Reduction of intermetallic compounds in ultrasonic-assisted semi-solid brazing of Al/Mg alloys

ABSTRACT

An ultrasonic-assisted semi-solid brazing method was used to control Mg/Al intermetallic compounds (IMCs). Zn–20.95Al, which has a large semi-solid temperature range of 60°C, was chosen as filler metal. Oxide films on the Al/Mg surfaces were removed despite the semi-solid state of the filler metal at 430°C. Al–Mg–Zn IMCs were observed inside the joint at 667?W and 5?s. The IMCs was reduced as ultrasonication time was prolonged. At high power of 1000?W, short ultrasonication time was needed to guarantee that no IMCs remained inside the joint. Otherwise, new IMCs formed because of serious cavitation erosion on substrates. Ultrasonication time of 20?s was unable to completely eliminate IMCs and solid phases at 423°C.     

Effect of trace Ag on the microstructure and precipitation of an Al–Cu–Mg alloy

Abstract

This paper investigated the effect of different amounts of Ag addition on the microstructure, properties and precipitation processes of Al–4·6Cu–6·9Mg(wt-%) alloy using various analytical methods. It was found that Ag addition stimulated new X′ 9 and Ω phases precipitated finely and dispersively in the matrix, as a result of Mg–Ag co-clusters; the volume fraction of precipitates increased with the content of Ag addition. Such precipitation improved the mechanical performance of the Al–Cu–Mg alloy significantly. The mechanism for the formation of new precipitates is also described in this paper.     

Studies on magnesium and its alloys in battery electrolytes

Abstract

Studies on the anode efficiency of magnesium and its alloys in battery electrolytes such as magnesium perchlorate, magnesium bromide, and magnesium chloride solutions were made by galvanostatic polarisation and corrosion rate measurements. The open circuit potential and corrosion rate of magnesium increase when lead is present in the alloy. The corrosion rates were observed to follow the order Mg> AP65 (Mg?(6·0?7·0) Al?(4·4?S·0)Pb) > AZ61(Mg?(5·8?7·2)Al?(0·5?1·5)Zn) > AZ31(Mg?(2·5?3·5)Al?(0·6?1·4)Zn) in all three electrolytes. Anode efficiency increased with increasing current density up to 40 mA cm^?2, but decreased above this level. Galvanostatic polarisation results indicated that corrosion of magnesium and its alloys in these electrolytes occurs under cathodic control. Of the tested alloys, AZ31 and AZ61 were found to be most suitable in respect of corrosion rate and anode efficiency in 2M Mg(ClO₄)₂ solution.     

Oxide film characteristics of Al–7Si–Mg alloy in dynamic conditions in casting

Abstract

'New' oxide film which forms in a very short time in the casting process was studied. Samples for the study were prepared based on a technique in which an oxide–metal 'sandwich' could be made. Alloy A356 (Al–7Si–0.4Mg) was selected for the study. Features such as thickness of the oxide film, its morphology, rigidity and presence of eutectic phase have been examined and shown by SEM study. Possible consequences of the morphology of the oxide film are discussed.     

Microstructure and mechanical properties of spot welded Al–5·5Mg–0·3Cu alloy

Abstract

The influences of spot welding on the microstructure and mechanical properties of an Al–5·5Mg–O·3Cu alloy have been investigated. Results showed that dendrites were formed with porosity and cracks in the nugget. Grain boundary melting occurred in the heat affected zone and wide grain boundaries appeared. The alloy exhibited low hardness in the nugget centre. Tensile cracks propagated at the edge of the nugget and mixed rupture with dimples and intergranular fracture occurred. Fatigue fracture initiated at the edge of the nugget and propagated perpendicularly to the tensile axis. Transgranular fracture with striations was also observed.     

