Corneliu M. Craciunescu

Copper Interlayer Contribution on Nd:YAG Laser Welding of Dissimilar Ti-6Al-4V Alloy with X5CrNi18-10 Steel

Welding of heterogeneous Ti-6Al-4V alloy and X5CrNi18-10 steel, using a continuous Nd:YAG laser was performed by inserting a 600-μm-thick Cu foil between the two components, thus enabling an improvement of the properties of the joint. The changes in the main laser welding parameters (power and speed), based on the microstructural observations and mechanical tests, show that the resulting properties are influenced by the nature and dimensions of the transitional zone between the unmelted Ti-6Al-4V and the melted bath, where the intermetallic phases accumulate.

Prediction of thermally-controlled actuation for shape memory alloy film-based bimorph cantilevers

A model is proposed for predicting the deflection of bimorph cantilevers made of a shape memory alloy film deposited onto solid substrates, by taking into account the changes in the thermal and elastic properties of the film during the martensitic phase transformation. The zero actuation reference temperature is the deposition or annealing temperature, with the film in the austenite phase. On cooling from this point, the thermoelastic stress building up in the bimorph causes bimetal-type behavior until the onset of the austenite–martensite transformation, which now controls the actuation up to its completion, when the bimetal actuation reappears. By adapting the classical bimetal thermostat model to the case when one component undergoes martensitic transformation, the deflection during heating and cooling is calculated by averaging the isostrain and isostress phase distribution bounding approximations; the case of a Ti50.1Ni49.9 film/Si was examined. Predictions in good agreement with the recorded deflection of a Ti49.2Ni50.8/Si cantilever (up to a temperature shift explained by composition change and stress developed in the film during the transformation) resulted, indicating a strong effect (attributed to the differences between the elastic and thermal constants of the two phases) of the martensitic phase transformation on the deflection slope and sign.

Pulsed Laser Processing of Dissimilar Ti-6Al-4V and X5CrNi18-10 Joints

Welding of Ti-alloys with stainless steels is a difficult task due to the formation of brittle intermetallic compounds, and the use of intermediate materials is a possible solution to surpass this problem. The joining of Ti-6Al-4V alloy with X5CrNi18-10 steel, using a Cu interlayer, was analyzed as a function of the Nd:YAG pulsed laser welding parameters, with focus on the effects on the integrity, microstructure, and the properties of the resulting joints. The quality of the joint is determined by the interface developed between the Cu interlayer and the Ti-alloy. An improvement of the properties was observed to be influenced by the limitation of the component mixing in the melt and by interfacial phenomena favoring the solidification process, especially at Ti-alloy interface.

Investigations on Joining Surface Hardened Steels by Friction Welding

Abstract The particularities of joining surface hardened steels (carburized, nitrided and induction hardened) using friction welding are analyzed in order to assess the influence of the wear protection layers on the welding process. The joint influence of the friction speed, friction pressure and time, as well as the upsetting pressure is evaluated for joints made out of carburized, nitrided and carburized with nitrided steels, respectively. The investigation of microstructure and properties allows an optimization of the friction welding parameters. The friction stroke appears to be the main parameter for welding carburized and nitrided steels, respectively. For welding carburized with nitrided parts, remains of carbon and nitrogen enriched materials are still present in the joining plane, regardless of the axial pressure and the friction stroke. Only a limited diffusion of the two components in the axial direction can be detected.

GRADED TRANSITIONS IN Ni-Ti SHAPE MEMORY ALLOYS PROCESSED BY SEVERE PLASTIC DEFORMATION

NiTi shape memory alloys were subjected to a single step severe plastic deformation via the equal channel angular pressing (ECAP). The properties were analyzed by comparing the transformation characteristics for sections across the thickness of the deformed sample, with focus on the deformation gradient in the outer layers of the materials and the front surface that gradually engages in the deformation. A gradient related to the crystallinity and the state of stress was observed across the thickness of the deformed sample.

Phase Transformations on Friction Welding of Dissimilar Thermochemically Treated Steels

The friction welding process provides a continuity of the joined materials at crystalline network level. This continuity is based on diffusion phenomena, on the forces of inter-atomic links as well as on the thermal and mechanical energy inputs. The action of the thermo-deformational field developed during the welding process is associated with solid phase transformations, influenced in terms of nature and intensity by the sensibility to mechanical hardening of the steel components, the critical quenching rate and the re-crystallization temperatures. The composition and microstructural gradients of thermochemically treated steels by carburization and nitriding leads to particularities in the morphology of the transformations induced in the welding plane. The work reports on the transformations observed in the joining plane explored by microstructural, hardness and impact testing.

A Study of Ti-Ta Library and Microstructure Manufactured by Magnetron Sputtering

Ti-Ta films were deposited out of Ti and Ta targets on glass and kapton substrates. The co-deposition leaded to the formation of a compositional spread in the Ti-Ta system, a continuous variation of the Ta/Ti ratio along the full range of the film. The typical microstructure observed for the films deposited at RT consisted of columnar grains with dotted Ta-rich particles, reflecting a relatively nonhomogenous structure. Annealing at 400 °C leaded to the individualization of grain boundaries and to a coalescence of the Ta-rich particles. Annealing at 500 °C further reduced the amount of particles, but also appeared to be associated with the cracking in the stressed area of the film/kapton structure.

Calorimetric Studies of the Martensitic Transformation in Ni-Mn-Ga Ferromagnetic Shape Memory Alloys

The thermal characteristics of the martensitic transformation in Ni-Mn-Ga alloys were studied using differential calorimetry in order to assess the influence of quenching and the heating rate on the resulting thermal properties. Following solution annealing at 850 °C and water quenching, sharper peaks are observed. The martensitic microstructure was observed by both optical and electron microscopy, showing typical needle like morphology and the martensitic transformation kinetics was analyzed using the Kissinger approach.

Superelastic Behavior in NiTi Shape Memory Alloy Wires and Ribbons

Shape memory alloy ribbons in austenitic state were studied in a tensile testing machine in order to assess their superelastic behavior. They were compared with conventional materials and hair wire. The shape memoy alloy ribbon shows a particular behavior, with an ultimate tensile stress of about 1450 MPa reached at 9.5 % strain. The superelastic plateau was recorded around 590 MPa on loading and around 350 MPa on the unloading branch. Following multiple loading and unloading cycles, the superelastic behavior was not affected, nor was affected the ultimate tensile strength, that remained in the same range as for the one tested before cycling. The advantages of the superelastic ribbon compared to the ones of conventional materials are discussed. The superelastic properties of the ribbon are in the range of single crystals on what concerns the recoverable strains. This is attributed to the particular fine microstructure of the NiTi ribbon.

Fracture modes during severe plastic deformation of NiTi shape memory alloys

The effects of severe plastic deformation on the surface micro-structural characteristics of NiTi shape memory alloys were observed after one single pass using the equal channel angular pressure technique. The analysis of the deformation and fracture showed distinct features related to the composition of the alloys, the temperature of the process, and the surface effects during the relative sliding in the die. In samples deformed at room temperature, the cracks initiated at the surface under tensile stress are amplified during the extrusion in the concurrent channel. The multiple cracks that develop during the friction process between the surfaces of the sample and channels of the die are the main cause for the fracture, even in materials that are less brittle, incorporating a smaller fraction of Ti2Ni precipitates and showing ductile fracture. A differential state of stress appears to exist in the deformed alloys starting from one surface to the other.