Selective laser melting manufactured CNTs/AZ31B composites: Heat transfer and vaporized porosity evolution
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ARTICLE Selective laser melting manufactured CNTs/AZ31B composites: Heat transfer and vaporized porosity evolution Jiaojiao Wub) and Linzhi Wanga),b) Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China; and Chongqing Key Laboratory of Additive Manufacturing Technology and Systems, Chongqing 400714, China (Received 28 April 2018; accepted 15 June 2018)
In this research, heat transfer analysis was operated by simulation to investigate the influence of carbon nanotubes (CNTs) on laser absorption and molten pool characteristic as well as the vaporization porosity of a typical magnesium alloy of AZ31B in the selective laser melting (SLM) process. It is concluded that the laser absorption is enhanced by 7.9% through mixing 1.5 wt% CNTs into AZ31B alloy powders. The full melting state of molten pools for CNTs/AZ31B composites was achieved by laser input energy densities (LIEDs) larger than 42 J/mm3. However, vaporization porosity has an ascendent tendency with LIED increasing, which leads to poor densities of manufactured parts. As a result, the optimal relative density and mechanical properties of composites are obtained by an LIED of 42 J/mm3. It may solve the problem of low laser absorption in laser processing for magnesium alloys and provide a referenced method to evaluate the vaporization porosity of the material in the SLM process.
I. INTRODUCTION
Magnesium alloys have excellent properties such as high strength-to-weight ratio and stiffness, good biological compatibility, and electromagnetic shielding ability.1 They are widely applied in fields of aerospace, automobile, electronics, and medical industries.2 However, because magnesium has a close-packed hexagonal crystal structure, the number of slipping surfaces is very limited.3 This leads to a bad performance on plastic deformation at ambient environment. Therefore, hot forming and pressure casting have been main methods for magnesium alloy production in recent decades.4,5 These conventional methods are difficult to fabricate precise and complicate structures. Besides, they generally cost a long time, which could not meet the requirement of the short design-to-manufacture period. It is necessary to develop a more advanced and efficient technology for magnesium alloy fabrication. At present, the selective laser melting (SLM) process becomes an advanced technology for precise and complicate structure fabrication.6 It needs no external machining tools or casting molds,7 besides, cooling rates may reach 106 K/s in molten pools.8 The grain refinement due to large cooling rates can significantly improve the mechanical properties of metal parts. As a result, the SLM technology is supposed to overcome the forming limit of magnesium alloys in conventional methods. a)
Address all correspondence to this author. e-mail: [email protected] b) These authors contributed equally to this work. DOI: 10.1557/jmr.2018.224 J. Mater. Res., 2018
Ng et al.9 and Zhang et al.10 studied the mechanical properties and relative density of S
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