Solidification characteristics of Al–9·6Mg alloy solidified under high pressure (4 GPa)

Abstract

The present paper studies influences of high pressure up to 4 GPa at 1273 K on the microstructures and the phases of Al–9·6Mg alloy by means of optical microscopy (OM), X-ray diffraction (XRD), energy dispersive X-ray microanalysis (EDX), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The results showed that fine dendrite with long primary arm formed at 4 GPa in comparison with the coarse discontinuous dendrite under normal pressure. Specially, a new high pressure metastable phase, Al_(63·7)Mg_(36·3), was produced besides a small amount of β-Al₃Mg₂ phases at 4 GPa, which was not found in other solidification conditions. Al_(63·7)Mg_(36·3) phase has bcc structure with the lattice constant a=8·495 Å. Evolution mechanism of microstructures and phases was studied in the present paper. Furthermore, Mg concentration in α-Al phase has increased by 42%. Correspondingly, the lattice constant of α-Al phase has increased by 0·44%. The nanohardness of α-Al phase in the alloy increased under high pressure, whereas that of the elastic modulus decreased.     

Anodic Behaviour of Iron in Sodium Sulphate Solutions Containing Fatty Acids

Abstract

The effect of some fatty acids (caproic, caprylic, capric) on the anodic behaviour of iron in de-aerated 0·05 M Na₂SO₄ solutions of pH 6–12 was studied by ellipsometry and by measurements of the double layer capacitance (dlc). At certain concentrations of added fatty acids (e.g., caprylic acid at a concentration of 2 × 10^?2 mole/l) the rate of anodic dissolution of iron and the dlc are both decreased.

The most efficient inhibition is observed at pH 6. The surface films formed in solutions at pH 6 exhibit complex indices of refraction, n = (2·6 ± 0·2) {1—(0·16i ± O·02i)} and grow according to a logarithmic law. Similar optical properties and film growth kinetics are found for films formed in similar solutions at pH 12. These films are composed of iron oxide. At potentials below 500 m V_NHE, an increase of pH in the range 6–10 causes a decrease in inhibitive efficiency of the fatty acids, an increase of dlc, and the formation of non-protective films with low refractive indices. At potentials above 500 mV the surface films are protective irrespective of the pH of the solution.     

Comparison of pitting susceptibility of passive films formed on iron with different inorganic inhibitors

Abstract

The pitting susceptibility of passive films on iron in chloride containing solution, formed using three different inorganic inhibitors, chromate, nitrite and bicarbonate, is determined using linear sweep voltammogram. It is found that the pitting susceptibility becomes stronger in the order of the passive films formed in 0 · 01M chromate, 0 · 1M nitrite and 0 · 5M bicarbonate. The difference in pitting susceptibility is explained by the difference in electronic property of the passive films, which is determined using Mott–Schottky measurements. The donor density of the passive films formed in bicarbonate is highest, followed by nitrite and then by chromate, although the concentration of bicarbonate is five times that of nitrite and 50 times that of chromate. This suggests that the passive film formed in bicarbonate is most susceptible to attack by chloride ions.     

Nature of the Oxide Film Present on Iron after Inhibition in 0·05 M Chromate Solutions

Abstract

Chemical and microprobe analyses have been used to study the composition of the oxide films formed on iron by 0·05 M potassium chromate, pH 4–8.

Chemical analyses have shown that the air-formed oxide film was thinned and evenly reinforced with a normal iron chromium spinel having a composition in the range Fe²⁺ (Fe³⁺_0·5 Cr³⁺_1·5)O₄ — Fe³⁺ Cr³⁺ O₃

Microprobe analyses indicated enrichment of chromium at scratch lines, but the effect was small compared with the overall thickening of the films.     

Chromate passivation of hot dipped Zn25Al alloy coatings

Abstract

A low concentration chromate passivation treatment has been successfully applied to a new type of hot dipped Zn2 5Al alloy coating, and the corrosion resistance of the chromate passive film has been assessed using the copper accelerated acetic salt spray (CASS) test, electrochemical measurements, and sea water immersion testing. The results showed that the corrosion resistance of the Zn2 5Al alloy coating was significantly better after the chromate passivation treatment. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses showed that the composition (at.-%) of the low concentration chromate passive film was: 5·5S–3·4Na–11·8C–7·9Ti–41·6O–1 3·7Cr–16·0Zn. Aluminium was not found in the film, which is attributed to the dissolution behaviour of the Zn2 5Al alloy coating in acidic chromate solution.     

