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  • 1. Liu, Xihe
    et al.
    Zhao, Congcong
    Zhou, Xin
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsinghua University, China.
    Liu, Wei
    Microstructure of selective laser melted AlSi10Mg alloy2019In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 168, article id 107677Article in journal (Refereed)
    Abstract [en]

    The influence of laser power during selective laser melting (SLM) on the grain morphology and texture component in AlSi10Mg alloy has been investigated, using electron backscattered diffraction (EBSD). Both equiaxed and columnar grainswere observed. The formation of equiaxed grainswas attributed to the huge thermal gradient on the border of melt pool and the columnar to equiaxed transition (CET) occurred in front of the columnar grains. The grain size of lowlaser power samplewas found smaller than that of higher ones. A fine pseudoeutectic structure, in which Si existed as fibrous, was observed because of the high cooling rate. This paper, from a new angle, explained the formations of three different zones across the melt pool, which were differentiated by the morphology of Si phase. The three zones correspond to the three temperature zones, whichwere divided by liquidus and solidus temperature, during the heating by laser beam. The coarse zones are formed by reheating the basic metal to semi-solid state when the temperature is lower than the liquidus temperature but higher than the solidus temperature.

  • 2. Saeidi, K.
    et al.
    Neikter, M.
    Olsen, Jessica
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Shen, Zhijian James
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Akhtar, F.
    316L stainless steel designed to withstand intermediate temperature2017In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 135, p. 1-8Article in journal (Refereed)
    Abstract [en]

    Austenitic stainless steel 316L was fabricated for withstanding elevated temperature by selective laser melting (SLM). Tensile tests at 800 degrees C were carried out on laser melted 316L with two different strain rates of 0.05 S-1 and 0.25 S-1. The laser melted 316L showed tensile strength of approximately 400 MPa at 800 degrees C, which was superior to conventional 316L. Analysis of fracture surface showed that the 316L fractured in mixed mode, ductile and brittle fracture, with an elongation of 18% at 800 degrees C. In order to understand the mechanical response, laser melted 316L was thermally treated at 800 degrees C for microstructure and phase stability. X-ray diffraction (XRD) and Electron back scattered diffraction (EBSD) of 316L treated at 800 degrees C disclosed a textured material with single austenitic phase. SEM and EBSD showed that the characteristic and inherent microstructure of laser melted 316L, consisting of elongated grains with high angle grain boundaries containing subgrains with a smaller misorientation, remained similar to as-built SLM 316L during hot tensile test at 800 degrees C. The stable austenite phase and its stable hierarchical microstructure at 800 degrees C led to the superior mechanical response of laser melted 316L.

  • 3. Wang, Dianzheng
    et al.
    Wang, Zhimin
    Li, Kailun
    Ma, Jing
    Liu, Wei
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsinghua University, China.
    Cracking in laser additively manufactured W: Initiation mechanism and a suppression approach by alloying2019In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 162, p. 384-393Article in journal (Refereed)
    Abstract [en]

    Cracking represents the main challenge for exploiting tungsten in additive manufacturing. In this study, laser powder-bed-fusion technique was applied to additively manufacture tungsten. In the built bulks, the grain boundaries were found to be rich in nanoscale gas pores. On the basis of that, a nanopore segregation induced cracking initiation mechanism was proposed. In order to control cracks, W-6wt.%Ta alloy was produced and the cracking suppression mechanism was investigated. The W-6Ta alloy is characterized by a submicron intragranular cellular structure, which composed large amount of interlocked dislocations as revealed by transmission electron microscopy. Owing to the cellular structure, the nanopores were trapped inside grains, which can reduce the cracking possibility. Moreover, the W-Ta alloy possesses higher strength (by 17%) and higher energy dissipation rate (by 52%) than pure tungsten, which both are beneficial for crack reduction.

  • 4. Wang, Dianzheng
    et al.
    Yu, Chenfan
    Ma, Jing
    Liu, Wei
    Shen, Zhijian
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Tsinghua University, China.
    Densification and crack suppression in selective laser melting of pure molybdenum2017In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 129, p. 44-52Article in journal (Refereed)
    Abstract [en]

    In this study, dense and crack-free pure Mo has been fabricated by selective laser melting. In order to obtain densification, the precursor powders were granulated and processed by plasma spheroidization. Parts of density of 10.16 g/cm(3) (99.1% of theoretical density) were obtained with spherical powders because of its increased laser absorptivity and packing density. The crack growth behaviors under various scanning strategies are analyzed by electron backscattered diffraction. The interlocking grain boundary structure, which increased the crack growth resistance and caused crack deviation, is formed under layer-wise rotated laser scanning. Cracks can be fully suppressed by applying the designed supporting structure. This study provides a novel route for the fabrication of complex Mo parts.

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