Turbocharger Wheels Research Articles

Precipitation behavior of hexagonal carbides in a C containing intermetallic γ-TiAl based alloy

Intermetallic γ-TiAl based alloys are currently used as structural materials in high-temperature applications such as turbocharger wheels, valves and turbine blades. Adding C yields solid solution and precipitation hardening of the TiAl alloys, which raises their service temperature. Within this work, the effect of C on the TNM alloying system is analyzed by complementary differential scanning calorimetry measurements and selected heat treatments in conjunction with detailed electron microscopy analysis. This allowed the determination of the transition temperatures and revealed the precipitation onset of hexagonal Ti2AlC carbides between 1450 °C and 1475 °C. The precipitation kinetics of the carbides were further investigated by laser scanning confocal microscopy and revealed a preferred and steady growth in the length direction. Further, orientation relationships between the TiAl phases and the carbides from Blackburn and Burgers type could be determined by electron back-scatter diffraction. The results of this study provide a better understanding of the effect of C addition on the TNM alloy and thus enable further improvement of the alloying design process.

Short-Time Creep Deformation of the Coarse-Grained Nickel-Base Alloy 247 and Its Implications on the High-Cycle Fatigue Behavior

Abstract Cast polycrystalline nickel-base superalloys are typically used for critical high temperature aerospace and automotive components, such as turbine blades or turbocharger wheels. These high temperature components can undergo creep damage caused by, e.g., centrifugal forces on the turbine rotor blades, as well as high-cycle fatigue (HCF) from, for instance, vibrations of the rotor blade. Therefore, both, creep resistance and fatigue strength are important mechanical properties of these materials. The study presented here addresses the introduction of creep damage and its influence on the high-cycle fatigue behavior. For this purpose, fatigue specimens of coarse-grained Alloy 247 LC CC blade root material were prestressed on a pneumatic creep test rig to achieve creep-induced microstructural damage. The isothermal 900 °C short-time creep investigations below 1000 h test duration were performed at different tensile stresses which result in different strain rates. After the creep tests, scanning electron- and optical microscopy investigations of metallographic cross section of selected specimens were performed to determine the creep-induced grain boundary damage such as pore count, pore size, and coarsening of the γ' structure. This was followed by uniaxial, stress-controlled isothermal 850 °C high cycle fatigue tests on undamaged and predamaged specimens at a frequency of 10 Hz and a load ratio of R = −1. Subsequently, fractographic and cross section investigations, which provide information on the fatigue cracking, the failure initiating defects, and the pore morphology, were performed. The results of this study show that the degree of creep induced porosity is a dominant structural parameter for the HCF behavior of the investigated nickel-base superalloy.

Engine performance improvements through turbocharger matching and turbine design

AbstractIt is interesting to improve engine system performance with turbocharger technologies. In this study, a systematic simulation for engine and turbocharger matching to provide full utilization of the turbocharger potential and improve the engine performance without sacrificing the emission is developed. A velocity ratio concept was proposed to count the turbocharger performance impacts due to the diameter ratio of compressor and turbine wheels. A design of experiments was used to optimize the turbocharger and engine performance for different turbocharger factors. A better‐matched turbocharger was obtained. A multidisciplinary optimization method was used to design a mixed flow turbine wheel to reduce the turbine velocity ratio at peak efficiency and increase the overall turbocharger efficiency. Results showed that about 0.4% torque improvements and 1.2% reductions in the engine brake‐specific fuel consumption were obtained without making any other changes to the engine. This study demonstrated that systematic simulations for engine systems and considering turbocharger wheel diameter ratio effects could further improve the turbocharged engine system matching and the engine performance.

