As important structural materials widely used in aerospace and automotive industries, aluminum alloys are perfect candidates for development of laser metal additive manufacturing (AM). Amongst AM aluminum alloys, laser powder bed fusion (LPBF) AlSi10Mg has received substantial attention due to its good printability and relatively low cost. Great efforts have been devoted to seek optimum process parameters that can enhance mechanical performance. However, a large scattering of material properties arises from the literature data, especially for the as built state, thus casting a shadow over further development of LPBF Al alloys. This review article aims to summarize the recent progresses on the characterization of microstructure, assessment of strengthening and damage mechanisms, evaluation of fracture and fatigue resistance, and attempts to build a primary comprehensive link between mechanical performance and microstructure for the as built state. Following the analysis of the state of the art, the review will finally provide an outlook on additional efforts needed to quantify the microstructure-property relation, based on which maximizing the potential of mechanical performance through optimizing microstructure may be achieved. 

Review on the Correlation Between Microstructure and Mechanical Performance for Laser Powder Bed Fusion AlSi10Mg

Aims Plant biomass allocation and elemental homeostasis have recently become a focus in ecological studies. Herbaceous plants are a major component of species diversity in desert ecosystems. Study on biomass allocation and leaf stoichiometry with growth will be beneficial to further understanding the survival strategy and functional features of desert herbs. Methods Four dominant species, including two ephemeral plants (Erodium oxyrrhynchum and Hyalea pulchella) and two annals (Agriophyllum squarrosum and Ceratocarpus arenarius), were selected for in situ and multiple- time investigations. Changes in the characteristics of biomass allocation and leaf N-P stoichiometry, as well as their relationships were analyzed. Important findings Root to shoot ratio decreased gradually during the accumulation of plant biomass in all the four species. The allometric relationships between above- and belowground biomass, however, varied among the four species; the allometric scaling exponents in H. pulchella and C. arenarius initially increased and then de- creased, and tended to become stable toward the later period, whereas the allometric scaling exponents in E. oxyrrhynchum and A. squarrosum initially increased and then tended to be isometry in later period. The leaf N and P contents tended to increase in H. pulchella, but declined in other three species with growth, indicating that the leaf N-P stoichiometry of the plants studied changed and showed weak correlations with biomass indices.

Biomass allocation and leaf stoichiometric characteristics in four desert herbaceous plants during different growth periods in the Gurbantünggüt Desert, China

Multiscale plasticity behavior and fatigue performance of laser melting multi-layer nickel-based superalloys upon heat treatments

Heterogeneous slip localization in an additively manufactured 316L stainless steel

The effects of different heat treatments on the microstructure and mechanical properties of the laser melting nickel-based superalloys were systematically investigated. The strength improvement of the heat-treated material was mainly derived from the aging treatment, which converted the γ matrix together with Niobium to the strengthening phases γ′′ and γ′. The solution treatment dissolved partial Laves phases to form the short-acicular δ phases in the remelting zone (RZ), whereas the homogenization treatment dissolved most precipitates including the Laves phase and δ phase, leading to significant grain coarsening in the multi-layer material. Moreover, based on the instrumented indentation reverse analysis, RZ shows consistent stress–strain relationships after the heat treatments, reaching the same level as the heat-treated base material (BM). The remelting-affected zone (RAZ) reveals more outstanding mechanical properties after the direct aging (DA) treatment for the work hardening contributed by dislocation strengthening. Additionally, the residual stresses and deformations in RAZ provide an extra thermodynamic driving force for the grain recrystallization and the formation of the short-clavate δ phase in the solution-treated RAZ. The remaining large grains and small Taylor factor in RZ are considered to lead to a reduction of the mesoscopic yield strength, which cannot be identified in microscopic indentation analysis. The DA heat treatment is recommended for the strength improvement of the laser melting materials. 

Effects of heat treatments on microstructure and mechanical properties of laser melting multi-layer materials

The correlation between mechanical behavior and heterogeneous microstructure is still unclear for laser powder bed fusion AlSi10Mg. The present work investigates strain hardening and damage mechanisms with different build platform temperatures (35 °C and 200 °C) and loading directions (horizontal and vertical, building direction being vertical) to probe the effects of microstructure size and melt pool border orientation. The 35 °C microstructure involves load direction independent properties, with ductility determined by damage initiation occurring extremely close to final failure. In contrast, the 200 °C microstructure exhibits anisotropic mechanical behavior, as revealed by higher strain hardening capacity, earlier damage initiation and more dispersed damage distribution for the vertical loading direction. Although the vertical samples generally involve strain localization and crack propagation along the melt pool border, their ductility is not compromised compared to the horizontal counterparts. We attempt to explain these hardening and damage behaviors with stress partition and trade-off between phase stress and strain gradient. 

Microstructure and loading direction dependent hardening and damage behavior of laser powder bed fusion AlSi10Mg

Drought stress severely affects plant growth,development and reproduction,and it is one of the most important stress factors studied by researchers.Drought stress influences plant morphological structures and functions.The variation in such indices as biomass,water efficiency,photosynthetic system,osmotic adjustment,stability of cellular membrane,defense capabilities of anti-oxidative systems and hormone levels is often used to determine the plant to drought resistance.Here,in order to have a more comprehensive and profound understanding of the mechanism of resistance to drought,the plant physiological,morphological responses to drought stress are reviewed,especially for the above-mentioned indices.

Effects of drought stress on plant growth and physiological traits

Laser powder bed fusion AlSi10Mg exhibits severe mechanical anisotropy, which hinders its applications in industrial field. In the present work, we investigated the mechanisms of anisotropy in the light of phase stress partition, using traditional crystal plasticity and mechanism-based strain gradient crystal plasticity models. Two representative volume elements of the Al-Si cellular structures, corresponding to 35 ℃ and 200 ℃ build platform temperatures, were built to assess the influences of Al matrix strength and strain gradient on phase stress partition and anisotropy. In addition, the maximum principal stress distribution of the Si-rich network was probed, based on which the potential damage sites and densities were evaluated. The results show that the anisotropic strength and damage both originate from the elongated Si-rich network. According to the comparison between the 35 ℃ and 200 ℃ materials, an increase in Al phase strength can change the phase stress partition and effectively mitigate the anisotropy degree. Moreover, it is revealed that the strain gradient tends to alleviate anisotropy and delay damage evolution. The findings of this work can pave the way to developing high-performance LPBF Al alloys with lower degrees of anisotropy. 

Lu Song

Papers

Review on the Correlation Between Microstructure and Mechanical Performance for Laser Powder Bed Fusion AlSi10Mg

Microstructure and loading direction dependent hardening and damage behavior of laser powder bed fusion AlSi10Mg

Effects of drought stress on plant growth and physiological traits

Deciphering phase stress partition and its correlation to mechanical anisotropy of laser powder bed fusion AlSi10Mg