Modelling yield strength of heat treated Al–Si–Mg casting alloys

Abstract

A model for the yield strength of artificially aged Al–Si–Mg casting alloys has been developed. The model includes Mg concentrations between 0·2 and 0·6 wt-% and aging temperatures between 150 and 210°C. Spherical precipitates with the composition Mg₅Si₆, which grow by diffusion of Mg from the surrounding α-Al matrix, are assumed in the model. Nucleation is assumed to be instantaneous and growth of the precipitates is modelled using Fick’s second law and mass balance. When supersaturation is lost the continued precipitate growth is modelled using the Lifshitz–Slyozov–Wagner coarsening law. An average precipitate radius is calculated and a precipitate size distribution is introduced by using a relation between the average radius and its standard deviation. The strength contribution from precipitates is calculated using coherency strengthening and Orowan strengthening. The agreement between the model and experimental data is generally good; however, modelling the underaged condition needs further refinement.     

Effect of alloying elements on heat treatment response of aluminium high pressure die castings

Abstract

High pressure die cast (HPDC) aluminium components that respond to age hardening cannot normally be solution treated at high temperatures because the presence of internal porosity and entrapped gases leads to the formation of surface blisters. Parts may also become dimensionally unstable due to swelling. These factors that prevent heat treatment present significant limitations to the utilisation of HPDC components. Now it has been found that blistering and dimensional change can be avoided by using much shorter solution treatment times and lower temperatures. Experiments with alloys 360 (Al–9·5Si–0·5Mg) and 380 (Al–8·5Si–3·5Cu) have shown that strong responses to age hardening are still possible following these modified solution treatments. In the current paper, the role of critical alloying elements is considered in both current specification Al–Si–Cu–(X) alloys, and also in newly developed alloy compositions. It is shown that 0·2% proof strengths over 400 MPa may be readily achieved by heat treating conventionally produced die castings.     

Thermally Formed Oxide Films on Al–XSi Alloys Heated in Different Gases

Thermogravimetric analysis (TGA) testing was used to measure the change in weight of polished samples of Al–XSi (X = 0 and 1.2 mass%) alloys. The samples were heated at 843 K for 6 h in dry air or nitrogen gas. X-ray diffraction was used to monitor the formation of the oxide films on the surface of the samples. The surface oxide films were more compact after the Al alloy samples were heated in air, and the oxide films showed some cracks after being heated in nitrogen gas. The thermally formed surface oxide films on the Al–1.2 mass% Si alloy samples heated in air and in nitrogen gas possessed loose structures, which comprised mainly γ-alumina, diaspore, and gibbsite, along with metallic silicon and/or aluminum. The weight variation curve of the films appeared serrated; this can be attributed to chain reactions (3Si + 3O₂ → 3SiO₂ + 4Al → 3Si + 2Al₂O₃) that occurred within the film.     

Role of decomposition of AlMgFeSi phase in mechanical properties and fracture behaviour of 357 alloys

Abstract

The π-AlFeMgSi iron intermetallic phase in Al–Si–Mg alloys is known for its detrimental effect on ductility and strength. An attempt was made to correlate the impact and tensile properties of heat treated Al–7Si–0·55Mg–0·1Fe alloy samples with the changes that occur in the microconstituents such as the eutectic Si and π phase with prolonged solution treatment. Impact and tensile tests showed that the first 20 h of solution treatment is more sensitive to the changes occurring in the Si particle morphology than that in the π phase. Decomposition of the π phase significantly reduces the impact properties after prolonged solution times. Fracture behaviour is controlled mainly by the eutectic silicon morphology. The π phase particles act as crack initiation sites and facilitate crack propagation in as cast and heat treated alloys. Quality index values show that optimum solution time for Sr modified alloys is 12 h. Impact properties are more sensitive to changes in the π phase during solution treatment than tensile properties.     

