M. Blicharski

Friction stir welding of aluminium 7136-T76511 extrusions

Abstract This research programme evaluates the as welded properties of Al 7136-T76511 extrusions joined through friction stir welding (FSW). Microstructural characterisation and mechanical testing were performed on the baseline material and on panels friction stir welded at 250 and 350 rev min–1 (all other weld parameters held constant). Transmission electron microscopy revealed the microstructural features in each of the unique weld regions and demonstrated that the precipitate density and morphology in these regions correlates with the temperature profile produced by the FSW process. A thermal model of FSW is developed that utilises an energy based scaling factor to account for tool slip. The slip factor is derived from an empirical relationship between the ratio of the maximum welding temperature to the solidus temperature and energy per unit length of weld. The thermal model successfully predicts the maximum welding temperatures and profiles over a range of energy levels. The mechanical behaviour after welding is correlated to the temperature distribution predicted by the model and to the observed microstructural characteristics. As welded mechanical properties of the alloy trended positively with the energy per unit length of weld, i.e. the highest joint efficiency was achieved at the highest welding temperature.

Influence of plastic deformation and prolonged ageing time on microstructure of a Haynes 242 alloy

The material used in this study was a commercial HAYNES® alloy 242™ with a nominal composition of Ni‐25% Mo‐8% Cr (in wt.%). In the standard heat treatment, the 242 alloy is annealed at a temperature between 1065 and 1095 °C and then water quenched. The ageing treatment is carried out at 650 °C for 24 h in order to develop the long‐range‐order strengthening. The alloy in the conventionally aged condition was additionally cold rolled to 50% reduction in thickness and subsequently subjected to prolonged ageing at 650 °C for 4000 h. The enhanced diffusion resulted in the decomposition of the Ni2(Mo,Cr) metastable phase into the stable Ni3Mo‐based phase. The presence of the new stable phase increased the yield and tensile strengths but deteriorated the ductility of the alloy at both room and 650 °C temperatures.

Microstructure and mechanical properties of friction stir welded 7136–T76 aluminium alloy

Abstract The microstructure of the weld was examined by light and electron microscopy (scanning and transmission). The various regions, i.e. thermomechanically affected zone, heat affected zone and unaffected base material, were studied in detail to better understand the microstructural evolution during friction stir welding and its impact on basic mechanical properties. The change in morphology of the strengthening phases reflected the relative temperature profile and the amount of deformation across the welded joint during the stir welding process. The centre of the weld was composed of fine grains and coarse particles identified mainly as MgZn2. In the thermomechanically and heat affected zones, the grain size was not uniform, and the strengthening phases filled the grain interiors, while grain boundaries were surrounded by precipitation free zones. The size of the strengthening phase decreased towards the base material. The hardness profile of the friction stir weld displayed the lowest hardness on the retreating side. Tensile properties of the weld itself were superior to those for material containing weld.

Electron Microscopy Investigation of Inconel 625 Weld Overlay on Boiler Steel

The investigation was focused on the microstructure characterization as well as changes in chemical composition and hardness of water wall tubing weld overlaid with Inconel 625. The analysis comprised studies in a light and electron microscopy scale that included the evaluation of weld overlays microstructure and microsegregation of alloying elements across the overlay and base metal interface. The particular attention was turned to the distribution of the main element content (Fe, Ni, Mo, Nb, Cr) in the base metal fusion zone as well as in the weld overlay itself. It was shown that the solidification process resulted in significant segregation in alloying elements giving rise to the substantial differences in chemical composition between dendrite cores and interdendritic spaces. It is believed that the microsegregation together with precipitation of secondary phases may contribute to the deterioration of corrosion resistance and overall mechanical properties of weld overlay including ductility and fracture toughness.

Microstructural and flow characteristics of friction stir welded aluminium 6061-T6 extrusions

Abstract Tin plated 6061-T6 Al extrusions were friction stir welded in a 90° butt weld configuration. A banded microstructure of interleaved layers of particle rich and particle poor material comprised the weld nugget. Transmission electron microscopy revealed the strong presence of tin within the particle rich bands, but TEM foils taken from other microstructural regions showed no indication of Sn containing phases. Since tin is limited to the surface of the preweld extrusions, surface material flowed into the nugget region to form the particle rich bands, while the particle poor bands originated from material within the extrusion thickness. The morphology of the principle strengthening phases indicated the relative temperature profile across the welded joint, and hardness profiles of as welded specimens consistently displayed the lowest hardness on the retreating side. Specimens that were solution heat treated and aged after welding revealed a normalised hardness profile across the weld.

