Microstructural analysis of stir zone of Al alloy produced by friction stir spot welding

Abstract

Friction stir spot welding (FSSW), which was recently developed as a spot joining technique, has been applied to a lap joint of Al alloy 6061 sheets, 1 mm in thickness, to clarify its microstructural features of the joint. A nugget shaped stir zone having finer grain size was observed around the exit hole of the probe. Crystallographic texture analyses using EBSD method suggested that the material flow occurred along the rotating direction of the FSSW tool in the wide region including the stir zone. In the periphery of the nugget shaped stir zone, which was characterised by finer grain size than the stir zone interior, no inclusions or precipitates were found on the SEM scale. A softened region was formed around the joint centre, which could be explained as resulting from dissolution and/or growth of the strengthening precipitates due to thermal cycle of FSSW.     

'Quench sensitivity' of 7075 aluminium alloy plates

Abstract

7075 aluminium alloy is widely used especially in those applications for which high mechanical performances are required. In the technical literature it is well known that the corrosion resistance and the mechanical properties of this material strongly depend on the cooling rate during quenching. This phenomenon is known as 'quench sensitivity'. The main aim of this paper is to investigate the influence of the cooling rate during quenching of samples taken from plates by varying the parameters of the heat treatment and the rolling direction (L, LT and ST). All the samples were heat treated in laboratory equipment to reach T6, T76 and T73 tempers. The samples were prisms (13 × 13 × 100 mm) and were quenched in water; the cooling rate was imposed by changing the water temperature or changing the polymer amount in a water agitated bath at 20°C. In each of the experimented condition, the cooling rate was measured by a thermocouple placed in the sample. Moreover, a finite element method (FEM) simulation was carried out in order to estimate the heat transfer coefficient during the cooling in all the experimented conditions. Tensile and intergranular corrosion tests were performed to point out the influence of the investigated cooling rates.     

Microstructural evolution of AS21X HPDC alloy during thermal treatment

Abstract

The microstructure of a High Pressure Die Cast Magnesium (HPDC) AS21X alloy was investigated after various heat treatments. The material, supplied in the as cast state, consisted of Mg-α grains separated by intermetallic particles such as Mg₁₇Al₁₂, Mg₂Si and Al–Mn. The alloy was subjected to solution treatment at 415° C for times ranging from 0.5 to 48 h and to aging to assess grain growth stability and precipitation hardening. Light microscopy showed that Mg-α grains increase slightly in size whereas intermetallic particles do not disappear but assume a more rounded shape. Static precipitation and/or dissolution were followed by electrical conductivity, hardness measurements and X-ray diffractometry. Tensile properties at room temperature were evaluated on both the as cast and solution treated samples. Density was used as an indicator of porosity to explain the scatter in elongation to fracture data. Study of the fracture surfaces revealed the morphology of porosity and the otherwise ductile fracture failure mechanism.     

Corrosion Behavior of Ti60 Alloy Coated with a Solid NaCl Deposit in O2 Plus Water Vapor at 500-700°C

The corrosion behavior of Ti60 has been studiedin oxygen containing water vapor at500-700^°C, with and without a solid NaCldeposit on the surface. No measurable corrosion of theTi60 alloy was observed in atmospheres without NaCl.Corrosion of Ti60 was accelerated in the presence ofNaCl because of the formation of a nonprotectiveTiO₂ oxide scale; the corrosion of Ti60 issignificantly aggravated due to the synergistic effect ofNaCl and water vapor. When the temperature was increasedup to the melting point of NaCl, the corrosion of Ti60coated with NaCl both in air-NaCl and inO₂-NaCl plus water vapor became more severe. Amechanism to explain the effect of water vapor and NaClon the corrosion of Ti60 is proposed.     

Effects of Chromia Coatings on the High-Temperature Behavior of F17Ti Stainless Steel in Air: Analytical Studies of the Effect of Rare-Earth-Element Oxides

The MOCVD deposition of neodymium oxide and/orchromium oxide provided beneficial effects both onisothermal- and cyclic high-temperature behavior ofcommercial F17Ti stainless steel. Fracture crosssections provided information about the morphology ofthe oxide scales formed on bare steel and coatedspecimens. The chromia scales developed small equiaxedgrains on the Nd₂O₃-coated samplesand columnar grains on the uncoated ones. Neo dymium segregatedwithin a surface layer composed ofMn_1.5Cr_1.5O₄ spineloxide. A complex phase (close to the structure ofCeTi₂₁O₃₈) was identified in thiszone. It could act as a source of neodymium ions, which couldsegregate to the grain boundaries of the chromia scale.Polished cross sections associated with X-ray mappingstudies confirmed the scale structure and the location of the rare-earth element in the outer part ofthe oxide layer.     

