Ever-increasing demands of industrial manufacturing regarding mechanical properties require the development of novel alloys designed towards the respective manufacturing process. Here, we consider wire arc additive manufacturing. To this end, Al alloys with additions of Zn, Mg and Cu have been designed considering the requirements of good mechanical properties and limited hot cracking susceptibility. The samples were produced using the cold metal transfer pulse advanced (CMT-PADV) technique, known for its ability to produce lower porosity parts with smaller grain size. After material simulations to determine the optimal heat treatment, the samples were solution heat treated, quenched and aged to enhance their mechanical performance. Chemical analysis, mechanical properties and microstructure evolution were evaluated using optical light microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray fluorescence analysis and X-ray radiography, as well as tensile, fatigue and hardness tests. The objective of this research was to evaluate in detail the mechanical properties and microstructure of the newly designed high-performance Al–Zn-based alloy before and after ageing heat treatment. The only defects found in the parts built under optimised conditions were small dispersed porosities, without any visible cracks or lack of fusion. Furthermore, the mechanical properties are superior to those of commercial 7xxx alloys and remarkably independent of the testing direction (parallel or perpendicular to the deposit beads). The presented analyses are very promising regarding additive manufacturing of high-strength aluminium alloys.

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A high speed steel, processed by two powder metallurgy routes and heat treated to give a range of microstructures, was investigated in 4-point bending at room temperature using smooth and precracked specimens. The finer microstructures were in the material from gas atomised powder which was hot isostatically-pressed, commercial ASP60 alloy, while the coarser microstructures derived from laboratory vacuum sintering of water-atomised powder. The resultant hardness values H , , , were in the rage 780 to 1050, prior autenite grain sizes, 5 to 25 pm and maximum carbide sizes, 6 to 32 pm.Only some of the uncracked samples exhibited macroscopic yielding, at stresses in the range 1.64 to 2.59 GPa; the finer microstructures being associated with the higher strengths. Macroscopic plastic deformation never exceeded 0.33%; fracture strengths were in the range 1.46 to 2.75GPa. Fracture toughness, Klc, varied from 12 to 17 M P a f i in the H , , , range 920 to 800 for the directly sintered steel and only from 10 to 12 M P a f i in ASP60. The insensitivity of K , , to macroscopic hardness in ASP60 is associated with the plastic zone size of 1.5 pm which approximates to the average carbide spacing.Nucleation and growth of natural, i.e. stress-induced, microcracks in unnotched specimens was studied by surface replica microscopy. Crack nucleation took place at stresses between 0.5 and 1.5 GPa, i.e. below those for yielding and for fracture, u,, and was by debonding of inclusions (alumina and calcium-alumino silicates) or cracking of carbides. In the coarsest microstructure monotonic stepwise subcritical crack growth was observed from stress levels of -1.3 GPa, i.e. -0.8 to 0.90,. Similarities to the behaviour of short fatigue cracks in metallic materials and the R-curve behaviour of ceramics are referred to.