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

The mechanical properties, corrosion behavior, and related microstructures of Al–Mg-Mn AA5083 rolled sheets microalloyed with Sc and Zr were investigated under various thermomechanical processing conditions. The results revealed that adding 0.05 wt.% Sc resulted in a minor change in the alloy performance relative to the base alloy. When the Sc content reached 0.1 wt.% it significantly improved the alloy strength and recrystallization resistance, and dramatically lowered the intergranular corrosion (IGC) susceptibility. Compared with the base alloy, the 0.1 wt.% Sc addition (along with 0.08 wt.% Zr) increased the yield strength (YS) by 8, 75, 25, and 33% for the cold-rolled (H18-), annealed (O-), extra-deformed (H116-), and stabilized (H321-tempers) conditions, respectively. In addition, the recovery and recrystallization resistances were significantly improved, as demonstrated by the retained deformed fibrous structure in the O- and H321-tempers. Furthermore, the IGC susceptibility was substantially decreased in the H321-temper due to the prevention of continuous β-Al3Mg2 precipitation along recrystallized grain boundaries. The YS of the final rolled sheets was modeled using constitutive analysis to predict the effects of various microstructural components on the strengthening contribution. Graphical Abstract

The impact of hot rolling temperature on the microstructure evolution and mechanical properties of hot/cold rolled AA5083 alloy containing Sc and Zr was studied. The results revealed that low hot-rolling temperatures (LHRT: 400–425oC) resulted in superior tensile properties compared to high hot-rolling temperatures (HHRT: 500–525 oC). The yield strength (YS) of the LHRT samples reached 450 MPa in the H18-temper and 291 MPa in the O-temper, which were respectively 20% and 39% higher relative to the HHRT samples. Microalloying with Sc/Zr and two-step homogenization promoted the formation of dispersed particles in the form of Mn dispersoids and nanosized Al3(Sc,Zr) precipitates. The rolling temperature was found to have a profound impact on the characteristics of these dispersed particles and on the recrystallization resistance. The HHRT caused the coarsening of both dispersed particles during rolling. The LHRT preserved the deformed elongated grain structure due to a stronger retardation of recovery and recrystallization during hot rolling and post-annealing. The strengthening mechanisms of the Mn-dispersoids and Al3(Sc,Zr) precipitates were quantitatively analyzed based on particle characteristics. The predicted YS contributions in the O-temper were in good agreement with the measured YS difference between the LHRT and HHRT samples.

An abnormally coarsened grain structure after brazing can negatively affect the in-service performance of a brazed heat exchanger. One of the numerous beneficial effects from the Sc addition to aluminum is its retardation effect on recrystallization and grain growth. In this work, dilute Al–0.07Sc and Al–0.07Sc–0.09Zr alloys were assessed with respect to the hot deformation and the post-brazing grain structure and compared with the base 1xxx alloy. The flow curves of the hot compression tests showed that the Al–Sc–Zr alloys exhibited higher flow stresses compared to the base alloy. The microstructure of the deformed samples in all three investigated alloys exhibited a recovered microstructure, but both Sc-containing alloys showed an improvement in the resistance to dynamic recovery. EBSD maps after a high temperature simulated brazing showed that the microstructure of the base alloy suffered from abnormal grain growth. Severe grain coarsening was still observed in the recrystallized microstructure of Al–0.07Sc alloy after brazing. The combined addition of Sc and Zr in Al–0.07Sc–0.09Zr alloy suppressed the abnormal grain growth, showing an improved control on the brazed microstructure. After post-braze aging at 350 °C for 4 h, the hardness of both Sc-containing alloys increased by 90–122% relative to the base alloy hardness.

