Transient liquid phase bonding of magnesium alloy (Mg – 3Al – 1Zn) using copper interlayer

Abstract

The mechanism, microstructure and mechanical properties of transient liquid phase (TLP) bonded magnesium alloy (Mg – 3Al – 1Zn) joints using copper interlayers in an argon atmosphere have been investigated. The formation process of the TLP joint comprises a number of stages: plastic deformation and solid state diffusion, dissolution of the interlayer and base metal, isothermal solidification and homogenisation. The composition profiles and microstructures of the joint depend on the bonding time at a temperature of 530°C. With an increase in bonding time from 10 to 60 min, the concentration of copper and the amount of CuMg₂ compound in the joint decrease. For longer bonding times, the most pronounced features of the joint are composition homogenisation, grain coarsening and elimination of the bond line within the joint centre. The presence of brittle CuMg₂ and grain coarsening of the joint are the main reasons for impairing the joint shear strength. A joint shear strength of 70.2 MPa, which is 85.2% of the base metal strength (82.4 MPa), can be achieved by bonding at 530°C for 30 min.     

Creep behaviour of AZ61 magnesium alloy at low stresses and intermediate temperatures

Abstract

Tensile creep response was investigated for AZ61 alloy (Mg - 6.4Al - 0.9Zn - 0.2Mn, wt-%) of mean linear intercept grain size ~ 25 μm at stresses in the range 0.9 - 4 MPa over the temperature range 250 - 346°C. Bingham behaviour is obtained with strain rate ? under stress σ given by ?∝σ - σ_o with a threshold stress σ_o decreasing from 1.25 MPa at 210°C to ~ 0.5 MPa at 346°C, which is similar to earlier work on pure magnesium. The corresponding Arrhenius plot of log (Td?/d σ) versus T^-1 indicates an activation energy comparable with that expected for the grain boundary self-diffusion coefficient D _B, and values of D _Bδ (where δ is the effective grain boundary thickness) derived from the Coble equation are also similar to those for pure magnesium. Grain elongation in the direction of the tensile stress is also consistent with the key indicative feature of diffusional creep: deposition of material at grain boundaries nearly transverse to the axis of tensile stressing. Strain rates versus stress are shown to be continuous with published results for superplastic flow of AZ61 at comparable temperatures but higher stresses.     

Superior mechanical properties of a selective-laser-melted AlZnMgCuScZr alloy enabled by a tunable hierarchical microstructure and dual-nanoprecipitation

Achieving high mechanical strength and ductility in age-hardenable Al7000 series (Al–Zn–Mg) alloys fabricated by selective laser melting (SLM) remains challenging. Here, we show that crack-free AlZnMgCuScZr alloys with an unprecedented strength–ductility synergy can be fabricated via SLM and heat treatment. The as-built samples had an architectured microstructure consisting of a multimodal grain structure and a hierarchical phase morphology. It consisted of primary Al₃(Sc_x,Zr_1−x) particles which act as inoculants for ultrafine grains, preventing crack formation. The metastable Mg-, Zn-, and Cu-rich icosahedral quasicrystals (I-phase) ubiquitously dispersed inside the grains and aligned as a filigree skeleton along the grain boundaries. The heat treated SLM-produced AlZnMgCuScZr alloy exhibited tunable mechanical behaviors through trade-off among the hierarchical features, including the dual-nanoprecipitation, viz, η′ phase, and secondary (Al,Zn)₃(Sc₉Zr), and grain coarsening. Less coarsening of grains and (Al,Zn)₃(Sc₉Zr) particles, due to a reduced solution treatment temperature and time, could overwhelm the more complete dissolution of I-phase (triggering more η′ phase), resulting in higher yield strength. Optimal combination of the hierarchical features yields the highest yield strength (∼647 MPa) among all reported SLM-produced Al alloys to date with appreciable ductility (∼11.6%). The successful fabrication of high-strength Al7000 series alloys with an adjustable hierarchical microstructure paves the way for designing and fine-tuning SLM-produced aluminum engineering components exposed to high mechanical loads.     

Microstructure and creep resistance of Mg–10Zn–4Al–0.15Ca permanent moulding alloy

Abstract

In this paper, the microstructure of Mg–10Zn–4Al–0.15Ca (wt-%) (ZAC104015) magnesium alloy was characterised in the as cast condition. Thermal analysis was performed to determine the liquidus, solidus, and intermediate reactions that occur upon cooling. The results show that this alloy is composed of α-Mg and two intermetallic phases containing calcium τ'₁ and τ'₂. These two intermetallic phases have the same type of crystal structure as that of phase τ (Mg₃₂(Al,Zn)₄₉). The thermal stability of the phases τ, τ'₁, and τ'₂, were also evaluated. This experimental study indicates that the phase τ'₂ is more stable than τ and τ'₁ at elevated temperature and directly related to creep resistance improvement.     

