </ul></div> <div class="htmlview paragraph">All of these programs contain partnership agreements that allow extensive development participation and all rely on a blend of developed hardware with infusion of modern technology. Each engine will provide a very competitive performance in its respective power class. The development progress within each program can be characterized as trouble-free and unburdened by technically-caused delay.</div> <div class="htmlview paragraph">The trend toward competitive procurement for propulsion systems can be used to advantage if addressed by a positive program structure that encourages participation by major partners during full-scale engine development.</div>

The advanced electron beam melting (EBM) system with the combination of vacuum refining and directional solidification (DS) performed the purification of large amounts of metallurgical grade silicon (MG-Si). In order to increase grain size or to align columnar grains being parallel to DS pulling direction in Si ingots, non-irradiated inner diameters in an EB pattern in the DS process were varied at a range of 5–35 mm. Average grain size increased with increasing non-irradiated inner diameter due to a smaller temperature gradient during the solidification of Si melts. However, the slope of the grain boundary inclined towards the ingot axis, which led to the formation of a triple junction in the ingot center in the case of large non-irradiated inner diameter. This happened despite there being a large temperature gradient due to the turbulent flow in the pool. This work reported that a purity of 99.8% for MG-Si was improved to above 99.999% with an ingot yield of 90% for 1 h.

<div class="htmlview paragraph">The feasibility of a simple-cycle gas turbine engine for automobiles is analyzed by means of comparison with Otto-cycle engines for automobiles. Applicable technology is reviewed with emphasis on centrifugal compressor technology, which has demonstrated pressure ratios of better than 10:1. The design point of a simple-cycle 150 hp automobile engine based on these components is presented, together with estimated torque and fuel-flow characteristics for the entire range of possible engine performance, derived from engine-cycle analysis programs.</div> <div class="htmlview paragraph">The results of a vehicle performance computer program designed to evaluate candidates for this application include time/speed/distance performance estimates for maximum acceleration as well as fuel economy and emissions estimates for pertinent driving cycles. Summaries of manufacturing cost estimates are presented for several candidate gas turbine engines.</div> <div class="htmlview paragraph">A speculative overall cost for both four and ten years of ownership is estimated based primarily on these fuel economy and manufacturing cost derivations.</div> <div class="htmlview paragraph">It is concluded that the simple-cycle gas turbine engine studied is sufficiently attractive as a low-pollution automobile engine for the post-1975 decade to warrant full-scale demonstration as soon as possible.</div>

<div class="htmlview paragraph">Renewed interest in a gas turbine as an alternative driveline for heavy trucks within Volvo, driven by the demands for lower emissions, resulted in a pre-study during 1990-1992 and the development and testing of a demonstrator engine between 1993 and 2000. To achieve the demanding goals for efficiency and emissions, the engine was designed as a recuperated and intercooled two shaft powerplant with a low-emission combustor.</div> <div class="htmlview paragraph">Following a comprehensive aerodynamic and concept study, engine components were rig tested and the engine mechanical design was finalized. Two engines were built and tested in a rig and in a truck. Very low exhaust emissions were demonstrated during rig testing, with NOx as low as 0,22 g/kWh in a 13-mode test. The efficiency goal of 42 % was not reached, but given more development time the measured efficiency of 38,6 % can most certainly be improved. Still, it will be the fuel consumption that is the major disadvantage compared to a diesel engine.</div> <div class="htmlview paragraph">The engine was also installed in a Volvo FH12 truck and tested for a limited time.</div> <div class="htmlview paragraph">The pros and cons of the gas turbines for trucks are discussed with some emphasis on emissions and fuel availability. Some areas where efforts must be made to improve the gas turbine are described and some suggestions of how to do it are given.</div>

The cost of the turbine blades for the gas turbine engines is increasing, because the latest superalloys for the turbine blades contain precious elements (i.e. precious metals, rate-earth metals) and the casting processes come to be complicated (i.e. directionally solidification, single crystal) to give higher creep resistivity against the increasing turbine inlet gas-temperature (TIT). Therefore, provision of a proper exchange period for the turbine blade can effectively reduce the life cycle cost in the gas-turbine engine operation. In this study, X-ray super-lattice reflections from the γ′ precipitate with L12 type crystalline structure were carefully evaluated, and the full width at half maximum (FWHM) of ω rocking curves of them were increased as the creep life fraction. On the other hands, θ-2θ profiles of the γ′ remained sharp even the creep ruptured sample. Therefore, the increase in the FWHM of the ω rocking curves was thought not to reflect the lattice disordering in γ′, but to reflect the degree of the misorientation among the γ′ precipitates. It was also supported by the electron backscatter patterns (EBSP) analysis. Although the reason of the relationship between creep life fraction and the γ′ misorientation must be revealed in further study, this evaluation method is expected as a non-destructive inspection to determine the creep damage level for the turbine blade.

Distributions of substitutional and interstitial impurities in silicon ingot with different grain morphologies

Performance deterioration in gas turbine engines (GTEs) depends on various factors in the ambient and the operating conditions. For example, humidity condensation at the inlet duct of a GTE creates water mist, which affects the fouling phenomena in the compressor and varies the performance. In this paper, the effective factors on the short-term performance deterioration of a GTE are identified and studied. GTE performance level is quantified with two physics-based performance indicators, calculated from the recorded operating data from the control system of a GTE over a full time between overhaul (TBO) period. A regularized particle filtering (RPF) framework is developed for filtering the indicator signals, and an adaptive neuro-fuzzy inference system (ANFIS) is then trained with the filtered signals and the effective ambient and the operating conditions, i.e., the power, the air mass flow, and the humidity condensation rate. The trained ANFIS model is then run to simulate the GTE performance deterioration in different conditions for system identification. The extracted behavior of the system clearly shows the dependency of the trend of performance deterioration on the operating conditions, especially the humidity condensation rate. The developed technique and the results can be utilized for GTE performance prediction, as well as for suggesting the optimum humidity supply at the GTE intake to control the performance deterioration rate.