Macrosegregation of ternary Al–4·5wt-%Cu–1·0wt-%Mg alloy in horizontal direct chill casting: implementation of non-equilibrium microsegregation model

Abstract

An extended and modified continuum mixture model for macrosegregation in solidifying alloys is applied to the case of horizontal direct chill casting (HDC) of a ternary Al–4·5Cu–1·0Mg (wt-%) alloy. A thermodynamic closure model for a Al–Cu–Mg alloy is coupled with an extended mixture continuum model to obtain steady state macrosegregation fields for the different casting parameters (e.g. casting speed, inlet superheat and inlet plenum position) in this study. Macrosegregation calculations were performed assuming that the solid forms a rigid and stationary network in all parts of the mushy zone. A microsegregation non-equilibrium model is used to calculate phase fraction compositions and phase distributions. The Cu exhibits more segregation compared to the previous study of a binary Al–Cu alloy, while Mg has a similar segregation profile in all cases but less pronounced. The phase compositions show that Cu is mostly distributed in secondary and ternary phases, while Mg is uniformly configured in the primary (Al) and ternary phase (Al₂CuMg). The primary Al phase is dominant (over 90%) in all cases, with highest peaks in depleted regions below the centreline of slab, except in the case where the inlet plenum is positioned on the bottom of the refractory plate due to a different flow pattern.     

Effect of ultrasonic melt treatment on microstructure of A356 aluminium cast alloys

Abstract

A systematic study of the effect of ultrasonic treatment on the structure of the A356 Al–Si–Mg alloy was carried out. Ultrasonic vibrations were applied to the melt at several temperatures (628 to 608°C) for different periods (0–180 s). The results showed that the ultrasonic treatment is effective in controlling the morphology and size of aluminium grains when applied between 619 to 626°C, and is very effective at 622–625°C, where fine non-dendritic grains are formed. Grain sizes smaller than 60 μm and roundness values of 0˙7 and higher were achieved by 15 s ultrasonic treatment at 623 and 620°C. At higher temperatures, the microstructures are dominantly rosette like and dendritic. At lower temperatures (below 617°C), the samples show inhomogeneous microstructures in terms of grain size and morphology. The current results are useful to set some process parameters for the ultrasonic treatment of liquid Al–Si–Mg foundry alloys prepared for semisolid processing.     

Oxidation of nickel base alloys strengthened by different hardening effects

Abstract

Three nickel base alloys strengthened by different hardening effects were investigated by thermogravimetry in air under isothermal conditions. The alloys investigated were γ′-Ni₃ (Al, Ti)-hardening alloy 80A (75Ni, 21Cr, 2·5Al, 1·7Ti, DIN No. 2·4952),solid solution hardened alloy C22 (59Ni, 21Cr, 13Mo, 3·5 Fe, 2·8W, DIN No. 2·4602) and a new high nitrogen containing and nitride hardening alloy N (61Ni, 27Cr, 10W, 1·4Ti, 0.2N). Tests were conducted in air between 900 and 1100° C for 48 h. Parabolic oxidationrates were determined and the formation of the oxide layer was investigated by optical microscopy and SEM. Oxidation data showed that the hardening mechanism has almost no influence on the oxidation kinetics. All of the alloys investigated formed chromia layers. After initial transient stateoxidation, the kinetics followed a parabolic law. Alloy 80A had the highest oxidation rate of the investigated alloys, which is attributed first to its lower chromium content and second to the formation of chromium carbides. At grain boundaries, internal oxidation, mainly of aluminium andtitanium, took place. The Al and Ti contents of alloy 80A were too low for the formation of a protective inner oxide layer of one of the two elements to take place. Alloy C22 showed the best resistance to oxidation since its chromium content of 21% is close to that for the minimum in the kineticsof oxide formation that has been found for binary Ni–Cr alloys. Additionally, there were no chromium rich precipitates to shift this chromium content to values that would result in higher oxidation rates. The nitride-containing alloy N contained a higher chromium content of 26%, whichled to a higher oxidation rate than that for alloy C22. A certain amount of inner oxidation took place, especially at coarse Cr₂N precipitates. Conclusions are presented about the optimised chemical composition of chromia laye-forming nickel base alloys for minimised oxidationrate.     