Interfacial characteristics of electron beam welding joints of SiCp/Al composites

Abstract

The electron beam welding of SiCp/101Al composites has been carried out. The influence of welding parameters on weldability and mechanical properties of the welded joints was discussed. The welding parameters were therefore optimised under the current experimental circumstance. Results show that only weak interfacial reaction between SiC particle and liquid aluminium occurred. Minute quantity brittle Al₄C₃ compounds and single phase Si were generated in the welded joint. The interfacial reaction between SiC particles and Al matrix could be greatly suppressed by adopting appropriate technique measures such as high welding speed and low heat input. The content of Al₄C₃ can be therefore greatly decreased in the welded joint. Moreover, modification welding and electron beam scanning could further improve the appearance of weld, and the welded joint with better quality could be obtained.     

Effect of Al and C on structure and mechanical properties of Fe–Al–C alloys

Abstract

Effect of aluminium and carbon content on the microstructure and mechanical properties of Fe–Al–C alloys has been investigated. Alloys were prepared by combination of air induction melting with flux cover (AIMFC) and electroslag remelting (ESR). The ESR ingots were hot forged and hot rolled at 1373 K. As rolled alloys were examined using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to understand the microstructure of these alloys. The ternary Fe–Al–C alloys containing 10·5 and 13 wt-%Al showed the presence of three phases: FeAl with disordered bcc structure, Fe₃Al with ordered DO₃ structure and Fe₃AlC_0·5 precipitates with L′1₂ structure. Addition of high concentration of carbon to these alloys resulted in excellent hot workability and superior tensile at room temperature as well as tensile and creep properties at 873 K. An increase in Al content from 9 to 13 wt-% in Fe–Al–C alloys containing the same levels of carbon has no significant influence on strength and creep properties at 873 K, however resulted in significant improvement in room temperature strength accompanied by a reduction in room temperature ductility.     

Formation of microlayers of clad residue on aluminium brazing sheet during melting and resolidification in brazing process

Abstract

The present paper is devoted to an analysis of clad residue formation during a controlled atmosphere brazing (CAB) process applied to composite aluminium brazing sheets. Evolution of the microstructure of the clad residue, and in particular the mass of resolidified clad formed, were studied. Observations confirmed that, even under optimal brazing conditions, a residue layer (formed away from the joint zone) always appears after brazing. It was established that the peak brazing temperature plays an important role in the process responsible for formation of the residue mass. However, dwell time at the peak brazing temperature does not have a significant influence on clad residue mass accumulation beyond its known influence on substrate dissolution and core metal erosion in the joint zone.     

Effect of heat input on the microstructure and mechanical properties of tungsten inert gas arc butt-welded AZ61 magnesium alloy plates

In this paper, the effects of heat input on the microstructures and mechanical properties of tungsten inert gas arc butt-welded AZ61 magnesium alloy plates were investigated by microstructural observations, microhardness tests and tensile tests. The results show that with an increase of the heat input, the grains both in the fusion zone and the heat-affected zone coarsen and the width of the heat-affected zone increased. Moreover, an increase of the heat input resulted in a decrease of the continuous β-Mg₁₇Al₁₂ phase and an increase of the granular β-Mg₁₇Al₁₂ phase in both the fusion zone and the heat-affected zone. The ultimate tensile strength of the welded joint increased with an increase of the heat input, while, too high a heat input resulted in a decrease of the ultimate tensile strength of the welded joint. In addition, the average microhardness of the heat-affected zone and fusion zone decreased sharply with an increase of the heat input and then decreased slowly at a relatively high heat input.     

Division of character zones and elements distribution of Fe3Al/Cr–Ni alloy fusion bonded joint

Abstract

A Fe₃Al/Cr–Ni alloy fusion bonded joint was divided into four character zones of a homogeneous mixture zone, a partial mixture zone, a partially fused zone and a heat affected zone. The microstructures, elements distribution and phase constitutions of the various character zones were analysed via metalloscope, SEM, electron probe microanalysis and X-ray diffraction. The results indicated that the microstructures were dissimilar in the different character zones. A 0·04–0·05 mm austenite rich band existed in the partial mixture zone. The diffusion of Fe, Al, Cr, Ni and C mainly occurred in fusion zone where Cr and Ni diffused into Fe₃Al to substitute some Fe on α ₁, α ₂, and β sublattices to form substitutional solid solution. The phase constitutions of Fe₃Al/Cr–Ni joint were Fe₃Al, γ-Fe, FeAl, NiAl, an unidentified Fe–C compound and an Fe–Cr–C compound (Cr₉Fe)₇C₃.     