Comparison of wrought and additively manufactured IN718 concerning crack growth threshold and fatigue crack growth behaviour

Rotary components in turbine machinery such as gas turbine blades and discs or turbocharger wheels need to be designed so that high cycle fatigue (HCF) loads are well below the fatigue endurance limit to ensure safe operation. This requires an accurate measurement of the fatigue endurance limit and the related intrinsic fatigue crack growth (FCG) threshold. Furthermore, knowing the FCG threshold is crucial in order to assess the criticality of initial defects (e.g. cavities, carbide nests, etc.). In this study, the influence of the manufacturing route on the FCG threshold and the FCG behaviour at 650 °C in air of the nickel based superalloy IN718 is investigated. For this, a per Laser Powder Bed Fusion (LPBF) process manufactured version is compared to a conventionally wrought IN718 material state, both in the short and long crack regime. To measure crack growth increments of about 1 μm, an alternative pre-cracking and threshold test procedure is proposed for high temperature testing. To assess the behaviour of both material states in the short crack growth regime, cyclic R-curves have been generated taking the influence of two different R ratios into account. A further comparison of the FCG behaviour of the LPBF and the wrought material state is made on the basis of da/dN-ΔKI plots, with a focus on the Paris regime created from both classical FCG tests as well as continued threshold tests. The results observed are discussed in relation to the initial characterization of the two material states, which includes tensile tests at room temperature and at 650 °C as well as microstructural investigations.

Effect of Eutectic Characteristics on Hot Tearing of Cast Superalloys

The occurrence of hot cracks in the blades of Ni-based superalloy turbocharger wheels is thought to be closely related to eutectic characteristics. Different alloys that are susceptible to hot tearing, namely K418, K419 and K424, were studied. SEM was used to obtain the microstructure of the alloys. DSC, isothermal solidification quenching process and EPMA were conducted to determine the relationship between the eutectic characteristics and solidification process, segregation behavior and dendrite coalescence. The results show that a large volume fraction of γ/γ′ eutectic indicates a wide-temperature solidification range and severe segregation. A large γ/γ′ eutectic size is a result of element segregation and poor dendrite coalescence. Ti is thought to promote the formation of a large isolated residual liquid pool and lead to a large eutectic size, leading to the poor coalescence behavior of the dendrites. The volume fraction and size of the γ/γ′ eutectic were considered, and a eutectic influence factor (E) was defined to evaluate the effect of eutectic characteristics on hot tearing in cast superalloys. It was found that a high E corresponds to high hot-tearing susceptibility.

CFD 시뮬레이션 공력 성능 기법을 이용한 터보차저 휠 공력 특성이 스러스트 베어링 추력에 미치는 영향에 관한 연구

터보차저란 엔진 배기가스 유동 에너지로 터빈을 돌린 후, 이 회전력으로 컴프레서를 돌려서, 흡입되는 공기를 대기압 보다 높은 압력으로 밀어 넣어 엔진 성능을 극대화 시키는 장치이다. 터보차저회전 시, 컴프레서 및 터빈 휠의 공력에 의한 추력이 발생되며, 로터다이나믹 시스템의 축 방향으로 추력이 전달된다. 이때 축 방향으로 발생된 추력을 지지해주는 스러스트 베어링이 필요하게 되며 스러스트 베어링 설계를 위해 지지 가능한 추력 산출이 요구된다. CFD 해석을 이용하여, 휠의 추력을 산출하고, 컴프레서와 터빈 휠의 매칭을 통해, 베어링 추력 예측 방법을 알아보고자 한다. 본 연구를 통해 휠의 추력이 스러스트 베어링에 미치는 영향에 대하여 개발 프로세스를 정립하고자 한다.

Additive manufacturing of Ti-45Al-4Nb-C by selective electron beam melting for automotive applications

Selective electron beam melting (SEBM) is shown to be a viable production route for titanium aluminides components. Fully dense and crack free parts can be produced. In the present paper a titanium aluminide alloy Ti-45Al-4Nb-C was investigated and the complete processing chain was developed, i.e. starting from the determination of the processing window, the evaluation of corresponding material properties for cube like specimens and finally the production of turbocharger wheels. The material properties were optimized by adjusting scanning strategy as well as heat treatment with particular consideration of the application to turbocharger wheels. The issue of dimensional accuracy and the feasibility of joining will be discussed and a proof test is performed.