Corrosion behaviors for peak-aged Mg–7Gd–5Y–1Nd–0.5Zr alloys with oxide films

The corrosion behaviors of T5 (225 °C, 6.5 h) and T6 (460 °C, 2 h + 225 °C, 12 h) peak-aged Mg–7Gd–5Y–1Nd–0.5Zr alloys with oxide films were investigated by optical microscope (OM), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The weight loss rates and electrochemical tests were also analyzed. The thicknesses of T5 and T6 oxide films are roughly 0.6 and 1.0 μm, respectively. The components of oxide films mainly consist of O, Mg, Y, Nd, and Gd, and the T6 oxide film results in surfaces with larger peaks than T5 oxide film. In addition, Y, Nd, and Gd peaks are all higher than those of Mg–7Gd–5Y–1Nd–0.5Zr alloys, but Mg peak is consistently far below than that of the alloys. The specimens could be arranged in decreasing order of corrosion rates and corrosion current densities: T6 oxide film < T5 oxide film < T6 without oxide film < T5 without oxide film. The oxide films are compact to increase the corrosion resistance for Mg–7Gd–5Y–1Nd–0.5Zr alloys, which will provide a guiding insight into the corrosion and protection of Mg–RE alloys in atmospheric environments.     

Effect of Low-Levels of Strontium on the Oxidation Behavior of Selected Molten Aluminum–Magnesium Alloys

The effects of small additions of strontium on the oxidation behavior of aluminum–magnesium alloy melts were investigated by thermogravimetry at 750 °C for times up to 48 h. Oxidized samples were examined by FEGSEM, and phases formed within the oxide layer and in the alloy were identified by EDS, WDS and low-angle X-ray diffraction techniques. In the absence of Sr, the Al–Mg samples gained substantial amounts of weight by formation of spinel (MgAl₂O₄) at the oxide–metal interface. Samples containing Sr had significantly lower weight gains. A very significant decrease (98%) of total weight gain was observed for small Sr additions in the low Mg-bearing Al–Mg alloys. This change in oxidation behavior was linked to the presence of Sr enrichment of the liquid beneath the initial MgO layer suppressing the formation of spinel crystals.     

Examination of Oxides on Tin Surfaces by Cathodic Reduction

Abstract

Oxides formed on tin surfaces were examined by electrochemical reduction in galvanostatic and potentiodynamic experiments and also by electron diffraction. Reduction of oxide occurred at three distinct potentials (NHE scale): ?0·115 ?0·06 pH, ?0·150 ?0·06 pH and ?0·345 ?0·06 pH, although sometimes only one or two of them were observed. The most negative reduction step rarely appeared for electrodeposited tin coatings oxidised in air but was well developed for pure tin sheet and for hot-dipped tin coatings. Impurities in tin modified both the speed of oxidation and the form of the oxide reduction curves but their effect cannot account for the difference between electrodeposiied coatings and other tin surfaces. This difference is probably due to exposure of differing crystal faces. The two less negative reduction steps appeared for all forms of tin, but were not always clearly separated and they cover the possible reduction of both SnO and SnO₂. The most negative step is associated with SnO₂ only and its development is favoured by chemical or electrochemical oxidation, which may produce it for electrodeposited tin, or by high humidity of air. SnO₂ is also formed on all surfaces during long term storage at room temperature. Some thicker oxide films, formed during long storage or by anodic oxidation, may be partly undermined during reduction and the total reduction charge is not an accurate measure of their character.