Paul Rometsch’s research while affiliated with Centre for Ecology Development and Research and other places

The evolution of discontinuous and continuous Al3(Sc,Zr) precipitation in an Al-Mg-MnAA5083 alloy during heat treatment and hot rolling was investigated. The results showed that, at a high Sc content (0.43 wt%), a large number of line/fan-shaped structures were formed as discontinuous Al3(Sc,Zr) precipitation during solidification, while no such discontinuous precipitation was observed when the amount of Sc added was low (0.15 wt%). During the three-step heat treatment (275 °C /12 h + 375 °C/48 h + 425 °C/12 h), two types of precipitates — Mn-bearing dispersoids and spherical Al3(Sc,Zr) precipitates — were formed as the main strengthening phases. In the high-Sc alloy, the discontinuous Al3(Sc,Zr) precipitates dissolved partially. However, the quantity of the spherical Al3(Sc,Zr) precipitates in the high-Sc alloy was much lower than that in the low-Sc alloy, which degraded its aging hardening response. During hot rolling, although the discontinuous precipitates were completely dissolved, the number density of the spherical Al3(Sc,Zr) precipitates in the high-Sc alloy was still lower than that in the low-Sc alloy. The tensile properties of the Sc-containing alloys improved significantly compared with those of the base alloy. However, the yield and ultimate tensile strengths of the high-Sc alloy were lower than those of the low-Sc alloy. This indicates that the discontinuous precipitation had a deleterious effect on the mechanical properties of the alloy.

Laser powder bed fusion (LPBF) is one of the major additive manufacturing techniques that industries have adopted to produce complex metal components. The scientific and industrial literature from the past few years reveals that there is a growing demand for the development of high-strength aluminium alloys for LPBF. However, some major challenges remain for high-strength aluminium alloys, especially in relation to printability and the control of defects. Possible strategies that have been identified to achieve high strength with printability include the adaptation of existing high-strength cast and wrought alloys to LPBF, the design of new alloys specifically for LPBF, and the development of aluminium-based composites to achieve unique combinations of properties and processability. Whilst review papers exist for aluminium alloys in general for the related work up to 2019, the purpose of this paper is to review the latest developments related to high-strength aluminium alloys for LPBF up to early 2022, including alloy and process design strategies to achieve high strength without cracking. It aims to provide fresh insights into the current state-of-the-art based on a review of extensive yield strength data for a wide spectrum of aluminium alloys and tempers that have been studied and/or commercialised for LPBF.

The Al-3.40Mg-1.08Sc alloy plates were manufactured by selective laser melting (SLM) at platform temperatures of 35 °C and 200 °C, respectively, and the corrosion performance of them was studied along height direction. The results show that the corrosion resistance of the alloy plate built at platform temperature of 35 °C along height direction is basically the same due to a uniform microstructure; While the corrosion resistance of the alloy plate built at platform temperature of 200 °C along height direction is different. The evolution of microstructure and the distribution of secondary phases are investigated, and the results show that the Cu-rich phases in alloy play a key role on corrosion performance. At higher platform temperature, the cooling rate is relative slow and a certain degree of in situ ageing leads to the significantly different distribution of Cu-rich phases along grain boundary. Specimens built at the platform temperature of 200 °C are inclined to locate at the crossed grain boundary, rather than continuous segregation of Cu-rich phases along grain boundary that is built at platform temperature of 35 °C. Therefore, the corrosion resistance of Al-3.40Mg-1.08Sc alloy plate manufactured at platform temperature of 200 °C is higher, and presents a gradually decreasing trend along height direction.

In the present work, we investigated the possibility of introducing fine and densely distributed α-Al(MnFe)Si dispersoids into the microstructure of extruded Al-Mg-Si-Mn AA6082 alloys containing 0.5 and 1 wt % Mn through tailoring the processing route as well as their effects on room- and elevated-temperature strength and creep resistance. The results show that the fine dispersoids formed during low-temperature homogenization experienced less coarsening when subsequently extruded at 350 °C than when subjected to a more typical high-temperature extrusion at 500 °C. After aging, a significant strengthening effect was produced by β″ precipitates in all conditions studied. Fine dispersoids offered complimentary strengthening, further enhancing the room-temperature compressive yield strength by up to 72–77 MPa (≈28%) relative to the alloy with coarse dispersoids. During thermal exposure at 300 °C for 100 h, β″ precipitates transformed into undesirable β-Mg2Si, while thermally stable dispersoids provided the predominant elevated-temperature strengthening effect. Compared to the base case with coarse dispersoids, fine and densely distributed dispersoids with the new processing route more than doubled the yield strength at 300 °C. In addition, finer dispersoids obtained by extrusion at 350 °C improved the yield strength at 300 °C by 17% compared to that at 500 °C. The creep resistance at 300 °C was greatly improved by an order of magnitude from the coarse dispersoid condition to one containing fine and densely distributed dispersoids, highlighting the high efficacy of the new processing route in enhancing the elevated-temperature properties of extruded Al-Mg-Si-Mn alloys.