Microstructural evolution in AZ91D magnesium alloy during electron beam welding

Vacuum electron beam welding can have a low heat input, which means there is a minimum heat affected zone during welding of AZ91D magnesium alloy. From the observed microstructure, the weld of the AZ91D magnesium alloy can be divided into four regions, which are the weld metal zone, a partially-melted zone adjacent to the fusion boundary, a partially-melted zone adjacent to the base metal and the base metal zone. A sharp transition from the fusion zone to the non-melted zone, especially the characteristic partial melting microstructure and nature of the alloy elements, was observed. It was found that significant partial melting had taken place in the very narrow region around the weld metal of the AZ91D magnesium alloy. The Al content of eutectic β-Mg₁₇Al₁₂ in the partially-melted zone adjacent to the fusion boundary was close to the content in the continuously precipitated eutectic β particles in the fusion zone and much lower than the eutectic β in the base metal. The fully melted eutectic β-phase coexisted with the partially melted eutectic β phase in the partially-melted zone adjacent to the base metal.     

Joining of γ-TiAl to low alloy steel by electron beam welding

Abstract

The weldability of cast γ-TiAl with a low alloy steel by electron beam welding was investigated in this study. It was found that when a higher heat input was employed, solid state cracking appeared in the welds due to a high thermal stress, as well as formation of a brittle TiC phase and a small fraction of Ti₃Al intermetallic. Crack free welding could be achieved if welding parameters were properly controlled such that thermal stress was released and formation of TiC and Ti₃Al was minimised.     

Process model for strength of age hardenable aluminium alloy welds

Abstract

An existing process model for hardness prediction in age hardenable aluminium alloy welds is presented and analysed. One of the key criticisms of this model is that its derivation assumes softening is due to precipitate dissolution alone. The influence of precipitate coarsening has been determined by developing an equivalent model for softening owing to coarsening. It is shown that the experimentally derived master curves that form the basis of the model are capable of representing softening by a mixture of precipitate coarsening and dissolution. Methods to predict post-weld natural aging are discussed, and a new method is presented based on direct prediction of the Guinier–Preston zone fraction. The model has been applied to friction stir welding. Model predictions agree well with measured hardness profiles, and the sensitivity of the predictions to temperature is discussed.     

Interfacial microstructure and mechanical property of resistance spot welded joint of high strength steel and aluminium alloy with 4047 AlSi12 interlayer

Dissimilar materials of H220YD galvanised high strength steel and 6008-T66 aluminium alloy were welded by means of median frequency direct current resistance spot welding with employment of 4047 AlSi12 interlayer. Effects of interlayer thickness on microstructure and mechanical property of the welded joints were studied. The welded joint with interlayer employed could be recognised as a brazed joint. The nugget diameter had a decreased tendency with increasing thickness of interlayer under optimised welding parameters. An intermetallic compound layer composed of Fe₂(Al,Si)₅ and Fe₄(Al,Si)₁₃ was formed at the interfacial zone in the welded joint, the thickness and morphology of which varying with the increase of interlayer thickness. Reaction diffusion at the steel/aluminium interface was inhibited by introduction of silicon atoms, which restricted growth of Fe₂(Al,Si)₅. Tensile shear load of welded joints experienced an increased tendency with increasing interlayer thickness from 100 to 300 μm, and the maximum tensile shear load of 6.2 kN was obtained with interlayer thickness of 300 μm, the fractured welded joint of which exhibiting a nugget pullout failure mode.     

Microstructures and mechanical properties of an investment cast gamma titanium aluminide

Abstract

Investment castings have been produced in γ-TiAl of composition Ti–48Al–2Nb–2Mn (at.-%) using induction skull melting. The microstructures of the bars were studied in the as cast condition and after hipping and heat treatment. Heat treatment at 1200°C led to a near γ structure whereas treatment at 1350°C resulted in a nearly lamellar structure. However, a duplex structure was retained after treatment at 1300°C. Tensile, fracture toughness, and fatigue crack growth resistance tests have been carried out on specimens machined from different sized bars. The tensile properties increased with decreasing bar diameter but, conversely, both the fracture toughness and fatigue crack growth resistance improved as the bar diameter increased. It has been found that the fracture toughness and fatigue crack growth resistance in nearly lamellar structures were better than those in near γ structures, whereas duplex structures had intermediate properties. However, the tensile properties of duplex structures were better than both near γ and nearly α₂ /γ lamellar structures, with optimum values at 35 ± 5% α₂ /γ lamellae of ～400 MPa 0·2% proof strength, 470 MPa tensile strength, and 0·9% elongation.     

