Emerging metallic systems for additive manufacturing : in-situ alloying and multi-metal processing in laser powder bed fusion

Additive manufacturing of metals: Microstructure evolution and multistage control

As a revolutionary industrial technology, additive manufacturing creates objects by adding materials layer by layer and hence can fabricate customized components with an unprecedented degree of freedom. For metallic materials, unique hierarchical microstructures are constructed during additive manufacturing, which endow them with numerous excellent properties. To take full advantage of additive manufacturing, an in-depth understanding of the microstructure evolution mechanism is required. To this end, this review explores the fundamental procedures of additive manufacturing, that is, the formation and binding of melt pools. A comprehensive processing map is proposed that integrates melt pool energy- and geometry-related process parameters together. Based on it, additively manufactured microstructures are developed during and after the solidification of constituent melt pool. The solidification structures are composed of primary columnar grains and fine secondary phases that form along the grain boundaries. The post-solidification structures include submicron scale dislocation cells stemming from internal residual stress and nanoscale precipitates induced by intrinsic heat treatment during cyclic heating of adjacent melt pool. Based on solidification and dislocation theories, the formation mechanisms of the multistage microstructures are thoroughly analyzed, and accordingly, multistage control methods are proposed. In addition, the underlying atomic scale structural features are briefly discussed. Furthermore, microstructure design for additive manufacturing through adjustment of process parameters and alloy composition is addressed to fulfill the great potential of the technique. This review not only builds a solid microstructural framework for metallic materials produced by additive manufacturing but also provides a promising guideline to adjust their mechanical properties. 

In-situ alloying in laser-based additive manufacturing processes: A critical review

Aviation and automobile industries demand high strength, fatigue resistant, and wear-resistant materials in combination with lightweight, especially for structural applications. On the other hand, biomedical applications demand materials with low modulus and stiffness for optimized implants better matching the modulus of human bone combined with enhanced strength and wear resistance. For all the aforementioned applications in various fields, the fabrication of parts with desired size and shape without the need for joining or welding operations is desired while simultaneously reaching improved mechanical properties and more resistance to environmental attack, which are stringent requirements for almost all the applications. To achieve all these demands, both material developments, as well as modification of process conditions and parameters, are essential. Along these lines, a lot of research work focusses on advanced or even disruptive manufacturing routes and proper alloy development (e.g. Ti, Al, steels, and so on) applying additive manufacturing (AM) techniques for various applications. Among different AM methods, selective laser melting (SLM) is in high demand and preferred for achieving fully dense products in the required dimensions. Titanium alloys designed for AM have replaced a variety of other alloys due to their superior properties such as lightweight or good fatigue and corrosion resistance, achievable through modified microstructures gained by the faster heating and cooling rates realized upon laser printing. Ti alloys with a single (α) or dual (α+β) microstructure are mostly implemented in the aviation and automobile industries, whereas β alloys with exceptionally low modulus close to that of human bone are intensively studied for bio and dental implants but have not been commercialized yet. The modification of microstructure and properties in Ti-based materials with the addition of suitable reinforcement is also a reliable method. In this article, critical aspects for the optimization of processing parameters affecting the properties of SLM manufactured Ti alloys and titanium matrix composites (TMCs) will be presented, and future prospects of such materials will be critically assessed. This work is expected to be helpful for future studies on Ti alloys and composites with enhanced properties processed by laser manufacturing.

Selective laser manufacturing of Ti-based alloys and composites: impact of process parameters, application trends, and future prospects

Additively manufactured metallic materials exhibit excellent mechanical strength. However, they often fail prematurely owing to external defects (pores and unmelted particles) that act as sites for crack initiation. Cracks then propagate through grain boundaries and/or cellular boundaries that contain continuous brittle second phases. In this work, the premature failure mechanisms in selective laser melted (SLM) materials were studied. A submicron structure was introduced in a SLM Ag–Cu–Ge alloy that showed semicoherent precipitates distributed in a discontinuous but periodic fashion along the cellular boundaries. This structure led to a remarkable strength of 410 ± 3 MPa with 16 ± 0.5% uniform elongation, well surpassing the strength-ductility combination of their cast and annealed counterparts. The hierarchical SLM microstructure with a periodic arrangement of precipitates and a high density of internal defects led to a high strain hardening rate and strong strengthening, as evidenced by the fact that the precipitates were twinned and encircled by a high density of internal defects, such as dislocations, stacking faults and twins. However, the samples fractured before necking owing to the crack acceleration along the external defects. This work provides an approach for additively manufacturing materials with an ultrahigh strength combined with a high ductility provided that premature failure is alleviated. An analysis of metallic alloys fabricated through layer-by-layer deposition processes has revealed critical factors in preventing these materials from unexpectedly breaking. In selective laser melting (SLM) technology, thin layers of metal powders are assembled into three-dimensional objects using rapid heating and cooling steps. Zhi Wang from the South China University of Technology in Guangzhou and colleagues now show that the microstructure of a silver-copper-germanium alloy formed through SLM can affect the material’s strength. Using optical and X-ray microscopy, the team found that regularly spaced precipitates formed inside the 3D-printed alloy prevented abrupt atomic sliding movements, giving the material a higher natural strength and good ductility. Fractures that occurred at lower than expected stress levels were identified as arising from errors in the printing process, such as pores and unmelted powder particles. A submicron structure strategy was introduced in a selective laser melted (SLM) Ag-Cu-Ge alloy, showing semi-coherent precipitates distributed in a discontinuous but periodic fashion along the cellular boundaries. It leads to a remarkable strength of ~410 MPa with ~16% ductility, well surpassing the strength-ductility combination of their cast counterparts. The hierarchal SLM microstructure and high density of internal defects leading to a high strain hardening rate and strong strengthening. Premature failure occurred due to the external defects, such as pores and unmelted particles. This work paves a way for additively manufacturing materials towards high strength–ductility synergy.

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Premature failure of an additively manufactured material

Comparison of additively manufacturing samples fabricated from pre-alloyed and mechanically mixed powders

Microstructure and mechanical property of bimodal-size metallic glass particle-reinforced Al alloy matrix composites

The invention belongs to the technical field of aluminum matrix composites and discloses an aluminum matrix composite with a good high-temperature deformation capability and a preparing method of thealuminum matrix composite. The composite is formed by preparing reinforced particles with the volume fraction being 2%-20% and aluminum alloys with the volume fraction being 80%-98%; the reinforced particles are nanocrystalline amorphous double-phase particles; and the reinforced particles are Ti-Cu-Zr series alloys. The invention further discloses the preparing method of the composite. The prepared nanocrystalline amorphous double-phase particle reinforced aluminum matrix composite is high in room temperature strength, high in specific strength and good in high-temperature deformation capability and has a very good application prospect.

Meishen Xie

Papers

Premature failure of an additively manufactured material

Comparison of additively manufacturing samples fabricated from pre-alloyed and mechanically mixed powders

Microstructure and mechanical property of bimodal-size metallic glass particle-reinforced Al alloy matrix composites

Aluminum matrix composite with good high-temperature deformation capability and preparing method of aluminum matrix composite