Al–Mn 3xxx alloys are widely used in the fabrication of multi-port extruded (MPE) tubes for aluminum heat exchangers. The industrial manufacture for those tubes consists of four steps: (1) homogenization, (2) extrusion, (3) straightening/sizing, and (4) brazing. The combination of cold work and brazing could result in abnormally coarsened grains that have negative impacts on the in-service performance of MPE tubes. A 3xxx alloy microalloyed with 0.08 wt.% Sc and 0.09 wt.% Zr was assessed with respect to the grain structures after hot deformation and brazing, and a comparison was made with the base 3xxx alloy free of Sc and Zr. The flow stress of the base alloy during hot compression at 500 °C and 1 s−1 was 48 MPa, while the alloy with added Sc and Zr showed a higher flow stress of 61.5 MPa. EBSD maps after hot deformation showed that the alloy with added Sc and Zr exhibited more resistance to dynamic recovery and dynamic recrystallization. After the high-temperature simulated brazing at 605 °C, the base alloy suffered from abnormal grain growth, whereas the alloy with added Sc and Zr exhibited an elongated recrystallized microstructure with a finer grain size.

The properties of 5xxx5xxxaluminum alloysAluminum alloys can be improved with small additions of Sc. When Sc and Zr are added to 5xxx alloys, the alloys become heat-treatable as Al3(Sc,Zr) nanoprecipitates form at 300–400 °C. However, the heat treatmentHeat treatment and thermo-mechanical processingProcessing (TMP) need to be adapted to maximize the value of the Sc addition. In this work, AA5083 slabs were DC cast with and without minor additions of Sc and/or Zr. The room/elevated temperature mechanical propertiesMechanical properties of the cast materials were evaluated after various annealingAnnealing and homogenizationHomogenization treatments. Samples were then hot and cold rolled with different TMP treatments. The microstructureMicrostructure, room temperature mechanical propertiesMechanical properties and corrosionCorrosion performance were evaluated for selected tempers. It was evident that the properties could be improved significantly with small additions of Sc, but that this depends very much on the amount of Sc and on the processingProcessing parameters.

5xxx5xxxaluminum alloysAluminum alloys are traditionally considered non-heat-treatable. With the addition of Sc/Zr and multistep heat treatmentHeat treatment, two kinds of dispersoidsDispersoids (AlMn and Al3(Sc,Zr)) were formed. The effect of Sc additions (0.08–0.43 wt.%) on dispersoidDispersoids formation and mechanical propertiesMechanical properties of hot-rolled sheets was investigated. The results showed that tensile properties initially increased with increasing Sc addition. The yield strength (YS) and ultimate tensile strengthTensile strength (UTS) of the alloy with 0.16 wt.% Sc reached 295 and 411 MPa, respectively, showing improvementsImprovement of 28% in YS and 8% in UTS compared to the base alloy. However, with a further increase of Sc, the tensile properties declined owing to the formation of a line/fan-shaped microstructureMicrostructure associated with discontinuous Al3(Sc,Zr) precipitationPrecipitation during solidificationSolidification. The evolution of Al3(Sc,Zr) and AlMn dispersoidsDispersoids during heat treatmentHeat treatment and hot rollingHot rolling was characterized using scanning and transmissionTransmission electron microscopies. Their influence on the mechanical propertiesMechanical properties of hot-rolled AA5083 alloys was discussed.

With the addition of Sc and Zr to AA5083 alloy, two populations of strengthening particles (submicron-sized AlMn dispersoids and nanosized Al3(Sc,Zr) precipitates) precipitate during three-step heat treatment. Here, their influence on the microstructure and mechanical properties of hot-rolled sheets at ambient and elevated temperatures was investigated. The results show that the low-temperature (25–200 °C) tensile properties of the rolled sheets were significantly improved by increasing the Sc and Zr contents. The yield strength (YS) and ultimate tensile strength (UTS) of the alloy with 0.16 wt.% Sc and 0.17 wt.% Zr at ambient temperature reached 295 and 411 MPa, respectively, showing improvements of 30% in YS and 11.8% in UTS compared to the base alloy. However, the YS of the Sc/Zr-containing alloys at high temperature (300–400 °C) were lower than that of the base alloy. The mechanical properties of both the base and Sc/Zr-containing alloys were thermally stable during long-term thermal exposure at 300 °C for 500 h, demonstrating the great potential of this alloy for various elevated-temperature applications. The characteristics of the AlMn dispersoids and Al3(Sc,Zr) precipitates after the heat treatment and hot rolling were examined and quantified using transmission electron microscopy. Their combined contributions toward the YS at 25 and 300 °C were analyzed with the aid of constitutive strengthening equations and compared with experimentally measured values.

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.