Thermal Stability of Aluminum Alloys

Recent Progress in Creep-Resistant Aluminum Alloys for Diesel Engine Applications: A Review

Diesel engines in heavy-duty vehicles are predicted to maintain a stable presence in the future due to the difficulty of electrifying heavy trucks, mine equipment, and railway cars. This trend encourages the effort to develop new aluminum alloy systems with improved performance at diesel engine conditions of elevated temperature and stress combinations to reduce vehicle weight and, consequently, CO2 emissions. Aluminum alloys need to provide adequate creep resistance at ~300 °C and room-temperature tensile properties better than the current commercial aluminum alloys used for powertrain applications. The studies for improving creep resistance for aluminum casting alloys indicate that their high-temperature stability depends on the formation of high-density uniform dispersoids with low solid solubility and low diffusivity in aluminum. This review summarizes three generations of diesel engine aluminum alloys and focuses on recent work on the third-generation dispersoid-strengthened alloys. Additionally, new trends in developing creep resistance through the development of alloy systems other than Al-Si-based alloys, the optimization of manufacturing processes, and the use of thermal barrier coatings and composites are discussed. New progress on concepts regarding the thermal stability of rapidly solidified and nano-structured alloys and on creep-resistant alloy design via machine learning-based algorithms is also presented.

Change in Mechanical Properties of Laser Powder Bed Fused AlSi7Mg Alloy during Long-Term Exposure at Warm Operating Temperatures

Al-Si-Mg alloys are most commonly used to produce parts by laser powder bed fusion for several industrial applications. A lot of papers have already focused on the effects induced by conventional heat treatments on the microstructure and mechanical properties of AlSi10Mg alloys, rather than on AlSi7Mg. Nobody has investigated thermal stability during long-term direct and artificial aging heat treatments of AlSi7Mg. This study investigates the changes in mechanical properties induced by long-term exposure (512 h) at 150 and 175 °C (the operating temperature of AlSi7Mg) after (i) the laser powder bed fusion process performed on a pre-heated build platform (150 °C), and (ii) heat treatments to the solution at 505 °C per 0.5 and 4 h. Thermal stability was evaluated through both Vickers microhardness measurements to obtain the aging profiles, and tensile tests to evaluate the mechanical properties in specific conditions. An optical microscope was used to investigate the microstructure. It was found that aging at 175 °C confers the same effects induced by a secondary aging heat treatment on as-built samples and, simultaneously, the worst effects on the solution heat treated AlSi7Mg alloy after long-term exposure. The AlSi7Mg DA at both 150 °C and 175 °C showed the same Vickers microhardness (~95 HV0.5), UTS (~300 MPa), and YS (~200 MPa) values for the longest exposure times because the fine and cellular α-Al matrix confers higher stiffness and strength despite the over-aged conditions. On the other hand, the coarsening effects that affected the precipitates during aging at 175 °C, as well as the formation of the precipitate-free zones along the grain boundaries, justified the highest detrimental effects induced on the SHTed samples.

Influence of Microstructure on Fracture Mechanisms of the Heat-Treated AlSi10Mg Alloy Produced by Laser-Based Powder Bed Fusion

Few systematic studies on the correlation between alloy microstructure and mechanical failure of the AlSi10Mg alloy produced by laser-based powder bed fusion (L-PBF) are available in the literature. This work investigates the fracture mechanisms of the L-PBF AlSi10Mg alloy in as-built (AB) condition and after three different heat treatments (T5 (4 h at 160 °C), standard T6 (T6B) (1 h at 540 °C followed by 4 h at 160 °C), and rapid T6 (T6R) (10 min at 510 °C followed by 6 h at 160 °C)). In-situ tensile tests were conducted with scanning electron microscopy combined with electron backscattering diffraction. In all samples the crack nucleation was at defects. In AB and T5, the interconnected Si network fostered damage at low strain due to the formation of voids and the fragmentation of the Si phase. T6 heat treatment (T6B and T6R) formed a discrete globular Si morphology with less stress concentration, which delayed the void nucleation and growth in the Al matrix. The analysis empirically confirmed the higher ductility of the T6 microstructure than that of the AB and T5, highlighting the positive effects on the mechanical performance of the more homogeneous distribution of finer Si particles in T6R.

Heat Treatment of Aluminum Alloys with the Natural Combination of Dopants

Aluminothermic reduction without the separation of individual metals is currently considered as a possible method for processing ferromanganese sea nodules and creating new alloys. In this study, the product of their reduction-a manganese-based polymetallic mixture-was added to pure aluminum, as a mixture of alloying elements in their natural ratios. After extrusion, two new aluminum alloys with a total percentage of metallic additives ranging from 1 to 6 percent were prepared. The possibilities of the precipitation strengthening of these aluminum alloys, especially those containing Mn, Fe, Si, Ni, and Cu, were investigated under a wide range of heat treatment conditions. After each tested combination of annealing and artificial aging temperatures, the phase composition and the microstructure changes were recorded by X-ray diffraction, optical, and scanning electron microscopy with EDS analysis. Under none of the tested heat treatment conditions is a significant hardening effect observed, even though the precipitate phases are observed by TEM. However, the changes in the morphology of the present intermetallic phases caused by the heat treatment are revealed, which highlights the further possible development of these multicomponent alloys.