Fabrication processing and mechanical properties of Si3N4 ceramic turbocharger wheel

A composite technique was chosen to fabricate Si3N4 ceramic turbine wheel based on modified investment casting (MIC), slip casting (SP) and mold constraint hot isostatic pressing (MCHIP) aided by a near-net dimension using the gypsum mold and multi-piece Y2O3 ceramic mold. The detailed fabrication processing of Si3N4 ceramic turbine wheel was described. And the flexural strength and fracture toughness after different work temperature and speed were discussed. The results showed that owing to occurrence of phase transformation and chemical reaction, excess temperature resulted in interface cracking and interface debonding, flexural strength and fracture toughness decrease. Thermal expansion and centrifugal force under excessive high speed brought many pores in the microstructure and resulted in crack initiation and crack propagation. The critical work temperature was 700 °C and critical work speed was 100,000 r/min, which were obtained from the test and simulation results.

A study of low cycle fatigue life and its correlation with microstructural parameters in IN713C nickel based superalloy

Up to date, IN713C Nickel-based superalloy has been continued to be the best alloy candidate for turbocharger wheel applications due to its adequate fatigue property and resistance to degradation under harsh operating environments. Throughout this study, three different batches of as-cast IN713C nickel based superalloys with different microstructures including columnar, equiaxed and transition microstructures were investigated. Strain control Low Cycle fatigue (LCF) tests were conducted for the three different microstructures, achieving fatigue life between 100 and runout at 100,000 cycles, depending on the testing parameters. The fracture mechanics and failure mechanism were correlated to the alloy's microstructure, texture and chemical composition under various LCF conditions using optical microscopy, SEM, EDX and EBSD. In the current study an exact correlation between alloy's microstructure/microtexture and LCF endurance is established. The results showed that equiaxed microstructure has a superior fatigue life than the transition microstructure by 10% and columnar microstructure by > 200% at a given temperature and strain rate. This large discrepancy was mainly due to the grain size differences between the studied microstructures. Here, it was evidenced that the grain size controls the dendrites length. It is also demonstrated that all microstructures exhibited a longer fatigue life at room temperature than at 650 °C, doubling or tripling the fatigue life of the tested IN713C. Furthermore, the high presence of precipitates between dendritic arms in all three microstructures was found to have great influence on crack propagation path. It was apparent that segregated carbides in between dendritic arms caused secondary crack initiation and crack path undulations during the LCF tests.

Development and validation of a radial turbine efficiency and mass flow model at design and off-design conditions

Turbine performance at extreme off-design conditions is growing in importance for properly computing turbocharged reciprocating internal combustion engines behaviour during urban driving conditions at current and future homologation cycles. In these cases, the turbine operates at very low flow rates and power outputs and at very high blade to jet speed ratios during transitory periods due to turbocharger wheel inertia and the high pulsation level of engine exhaust flow. This paper presents a physically based method that is able to extrapolate radial turbines reduced mass flow and adiabatic efficiency in blade speed ratio, turbine rotational speed and stator vanes position. The model uses a very narrow range of experimental data from turbine maps to fit the necessary coefficients. By using a special experimental turbocharger gas stand, experimental data have been obtained for extremely low turbine power outputs for the sake of model validation. Even if the data used for fitting only covers the turbine normal operation zone, the extrapolation model provides very good agreement with the experiments at very high blade speed ratio points; producing also good results when extrapolating in rotational speed and stator vanes position.

Creep Properties of Ti-48Al-2Cr-2Nb Produced by Selective Electron Beam Melting

Titanium aluminides are highly attractive for high temperature applications involving dynamic components, e.g. turbine blades or turbocharger wheels, due to their high strength-to-weight ratio. The drawback is the difficult manufacturing of this material class due to the low toughness and high sensitivity to oxygen. Selective electron beam melting SEBM shows a new approach of producing complex titanium aluminide parts without a major oxygen pick up and avoiding problems with brittleness. The high cooling rates of this process lead to a very fine microstructure, which is not fully understood up to now. The microstructure determines the creep properties and therefore defines the performance of this material in high temperature applications. In this contribution, the creep properties of Ti-48Al-2Cr-2Nb fabricated by SEBM are investigated. The influence of the processing parameters and the building direction on the microstructure and the creep properties are discussed and compared to cast material.