Scanning and transmission electron microscopy microstructure characterization of mechanically alloyed Nb-Ti-Al alloys.

Results are presented of an investigation of the microstructure development during mechanical alloying and following consolidation of an Nb15Ti15Al alloy. The alloy was synthesized from elemental as well as pre‐alloyed powders. The microstructure of this material was examined by transmission electron microscopy, scanning electron microscopy and X‐ray diffraction. The use of pre‐alloyed TiAl powder for synthesis of the Nb15Ti15Al alloy meant that a much shorter time was required to complete the mechanical alloying process compared with the synthesis of elemental powders. The investigation indicates that three phases were present in the consolidated materials: the Nb solid solution, the Nb3Al intermetallic phase and the dispersoid.

Microstructure and mechanical properties of Nb15Al10Ti alloy produced by mechanical alloying and high temperature processing

In this work, an Nb15Al10Ti alloy produced by mechanical alloying was investigated. The milling of elemental powders of Nb, Al as well as TiAl intermetallic phase resulted in the formation of homogenous niobium solid solution, Nbss, and refinement of powder particles. Powder after milling was consolidated by conventional hot pressing at 1300°C under pressure of 25 MPa as well as by hot isostatic pressing at 1200°C under pressure of 1 GPa. Microstructure of consolidated material was examined by transmission electron microscopy, scanning electron microscopy and X‐ray diffraction. Materials after consolidation were composed of three phases: niobium solid solution Nbss, Nb3Al intermetallic phase and titanium oxide dispersoid TiO. The analysis of the mechanical properties indicated that both refinement of microstructure as well as introduction of ductile Nbss into the microstructure contributed to very high yield strength and fracture toughness satisfactory for this strength.

TEM investigation of ductile iron alloyed with vanadium

This article presents results of the processing and microstructure evolution of ductile cast iron, modified by an addition of vanadium. The ductile iron was austenitized closed to the solidus (1095°C) for 100 h, cooled down to 640°C and held on at this temperature for 16 h. The heat treatment led to the dissolution of primary vanadium‐rich carbides and their subsequent re‐precipitation in a more dispersed form. The result of mechanical tests indicated that addition of vanadium and an appropriate heat treatment makes age hardening of ductile iron feasible. The precipitation processes as well as the effect of Si content on the alloy microstructure were examined by scanning and transmission electron microscopy. It was shown that adjacent to uniformly spread out vanadium‐rich carbides with an average size of 50 nm, a dispersoid composed of extremely small ∼1 nm precipitates was also revealed.

Mechanical alloying and high pressure processing of a TiAl‐V intermetallic alloy

An alloy with a chemical composition of Ti‐45Al‐5V (at.%) was synthesized by mechanical alloying in a Szegvari‐type attritor from elemental powders of high purity. Before compaction, the powders were characterized by X‐ray diffraction and scanning as well as transmission electron microscopy. The compaction of powders was carried out by hot isostatic pressing and hot isostatic extrusion. The resulting material was subjected to microstructural and mechanical characterization. The microstructure investigated by transmission and scanning electron microscopy supplemented by X‐ray diffraction revealed that the bulk material was composed of a mixture of TiAl‐ and Ti3Al‐based phases, however, the typical lamellar microstructure for such alloys was not observed. The materials exhibited exceptionally high yield strength together with satisfactory ductility and fracture toughness. The high strength was unequivocally due to grain refinement and the presence of oxide dispersoid.

Electron Microscopy Investigation of Ageing Behavior in a Cu–Ni–Si Alloy

The investigated material was a commercial CW111A (CuNi2Si) copper alloy containing nominally 1.6–2.5 wt% Ni and 0.4–0.8 wt% Si. The thermo-mechanical treatment consisted of three-stage forging beginning at 900°C and immediate quenching followed by ageing at 500°C for 5 hours bringing about a balance of strength and electrical conductivity suitable for application of the alloy as electrical connectors. The main strengthening phase in the CW111C (CuNi2Si) copper alloy is the β-Ni3Si phase.