Current Trends in Automotive Lightweighting Strategies and Materials

The automotive lightweighting trends, being driven by sustainability, cost, and performance, that create the enormous demand for lightweight materials and design concepts, are assessed as a part of the circular economy solutions in modern mobility and transportation. The current strategies that aim beyond the basic weight reduction and cover also the structural efficiency as well as the economic and environmental impact are explained with an essence of guidelines for materials selection with an eco-friendly approach, substitution rules, and a paradigm of the multi-material design. Particular attention is paid to the metallic alloys sector and progress in global R&D activities that cover the “lightweight steel”, conventional aluminum, and magnesium alloys, together with well-established technologies of components manufacturing and future-oriented solutions, and with both adjusting to a transition from internal combustion engines to electric vehicles. Moreover, opportunities and challenges that the lightweighting creates are discussed with strategies of achieving its goals through structural engineering, including the metal-matrix composites, laminates, sandwich structures, and bionic-inspired archetypes. The profound role of the aerospace and car-racing industries is emphasized as the key drivers of lightweighting in mainstream automotive vehicles.

Improvement of Mechanical Properties and Forming Efficiency during Hot Gas Forming of CFRP Curved Surface Components

Carbon fiber reinforced plastics (CFRP) are widely used in aerospace and new energy vehicles due to their high specific strength and flexible design ability. At present, the traditional forming process of CFRP curved surface components has problems of low mechanical properties and long processing time. In this paper, a new method of hot gas forming was proposed to obtain CFRP components. By applying high temperature and high-pressure gas on one side of CFRP, the material was forced to deform and solidify at the same time. A special device for hot gas forming was designed and developed. The curing behavior and mechanical properties of original CFRP plates were studied. The main defects and the corresponding control methods of hot gas forming parts were analyzed by forming spherical parts, and the feasibility of the hot gas forming process was verified. Taking the battery cover plate of a new energy vehicle as the research object, the influence of forming temperature, gas pressure, pressurization rate and other process parameters on the mechanical properties of complex CFRP components were analyzed. The mechanism of both strength and efficiency improvement was analyzed. The results showed that with the increasing of gas pressure, the tensile strength and forming efficiency of the CFRP curved components were improved obviously. Under reasonable forming parameters, the tensile strength of the obtained parts was increased by 37%, and the forming efficiency was increased by 58%. The fiber bundles were distributed more evenly and compactly under the hot gas forming. This showed that the use of hot gas forming had good potential in the preparation of high-performance CFRP parts, which was helpful to improve the processing efficiency and forming quality of CFRP curved parts in the aerospace and new energy automotive fields.

On the Al-Al11Ce3 Eutectic Transformation in Aluminum-Cerium Binary Alloys

The L ↔ Al + Al₁₁Ce₃ technologically important eutectic transformation in Al–Ce binary alloys, containing from 5 to 20 wt.% Ce and ranging from hypo- to hypereutectic compositions, was examined along with the microstructure and properties of its solidified product. A combination of thermal analysis and metallography determined the coordinates of the eutectic point at 644.5 ± 0.6 °C and 10.6 wt.% Ce, clarifying the existing literature ambiguity. Despite the high entropy of melting of the Al₁₁Ce₃ phase, in hypoeutectic alloys the eutectic was dominated by the regular morphology of periodically arranged lamellae, typical for non-faceted systems. In the lamellar eutectic, however, the faceting of Al₁₁Ce₃ was identified at the atomic scale. In contrast, for hypereutectic compositions, the Al₁₁Ce₃ eutectic phase exhibited complex morphology, influenced by the proeutectic Al₁₁Ce₃ phase. The Al₁₁Ce₃ eutectic phase lost its coherency with Al; it was deduced that a partial coherency was present only at early stages of lamellae growth. The orientation relationships between the Al₁₁Ce₃ and Al in the eutectic structure, leading to partial coherency, were determined to be [0 0 1]_Al ║ [1¯ 1 1]_Al11Ce3 with (0 4 4¯)_Al ║ (2¯ 0 0)_Al11Ce3 and [0 1 1]_Al ║ [3¯ 0 1]_Al11Ce3 with (2¯ 0 0)_Al ║ (0 6 0)_Al11Ce3. The Al₁₁Ce₃ phase with a hardness of 350 HV and Al matrix having 35 HV in their eutectic arrangement formed in situ composite, with the former playing a role of reinforcement. However, the coarse and mostly incoherent Al₁₁Ce₃ eutectic phase provided limited strengthening and the Al–Ce alloy consisting of 100% eutectic reached at room temperature a yield stress of just about 70 MPa.

Optimizing Heat Treatment Parameters for the W-Temper Forming of 7xxx Series Aluminum Alloys

A major challenge in processing 7xxx series aluminum alloys is their limited formability at room temperature. In this paper, for the alloys EN AW-7020 and EN AW-7075, various variants of the W-temper forming process are investigated. For both alloys, a good cold formability and a high strength after aging can be achieved. The effects of solution heat treatment or retrogression temperature and holding time, as well as the influence of plastic deformation after quenching, were studied. For various combinations of process parameters, the formability of the as-quenched materials and the hardening performance during artificial aging were examined. For this, hardness measurements and differential scanning calorimetry (DSC) experiments were performed along the entire process chain, to reveal the development of the hardening precipitates. After solution heat treatment and quenching, the yield stress and hardness of both investigated alloys were drastically reduced in comparison to their initial T6 states, while the ductility was significantly increased. By a subsequent simple artificial aging treatment, the same hardness as in the T6 state could be restored. It was observed that plastic deformation immediately after quenching significantly decreased the artificial aging time to achieve the peak hardness. Besides the conventional solution heat treatment process, an alternative retrogression and re-aging procedure was identified for the alloy EN AW-7020. While the heat treatment temperature can be reduced as compared to the conventional solution heat treatment, the formability and hardenability are equally good. In contrast, no such alternative process could be identified for the alloy EN AW-7075.