Melting and solidification of TiNi alloys by cold crucible levitation method and evaluation of their characteristics

Abstract

The addition of Re, Fe and Cr into Ti–50 mol.-%Ni has been carried out to improve the oxidation and mechanical properties. The mono phase consisting of TiNi with the B2 type structure was identified in micro-alloyed materials proposed on the basis of the d-electrons concept. Experimentally, TiNi alloys were melted and solidified by the cold crucible levitation melting (CCLM) method. The TiNi–(Cr, Fe, Re) alloys with high purity and without contamination from a crucible were prepared, and the homogeneous microstructure was achieved by the diffusion mixing effect of CCLM even in the as-cast alloys which contained Re and Cr with higher melting temperatures and different specific gravities. The transformation from austenite to martensite phases occurred in all alloys below or above room temperature. Some alloys had the ability of shape memory even at room temperature. Ternary alloys showed a higher flow stress level compared with the binary TiNi alloy. On the other hand, the oxidation at 1273 K was promoted by the formation of titanium oxides (TiO₂) on the alloy surfaces. The oxidation resistance was improved by the formation of the continuous Cr₂O₃ film in TiNi–Cr alloys. The alloying effects by ternary elements (Re, Fe, Cr) in the intermetallic TiNi as well as metallic materials were explained well using two parameters used in the d-electrons concept.     

Laser welding of copper–nickel alloys: a numerical and experimental analysis

Abstract

Melt run trials were carried out on Cu–Ni bars using a CO₂ laser source in order to analyse the effects of welding parameters (i.e. laser power, welding speed) on geometrical characteristics and on the microstructure of the bead. Experimental results were then used to determine the source parameters to be employed in a finite element model (FEM) of the welding process, with particular attention paid to the thermal field induced by the laser beam. A specific procedure, named 'automatic remeshing technique', was used in order to minimise the computation time. The aim was to create a reliable numerical model, suitable for the optimisation, in practical cases, of welding processes of these kinds of materials. A good correlation, in terms of predicted cooling rates, with the values calculated from SDAS measurements, was observed.     

Compression behaviour of semisolid YL112 die casting aluminium alloy following isothermal heat treatment

Abstract

The semisolid compression deformation behaviour of YL112 die casting aluminium alloy with the non-dendritic and dendritic structures has been compared in tests using a Gleeble-1500 thermomechanical simulator. The non-dendritic structure was obtained by isothermal treatment at 570°C for 120 min. In tests up to compressive strains of 0·8, the semisolid compression stress of the alloy with non-dendritic structure initially increases rapidly with increasing strain, then decreases, before reaching a plateau value. Under different deformation temperatures and deformation rates, the maximum compressive stresses are obtained in all cases at strain values of ～0·07. The semisolid deformation stress decreases with increasing deformation temperature and increases with increasing deformation rate. The compression behaviour of the two types of alloy differs. The semisolid compression stress of the alloy with dendritic structure is higher than that of the alloy with non-dendritic structure; and for strains >0·07, the semisolid compression stress increases and decreases with increasing strain for alloys with dendritic and non-dendritic structures respectively.     

Observations of the phenomenon of abnormal arc voltage occurring in pulsed metal inert gas welding of aluminium alloy

Abstract

At the time of arc reignition after short circuiting during electrode positive polarity, cathode spots are newly formed in the centre of the weld pool surface, where oxides scarcely exist. The work function of the cathode surface increases and the cathode spots concentrate because of the lack of sufficient oxides, leading to an increase in the potential gradient across the cathode fall space and the adjoining contraction space. Consequently, the arc voltage becomes abnormally high in spite of the short arc length. Moreover, when electrode polarity is switched from positive to negative immediately after a droplet has detached from the wire tip, cathode spots are newly formed on the surface of the molten metal remaining at the wire tip, where little oxide exists, leading to an abnormal increase in arc voltage as well. Therefore, the change in arc voltage does not necessarily indicate a fluctuation in the arc length.     

Fatigue properties of Al–Si casting alloy with cold sprayed Al/SiC coating

Abstract

The fatigue properties of Al–Si alloy cold sprayed Al and Al–SiC composite coatings have been studied. The specimens coated with composites reinforced with a large volume (25%) of fine SiC particles exhibited improved adhesion strength at the interface due to crater formation, and cyclic fatigue lives at room temperature more than three times those of uncoated specimens. In high temperature low cycle fatigue tests at 250°C, the pure Al coatings showed longer fatigue lives than the Al–SiC composite coatings, which is attributed to an increment in ductility at the surface retarding fatigue crack initiation.