Low temperature aluminisation of alloy steels by pack cementation process

Abstract

The pack aluminisation process is normally applied at temperatures >973 K at which the mechanical properties of alloy steels would degrade. Thus, the present study was undertaken to apply this process to aluminising the alloy steels at temperatures <973 K in order to increase their high temperature oxidation resistance while maintaining their microstructure and hence mechanical strength and creep resistance. A type of commercial alloy steel P92 (9Cr–1Mo) was used for the present study. Pack powder mixtures consisting of Al, AlCl₃ (anhydrous) or NH₄Cl and Al₂O₃ were used to carry out the process. The aluminising temperature was varied from 773 to 973 K, pack Al content from 1 to 30 wt-% and aluminising time from 1 to 16 h to investigate their effects on the coating growth kinetics in the AlCl₃ activated packs. It was observed that all the coatings formed in the AlCl₃ activated packs were of a single layer structure with Fe₂Al₅ as the main coating phase. It was established that the interrelationship between the thickness h (in μm) of this coating layer and aluminising temperature T (in K), time t (in h) and pack Al content W (in wt-%) can be described by h=83005·9W^1/2t^1/2e^?73330/(RT). In the NH₄Cl activated packs, it was found that coating formation and dissolution took place simultaneously at 923 K and stable growth of a coating layer was only possible when the pack Al content was sufficiently high. However, the coatings formed in these packs had highly uneven regions.     

Hard coatings on aluminium alloy surfaces produced using microplasma oxidation

Abstract

The present paper reports on the production of oxide coatings on an aluminium alloy by microplasma oxidation and on their properties. The surface characteristics of the coatings were determined by surface and structural analytical techniques, i.e. SEM and X-ray diffraction. Thermo-analysis of the coatings was evaluated by means of differential scanning calorimetry and thermogravimetric analyses. It was ascertained that the coatings with a mixture of crystallised γ-Al₂O₃ and α-Al₂O₃ have significant microhardness, good electric resistance and good thermostability.     

Vacuum brazing of NiTi alloy by AgCu eutectic filler

Abstract

Joining of NiTi alloy to itself has been realised by vacuum brazing process using AgCu28 eutectic as filler metal. Microstructures, mechanical and shape memory behaviour have been investigated. The shearing strength of the brazed joint exceeds 100 MPa, and rupture occurs at the diffusion layer of parent metal beside brazing metal. The brazed joint will be stronger than parent metal on condition of the specimen with a joint of lap length 10 times of plate thickness. The brazed specimen shows a good shape memory behaviour. From the point of view of practice, the brazing joint design principle and brazing quality improvement have been discussed.     

Isothermal nanocrystallisation behaviour of melt spun Al86Ni9 Mm5 (Mmmischmetal)amorphous alloy

Abstract

The structure and mechanical properties of melt spun Al₈₆Ni₉Mm₅ alloy ribbons in both as solidified amorphous and heat treated nanocomposite conditions were investigated using DSC, XRD, TEM, and Vickers microhardness techniques. Primary crystallisation of the amorphous alloy resulted in the formation of fine nanocrystalline fcc-Al particles embedded in an amorphous matrix forming a nanocomposite. The growth behaviour of the primary fcc-Al particles under isothermal conditions was investigated. The hardness ofthe composite varied with the solute content in the amorphous phase and the microstructure after heat treatment. The hardening in these nanocomposites was quantitatively explained using a rule of mixtures model based on the volume fraction of the amorphous matrix and the Al particles. The nanometre sized particles were treated as perfect materials and the matrix was treated as an amorphousmaterial, in which the solute concentration increased as the volume fraction of the Al particles increased. The calculated results for the heat treated specimens using the rule of mixtures based on the isostress model have been found to be in good agreement with the experimentally obtained results.     

Coarsening of particulate silicon in aluminium based matrices

Abstract

Published measurements of the coarsening parameter K = (r³2r³_o)/t of particulate silicon in Al – Si alloy base matrices are compared with values predicted for volume diffusion control according to the LSW model corrected for finite volume fraction. While some measurements are within a factor of 10 of LSW predictions, others exceed these by factors as high as 10³ to 10⁴. Possible reasons for such large apparent enhancements of coarsening rate are discussed.     

Interfacial reaction and its effect on strength of Al18 B4O33 /Al composites

Abstract

Aluminium alloy 6061, AC8A, Al–1Mg, Al–9Cu and pure aluminium composites reinforced with aluminium borate whiskers were fabricated by a squeeze casting process. The interfacial reaction in the composites and its effect on the bending strength are discussed, together with the results from SEM, TEM, and X-ray diffraction. A slight interfacial reaction is favourable for composite strength as it has the effect of anchoring the whiskers. A T6 treatment can enhance the strength of an Al–9Cu matrix composite, but is not efficient for magnesium containing 6061 and AC8A matrix composites. Furthermore, if heated at temperatures higher than 793 K for a long time, the composite strength drops rapidly owing to whisker damage and shortening during the interfacial reaction. It is suggested that the interface in an Al₁₈ B₄O₃₃ /Al alloy composite is stable below 623 K which is the temperature requirement for automobile engine components.