The long production time required for large-scale parts fabricated by laser powder bed fusion (LPBF) tends to induce cracks, distortions, and overheating problems. In this work, to address these challenges, we explored and established a suitable strategy for producing large AlSi10Mg components. The platform temperatures to prevent cracks and distortions were firstly determined. Then, the in situ aging behavior was investigated for samples under various platform temperatures and holding times. Our results revealed that platform temperatures of 150°C and 200°C can effectively prevent cracks and minimize distortions. Besides, using 150°C, samples can reach peak hardness with a holding time less than 13 h. In comparison, those samples produced with a holding time longer than 13 h at 150°C and 200°C show obvious over-aging responses and thus lower hardness. However, such a hardness impoverishment can be recovered by using a T6 post-process heat-treatment.

The feasibility and efficacy of improving the mechanical response of Al-Mg-Si 6082 structural alloys during high temperature exposure through the incorporation of a high number of α-dispersoids in the aluminum matrix were investigated. The mechanical response of the alloys was characterized based on the instantaneous high-temperature and residual room-temperature strengths during and after isothermal exposure at various temperatures and durations. When exposed to 200 °C, the yield strength (YS) of the alloys was largely governed by β" precipitates. At 300 °C, β" transformed into coarse β', thereby leading to the degradation of the instantaneous and residual YSs of the alloys. The strength improvement by the fine and dense dispersoids became evident owing to their complementary strengthening effect. At higher exposure temperatures (350-450 °C), the further improvement of the mechanical response became much more pronounced for the alloy containing fine and dense dispersoids. Its instantaneous YS was improved by 150-180% relative to the base alloy free of dispersoids, and the residual YS was raised by 140% after being exposed to 400-450 °C for 2 h. The results demonstrate that introducing thermally stable dispersoids is a cost-effective and promising approach for improving the mechanical response of aluminum structures during high temperature exposure.

The effects of Al(MnFe)Si dispersoids, with different sizes and number densities, on the evolution of microstructure and ambient/elevated-temperature mechanical properties of extruded AA6082 alloys, with varying Mn content, under T5 conditions, were investigated. Compared to the low density of coarse dispersoids formed during conventional homogenization, the high density of fine dispersoids formed during a new low-temperature homogenization was more effective in increasing the material’s resistance to plastic deformation during extrusion, resulting in the dissolution of more constituent Mg2Si particles into the α-Al matrix. A large amount of β", some β' precipitates and fine dispersoids co-existed in the α-Al matrix of 0.5% Mn containing alloy, which afforded this alloy a substantial increase in ambient-temperature yield strength of 65-75 MPa under T5 conditions compared to the base alloy without dispersoids. A further increase in the Mn content decreased the number density of the β" precipitates, resulting in a decline in the mechanical properties. Upon thermal exposure at 300 °C for 100 h, β"/β' fully transformed into an undesirable equilibrium β phase and lost their strengthening effect, while fine and dense dispersoids became the dominant strengthener, leading to a 55-70% increase in the elevated-temperature yield strength relative to the alloys either without dispersoids or with coarse dispersoids. Dispersoid strengthening was more pronounced at 0.7% Mn addition as further increasing the Mn content mainly contributed to the fraction of insoluble Mn-containing intermetallics.

The dynamic metallurgical characteristics of the selective laser melting (SLM) process offer fabricated materials with non-equilibrium microstructures compared to their cast and wrought counterparts. To date, few studies on the precipitation kinetics of SLM processed heat-treatable alloys have been reported, despite the importance of obtaining such detailed knowledge for optimizing the mechanical properties. In this study, for the first time, the precipitation behavior of an SLM fabricated Al-Mn-Sc alloy was systematically investigated over the temperature range of 300°C to 450°C. The combination of in-situ synchrotron-based ultra-small angle X-ray scattering (USAXS), small angle X-ray scattering (SAXS) and X-ray diffraction (XRD) revealed the continuous evolution of Al6Mn and Al3Sc precipitates upon isothermal heating in both precipitate structure and morphology, which was confirmed by ex-situ transmission electron microscopy (TEM) studies. A pseudo-delay nucleation and growth phenomenon of the Al3Sc precipitates was observed for the SLM fabricated Al-Mn-Sc alloy. This phenomenon was attributed to the pre-formed Sc clusters in the as-fabricated condition due to the intrinsic heat treatment effect induced by the unique layer-by-layer building nature of SLM. The growth kinetics for the Al6Mn and Al3Sc precipitates were established based on the in-situ X-ray studies, with the respective activation energies determined to be (74 ± 4) kJ/mol and (63 ± 9) kJ/mol. The role of the precipitate evolution on the final mechanical properties was evaluated by tensile testing, and an observed discontinuous yielding phenomenon was effectively alleviated with increased aging temperatures.

Using gas flow to reduce laser plume attenuation is critical in the process control of laser powder bed fusion (LPBF) of metal powders. First, this work investigated Hastelloy X (HX) samples built at different gas flow speeds. Higher porosity with lack of fusion defects was found in the samples built at lower gas flow speeds, which indicates a significant influence of laser plume attenuation. Then, particle pickup experiments were conducted to investigate the limit of further increasing the gas flow speed without disturbing the powder bed. Eight different powders of four alloys (Al, Ti, steel, and Ni) with mean sizes ranging from 25 µm to 118 µm were studied. A model was introduced to predict the pickup speeds of different powders. Lastly, a method based on porosity and particle pickup speed was proposed for the reference of setting the lower and upper limits of gas flow speed in LPBF.