Failure analysis in aluminium turbocharger wheels

Abstract This paper presents a failure analysis conducted in aluminium compressor wheels used in diesel turbocharged engines. These wheels were made with machined AA 2618T652 alloy and installed in light truck engines, which were used in rural and industrial atmospheres. The premature failures of the wheels happened after life between 40,000 km and 300,000 km, while the expected life was about 1,000,000 km. The present investigation showed that a fatigue process was triggered by intergranular corrosion on the upper camber surface of the wheel. A set of immersion corrosion tests was carried out to evaluate intergranular corrosion susceptibility of the alloy. The root-cause for the formation of the intergranular corrosion cracking in the compressor wheels could not be identified in the present investigation. The overall results indicated that an “in service” contamination of the compressor surface started the intergranular corrosion cracking, promoting the fatigue failure of the wheels.

Effect of Process Parameters on Roughness and Hardness of Surface and Dimensional Accuracy of Lost Wax Process Casting

We know different casting processes like sand casting, gravity die casting, low pressure die casting, pressure die casting, counter pressure die casting, and centrifugal casting, spin casting, lost wax and lost foam casting. But lost wax or lost foam casting process are going to be most acceptable way of castings to get the intricate and complex shaped castings. Lost wax process is generally known as “Investment casting process”. About 5000 years ago the Egyptians used this process in the time of Pharaohs to make gold jewellery, which was the way of their investment hence the names comes. Ancient “lost wax process” domination for producing beautiful sculptures turns into a present day near net shape manufacturing of intricate engineering components. The journey begins with the development of Investment casting process by Dr. William H. Taggart of Chicago who wrote a detailed study on Investment casting process in 1907. Today’s Investment casting is a versatile process. It is used to manufacture parts ranging from turbocharger wheels to golf stick heads, from electronic boxes to hip replacement implants, General engineering to aerospace, automobiles to pump industries particularly impellers. In this paper we are discussing about the key parameters of the investment casting process which will give a more dimensional accuracy and best surface smoothness and hardness for the mold. The pre-conditions for the mold are discussed and possible defects can occur in the mold and the rectification also analysed experimentally.

Qualification of an Investment Casting Process for Production of Titanium Aluminide Components for Aerospace and Automotive Applications

Over the past years significant efforts were dedicated to developing production processes for aerospace and automotive parts from light weight titanium aluminides. Considerable progress was achieved worldwide in the field of near net shape casting. Access has developed an own production route, based on centrifugal investment casting. Various components from TiAl-based alloys, including turbocharger wheels and stator vanes were successfully cast and supplied for engine tests. A newly developed automated centrifugal casting line was recently taken into operation. This presentation will describe our activities that aimed to qualify the casting line for series production of turbocharger wheels: apart from basic issues of casting cluster design, process control and quality inspection we will discuss the casting process from the perspective of its techno-economic characteristics

Solidification of high Nb containing TiAl based alloys

Casting of titanium aluminides is an attractive processing route for production of near net shape components: turbocharger wheels, valves and aero-engine components are presently at the heart of casting developments. Among the casting alloys under consideration are a number of niobium rich TiAl based alloys that contain low boron additions for grain refinement and minor additions of other elements to enhance creep resistance. An essential condition that must be met to achieve grain refinement is a solidification pathway competed via β-(Ti), e.g. a pathway that avoids peritectic growth of α-Ti. In this contribution we describe the microsegregation analysis of a unidirectionally solidified sample from the ternary alloy Ti–45Al–8Nb. The corresponding solidification path is discussed on the basis of thermodynamic calculations and is shown to closely follow Scheil predictions with some amount of back-diffusion for aluminium. The analysis indicates that the nucleation undercooling for peritectic α (Ti) in the deep mushy zone is significant.

