Alexandru Paraschiv

Effect of niobium alloying level on the oxidation behavior of titanium aluminides at 850°C

This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α2 + γ(Ti3Al + TiAl) and orthorhombic Ti2NbAl, recognized as candidates for high-temperature applications, was investigated by exposure of the alloys for 100 h in air. Thus, oxidation resistance was expressed as the mass gain rate, whereas surface aspects were analyzed using scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and the type of oxidation products was analyzed by X-ray diffraction and Raman spectroscopy. The orthorhombic Ti2NbAl alloy was embrittled, and pores and microcracks were formed as a result of oxygen diffusion through the external oxide layer formed during thermal oxidation for 100 h.

Investigation of some Intrinsic Properties of Thermal Sprayed Molybdenum Coatings for Railway Axle Applications

Thermal sprayed Mo coatings deposited on steel support deposited by electric arc and atmospheric plasma spray were investigated. Microstructural investigation of the coatings showed that in both variants the splats formed by rapid solidification and splashing during the impact with the substrate have irregular shape, exhibit cracks and scattered debris. The measured average equivalent diameter of the splats and the calculated average diameter of the particles that generated the splats are higher for electric arc spraying. Using the measured and calculated data it was descibed the relathionship between splats diameter, splats thickness and flattening degree. Some intrinsec properties of the coatings as roughness, hardness and bond strength were measured. All these properties are influenced by the splat size. Lower size of the splats lead to reduced surface roughness, higher hardness and higher bond strength of the coatings.

Study on Processing of Titanium Aluminide Alloy Using Electrical Discharge Machining

Currently there is a continuous extension of application areas of titanium aluminides but their processing encounters a number of difficulties such as the precision of machining and modifying the properties of the surface layer. In the specialist literature [1, 2, 4, 7] different authors address the processing of these materials by conventional methods. In this context, this paper aims at finding the optimal regimes for the electrical discharge machining (EDM) of titanium aluminides Ti-40Al-5NB-3V and provides the analysis of the process parameters effect on cutting through wire EDM technological characteristics. In the case of wire EDM a relatively good output has been obtained as well as very good roughness of the processed surfaces. On the contrary, when considering the die sinking edm, due to very small output, several experimental measurements were performed on three machines – Sodick, ONA and ELER. Based on the experimental data, it has been established that the best output is to be obtained by reverse polarity machining.

Evaluation of Low Velocity Impact Response of Carbon Fiber Reinforced Composites

The analysis of damage resulted from concentrated out-of-plane impact forces is a concern in case of fiber reinforced polymeric composite aerostructures. Thereby, it can be quantified in terms of resulting size/damage type by drop weight impact testing. The aim of the present research was to evaluate the impact response of CFRP laminates by FEA and experimental tests. For FEA, two failure criterions were used, energetic criterion and Tsai Wu criterion. In both modeling scenarios the laminate was fractured. In case of the energetic criterion it was observed a meandering crack along with internal delamination of the material plies, while in case of the Tsai Wu criterion the impactor passes through the specimen, destroying the impacted area. Experimental tests performed at 0.7 meters height and 33 J of impact energy showed that all material plies were visible damaged, but the impactor didn’t pass through all specimen thickness. Different damage modes were observed on the tested specimens, the impacted face of the specimen presented a combined damage consisting in large cracks with fiber breakage indentation/puncture and a dent while on the non-impacted side of the specimen splits, cracks and a small swelling were observed. Considering the results from FEA and experimental tests, it can be said that regarding the damage shape, the experimental results are in a good agreement with a combination of the results obtained from the two failure models of FEA.

High Temperature Behavior of Two Titanium Aluminides for Blade Engine Applications. Preliminary Study

The developmental tendencies in obtaining high performance of gas turbines are chiefly connected with an increase in the engine’s capacity, its efficiency, lifetime, and reduce the deleterious emissions. One way to achieve these targets is the usage of lighter and stronger materials and enhancing the method of parts manufacture. The present work is a part of a national project whose main goal is to develop a new class of light alloys-coatings structures for aerospace, energy and the automotive industry, fated to reduce pollution and to contribute to a friendlier environment. One way to enhancing the high temperature response against the severe environmental conditions, is by development of titanium aluminides with increased niobium content and two different structures: α2 + γ (Ti2Al + TiAl), and Orto AlNbTi2. The paper present the preliminary results on mechanical properties modification because of niobium content increased from 10 at.% to 25% and on beneficial influence of Thermal barrier coating.

An Investigation of the Thermal Sprayed Molybdenum Coatings Behaviour to Micro-Abrasion Wear

The wear behaviour of thick molybdenum coatings deposited by electric arc thermal spray on steel support was investigated by micro-abrasion, a relatively recent introduced method for small scale wear testing. The wear mechanisms and wear rates without coatings penetration were investigated with respect of time corresponding to primary and secondary wear stages. The micro-abrasion of Mo coatings using SiC abrasive slurry have been discussed and wear scar characteristics were evaluated based on the experimentally results. The worn surfaces of the tested specimen were examined by SEM and the specific wear rate was calculated from experimental data. For the testing durations used it was identified the change from grooving to rolling wear corresponding to the transition of wear mode from two-body to three body-abrasion.

Non-Contact Roughness Investigation of Ball-Cratered Molybdenum Thermal Spray Coatings

The effects of micro-abrasion wear on the surface roughness of molybdenum coatings deposited by electric arc thermal spray on steel support were investigated. The 2D surface roughness was measured and correlated with the experimental results of the micro-abrasion tests. Different worn surfaces which correspond to running-in and steady stage of wear were investigated in terms of the microstructure, 3D image and 2D surface roughness. The micro-abrasion tests were made in the presence of a SiC abrasive slurry for test durations between 200 – 1600 s and the worn surfaces were evaluated by scanning electron microscopy and stereomicroscopy.

Gamma Titanium Aluminides Behavior at High Temperature Static Short-Term Stress

Today conventional titanium-based alloys represent one third of the weight of modern aircraft engines and, are the second most used engine material following Ni-based superalloys. [1] Titanium aluminide alloys based on intermetallic phases γ (TiAl) and α2 (Ti3Al) and the most recent – orthorhombic titanium aluminide, are widely recognized as having the potential to meet the design requirements for high temperature applications. The outstanding thermo-physical and mechanical properties of these materials rely mainly on the strongly ordered nature and the directional bonding of the compounds. These involve: high melting point, above 1460°C, low density of 3,9-5 g/cm3, according the alloying degree, high elastic modulus (high stiffness), high yield strength and good creep resistance at high temperature, low diffusion coefficient, good structural stability at high temperature. The main objective of our paper are focussed on the short-term mechanical properties if Titanium niobium aluminide at 850°C. High temperatures mechanical properties evaluation was performed by tensile testing at temperature of 850°C on universal static and dynamic testing machine Instron 8802, equipped with high temperature system, for maximum 1000°C, and extensometer with a measuring basis of 40 mm. The mechanical tensile test was performed according the ASTM E8, with control of deformation and a testing rate of 10-4 mmsec.-1. Short-term behavior request of the support uncovered alloys, at 850°C has proved to be modest and it seems obvious that the alloys based on titanium aluminides cannot be used without protective coatings. Key words: titanium aluminides, high temperatures, mechanical properties