42 P. Thompson et al. agglomeration of CNTs was also observed. Lahiri et al. [6] fabricated Al-CNT MMCs via roll bonding of Al foils coated with CNTs and studied the various strengthening mechanisms induced by CNTs. It was observed that uniform dispersion of CNTs leads to improved elastic properties, while agglomeration of CNTs yields improved strain hardening and ultimate tensile strength. In a relatively unique approach, Patel et al. [7] fabricated stainless steel-CNT MMCs by chemical vapor deposition (CVD) of CNTs directly on to stainless steel pellets. This study saw a significant increase in yield strength, elastic modulus and hardness. It also removed the step of mixing/ combining CNTs with metal particulates. Given the rising popularity of AM and CNT-MMCs, there are surprisingly few results reported on the fabrication/ performance of CNT-MMCs prepared by AM. In one study, Wang et al. [8] fabricated Inconel 625-CNT MMCs via Selective Laser Melting (SLM). In this study, CNTs were coated on IN625 powder via molecular level mixing. This method allowed for reduced agglomeration and improved mechanical properties, like yield strength and ultimate tensile strength. The present study examined the feasibility of preparing Al-CNT MMCs via CVD growth of CNTs directly onto AlSi10Mg powders, followed by Direct Metal Laser Sintering. 5.2 Experimental Procedure 5.2.1 CVD Growth of CNTs on AlSi10Mg Powders Synthesis of the composite powder was carried out in a similar fashion to that found in literature [7]. This procedure is outlined, schematically in Fig. 5.1a. The process began by loading AlSi10Mg powder into a CVD quartz tube, which laid horizontally inside of a furnace, equipped with precision temperature control. After the tube was airtight-sealed, a mechanical pump brought the system to a vacuumed state, and the system was purged further by the flow of argon and carbon monoxide at constant flow rates for a designated amount of time. The vacuum valve was closed during the gas purging phase. After the system was completely purged, the temperature in the furnace was elevated at a constant rate until it reached a dwell temperature below the solidus line indicated by the AlSi10 phase diagram, which has been analyzed in previous work dealing with DMLS [9]. Once dwell temperature was reached, acetylene was introduced into the system at a constant flow rate and the system was held at dwell temperature. After the dwell period, the carbon monoxide and acetylene vents were closed, and the furnace was opened to allow cooling. Argon flow continued until the powder was ready to be removed from the CVD tube. Figure 5.1b contains an SEM image of AlSi10Mg powders with CNTs grown on their surface. In order to obtain sufficient powder with CNT content below 1 wt.%, the post-processed powder was mixed with virgin AlSi10Mg in a 1:1 ratio using a Resodyn RAM 5 acoustic mixer. This ensured homogeneous distribution and reduction of potential agglomeration. The resulting CNT-reinforced AlSi10Mg powder for use in the EOS printer contained 0.67 wt.% of CNTs after weighing all powder amounts before and after CVD. 5.2.2 DMLS of Specimens and Heat Treatment Material testing coupons and representative parts were built by an EOS M290 system equipped with a 250×250×300mm work envelope and an up to 400 W continuous Yb: YAG fibre laser. Two builds were performed. The first build, which served as a control build, contained specimens fabricated from virgin EOS AlSi10Mg powder (P/N: 9011-0024) from a single powder lot. The second build contained specimens fabricated from CNT-infused EOS AlSi10Mg powder from the same virgin powder lot. The targeted spot size of the machine was 80 μm in diameter. A Hatching Distance of 190 μmwas utilized with a Stripe Width of 7 mm and an Overlap of 20μm. The hatching strategy utilized was the strip scanning strategy with a 67◦ rotation of the laser between successive layers. The Layer thickness prior to melting was 30μm. The builds were performed under an Argon atmosphere with maximum oxygen content of 0.1%. The build plate was kept at a temperature of 35 ◦C during printing. Both the Al-CNT MMC and AlSi10Mg builds, depicted in Fig. 5.2a, b, received stress relief heat treatment. This consisted of placement in a static air environment with nitrogen for 2 h at 300◦C.
RkJQdWJsaXNoZXIy MTMzNzEzMQ==