Centrifugal precision cast TiAl turbocharger wheel using ceramic mold

It is the potential candidate for gamma TiAl-based alloys that would replace Ni-based superalloys, which are being used in fabrication of the turbocharger wheel currently. Cast Ti–47Al–2Cr–2Nb alloys due to their specific performance requirement are now on the verge of a commercial application. A novel precision casting technique, which combines the ceramic mold casting with centrifugal casting, was described in this work. Multiple-section ceramic mold was designed and fabricated successfully. All the mold sections fit together to form a cavity and then open in many directions to eject the molded part. Microstructures and mechanical properties of cast Ti–47Al–2Cr–2Nb alloys under the action of centrifugal force are discussed.

Vacuum Induction Melting and Investment Casting Technologies Tailored to Near-Gamma TiAl Alloys

The present study focuses on vacuum induction melting and investment casting of neargamma TiAl intermetallic alloys. The attention is mainly given to a cost-effective melting process in which a primary alloy ingot is re-melted in a ceramic crucible and cast into a ceramic shell mould. Two types of crucibles (based on Al2O3 and Y2O3) are considered. The most detrimental reactions that govern the contamination of the molten alloy with ceramic particles were determined. Results suggest that the crucible wall attack can be considerably limited by using either the Y2O3 (with no SiO2-type binder) or Al2O3 crucibles with a suitable coating. After pouring, a mechanical interaction associated with different thermal expansions of TiAl casts and ceramic shell moulds can result in serious product damage. A simple 1D-1D model of the cooling process was formulated and the heat flow as well as stress states in the cast-mould system were numerically solved. Process parameters (melt superheat, initial mould temperature, cooling kinetics and mould composition) were optimized in order to reduce the stress in the casts. The optimized parameters delimited a processing window in which complex-shaped TiAl castings like turbocharger wheels can be fabricated.

Microstructure and mechanical properties of precision cast TiAl turbocharger wheel

From a technical perspective, many of the unique properties of TiAl-based aluminides making them attractive for high-temperature structural application also assign these alloys a challenge to process into useful products. Cast TiAl alloys due to their relatively low production cost are on the verge of a commercial application, especially in vehicle industry. The results concerning technology of precision casting together with microstructural and mechanical tests examinations of a turbocharger TiAl-based wheel prototype have been described. The processing technology of “self-supporting” ceramic shell molds was successfully verified during precision casting of turbocharger wheels. According to all results, a processing window for precision casting was established.

An application of a recursive Kalman filtering algorithm in rotating machinery fault diagnosis

In this paper, an application of adaptive order tracking fault diagnosis technique based on recursive Kalman filtering algorithm is presented. Order tracking fault diagnosis technique is one of the important tools for fault diagnosis of rotating machinery. Conventional methods of order tracking are primarily based on Fourier analysis with reference to shaft speed. In this study, a high-resolution order tracking method with adaptive Kalman filter is used to diagnose the fault in a gear set and damaged engine turbocharger wheel blades. The adaptive Kalman filtering algorithm can overcome the problems encountered in conventional methods. The problem is treated as the tracking of frequency-varying bandpass signals. Ordered amplitudes can be calculated with high resolution after experimental implementation. Experiments are also carried out to evaluate the proposed system in gear-set defect diagnosis and engine turbocharger wheel blades damaged under various conditions. The experimental results indicate that the proposed algorithm is effective in fault diagnosis of both cases.

New surface treatment using shot blast for improving oxidation resistance of TiAl-base alloys

A new surface treatment using shot blasts with WO 3 powder has been developed. This has been most effective in improving the cyclic oxidation resistance of TiAl-base alloy in air and in a diesel engine exhaust gas atmosphere of up to 1223 K. This effect is attributed to the formation of an Al 2O 3 layer in an oxidation atmosphere. The above layer is supposed to be formed as follows: Since the WO 3 powder is shot repeatedly during the surface treatment, a WO 3 layer adhering to the substrate is formed on the surface. This layer causes the preferential oxidation of aluminum at the early stage of oxidation, resulting in the formation of a sound and continuous Al 2O 3 layer. The developed surface treatment gives the same oxidation resistance as that of nickel-base superalloy Inconel 713C up to 1223 K, and is applicable to complex-shape parts with as-cast skin surfaces, such as turbocharger wheels.