Laser surface treatment techniques, including laser surface melting (LSM) and laser surface alloying (LSA), have been the subject of considerable interest as a means of enhancing the corrosion performance of aluminium and its alloys. Microstructural modification together with the incorporation of non-equilibrium concentrations of alloying elements resulting from relatively rapid rates of cooling compared with conventional surface treatment techniques provide the basis for property enhancement. This paper considers microstructural evolution in a range of laser surface treated aluminium alloys including LSM of Al–Cu, Al–Si, Al–Zn, Al–Fe and Al–transitional element alloy systems; LSA of Al–Ni, Al–Cr and Al–Mo. Where reported, corrosion and other property determination in these systems is discussed. It is shown that surface alloys with unique microstructural and compositional characteristics have been produced by these techniques and that in many cases promising improvements in hardness and critical pitting potential compared with conventional alloys have been reported.

Microstructure and corrosion properties of laser surface processed aluminium alloys: a review

Influence of oxygen content on microstructure and mechanical properties of Ti–Nb–Ta–Zr alloy

Strain rate dependence of deformation-induced transformation and twinning in a metastable titanium alloy

Laser cladding of MMC coatings on aluminium substrate: influence of composition and microstructure on mechanical properties

As a crucial branch for titanium industry, high-strength titanium alloys (HS-TAs, with UTS ≥ 1100 MPa) are indispensable structural materials for advanced engineering applications such as aerospace and marine fields. Along with the expansion of HS-TAs’ market, achieving satisfying synergies of high strength, high ductility (elongation ≥ 6%) and high toughness (KIC ≥ 50 MPa⋅m1/2) has been identified as the uppermost technical bottleneck for their research and development. To overcome the challenge, two primary strategies have been initiated by the titanium community, developing novel alloys and innovating processing technologies. For the former, a dozen of newly-developed alloys were reported to exhibit excellent strength-ductility-toughness combinations, including Ti-5553, BT22, TC21 and Ti-1300, for which the ideal mechanical performances were based on specific microstructures realized by low impurity rate (e.g. oxygen content ≤ 0.15 wt%), complicated processing and complex heat treatment. For the latter, several innovatory forging and heat treatment technologies were originated for the mature alloys to optimize their balanced property by extraordinary microstructural characteristics. In this review, we provide a comprehensive overview over the research status, processing and heat treatment technologies, phase transformation, processing-microstructure-property correlation and strengthening-toughening mechanism of HS-TAs for aerospace engineering applications manufactured via melting-forging process. Finally, the prospects and recommendations for further investigation and development are proposed based on this review. 

High-strength titanium alloys for aerospace engineering applications: A review on melting-forging process

Surface alloying of a commercially pure aluminium with nickel was carried out using a pulsed Nd-YAG laser. The distribution of the alloying element (nickel) in the alloyed layer was found to be highly non-uniform irrespective of the depth of alloying. The microstructure of the alloyed layer was found to consist mainly of a cellular solid solution phase of nickel in aluminium (α-Al), a lamellar eutectic of α-Al and Al 3 Ni phases, and primary Al 3 Ni dendrites. The microstructure of the alloyed layer is explained in terms of inhomogeneity of the nickel distribution. The effects on the microstructure of inherent rapid solidification under the conditions of laser surface alloying were also examined.

Evolution of microstructure in laser surface alloying of aluminium with nickel

The effect of β grain size on trigger stress for stress-induced martensitic transformation during tensile deformation in Ti-Al-Nb alloys was investigated. The trigger stress for stress-induced martensitic transformation (SIMT) in Ti-Al-Nb alloys exhibited a U-shaped behavior with variation in grain size. The variation of trigger stress with grain size was explained qualitatively, in terms of the contrasting change in the internal elastic energy stored in matrix due to formation of martensite (ΔEel) and the irreversible work done in overcoming the internal frictional resistance to phase boundary movement (∂Eirr) with β grain size.

Trigger Stress for Stress-Induced Martensitic Transformation during Tensile Deformation in Ti-Al-Nb Alloys: Effect of Grain Size

The effect of composition on trigger stress for stress-induced martensitic transformation (SIMT) during tensile deformation in Ti–Al–Nb alloys was investigated. The trigger stress in these alloys was found to decrease with an increase in Nb as well as Al content in the range of Nb (8–12 at.%) and Al (15–18 at.%) studied. The effect was explained on the basis of effect of these elements on the critical free energy required for the SIMT.

Effect of Al and Nb on the trigger stress for stress-induced martensitic transformation during tensile loading in Ti–Al–Nb alloys

Ti–Al–Nb alloys in the present range of composition were found to exhibit a typical four-stage behaviour observed in alloys undergoing stress-induced martensitic transformation (SIMT) in β as well as α 2 –β heat-treated condition. Intermittent unloading–reloading during tensile test was used to measure the apparent modulus at regular strain intervals. This coupled with the observation of microstructure of the samples from tensile tests interrupted at each of the four stages was used to identify the operative mechanism of each stage.

Archana Paradkar

Papers

Evolution of microstructure in laser surface alloying of aluminium with nickel

Trigger Stress for Stress-Induced Martensitic Transformation during Tensile Deformation in Ti-Al-Nb Alloys: Effect of Grain Size

Effect of Al and Nb on the trigger stress for stress-induced martensitic transformation during tensile loading in Ti–Al–Nb alloys

Various stages in stress–strain curve of Ti–Al–Nb alloys undergoing SIMT

The effect of strain rate on trigger stress for stress-induced martensitic transformation and yield strength in Ti–18Al–8Nb alloy