| Currently, the overseas and domestic SLS coated sand mold/core has been used in the materialparts such as aluminum, iron and steel. But when used it in magnesium alloys there are localoxidation combustion, because magnesium is very easy to oxidation burning. Direct deposit existingSLS coated sand mold (core). Flame retardant just depend on coating products, without other specialmeasures to prevent the reaction of magnesium alloy with sand mold (core). However, the coatingproducts’ flame retardant effect is not good,the casting surface exist partial oxidation burning andablation phenomenon. And the casting surface roughness, internal sand inclusion, oxide inclusionand lower mechanical performance, etc. Because SLS coated sand mold (core) tend to have interfacereaction with magnesium alloy melt. As a result, the SLS coated sand mold (core) using inmagnesium alloy casting is limited.In this paper, we select carbon powder, pyrite and boric acid as flame retardant which particlemesh size≤150.Using them designed and manufactured different content of flame retardantmaterials. Its based on140/270mesh SLS coated sand material which developed by our group. Andcarried the flame retardant coated sand on the experiment of laser sintering forming. This paperstudied the influence of different flame retardant and its content on the early strength,post-processing strength and gas evolution of coated sand, explored flame retardant’s effectmechanism. And further carry out ZM2magnesium alloy casting experiment, compared the differentflame retardants’ flame retardant effection on magnesium alloy, explored the mechanism of flameretardant, obtained the following main conclusions:Adding the above three kinds of single flame retardant to the SLS coated sand material had asignificant effect on the early strength. Among them, adding boric acid to the SLS coated sandmaterials have a larger negative impact, when adding proportion is1wt.%or higher. With theimprovement of its content, at the beginning of laser sintering strength of coated sand dramaticallyplummeted. It is because the boric acid powder break up into unconsolidated metaborate (HBO2)under the action of high laser energy, caused laser sintering forming difficulties. Pyrite by laserirradiated generating exothermic reaction, when the content is less2wt.%, the early strength ofcoated sand increases gradually. But as the content rises,the early strength has a downward trend.And carbon powder content is less0.2wt.%which can have a modest increase of early strength. But those three kinds of flame retardant for post-processing strength and the gas evolution of coated sandhave little impaction. The post-processing strength of containing flame retardant’s resin coated sandmold (core) is more than2Mpa. And its gas evolution have a little chang with the original SLScoated sand material. Considering the impaction of containing flame retardants’ laser sinteringforming coated sand, the optimum adding amount of carbon powder, boric acid and pyrite arerespectively0.1wt.%、0.5wt.%、2wt.%. But a single flame retardant additives is restricted, flameretardant effect is not obvious, when magnesium alloy casting process exsisting local oxidationburning. The ZM2magnesium alloy casting surface is black. The casting surface is rough andobvious ablation area. However, the boric acid has a better flame retardant effection than pyrite andcarbon powder.We further discussed composite flame retardant effects on laser sintering of coated sandforming. The study found that the pyrite ore powder and carbon powder composite material has thebest laser formability to coated sand,2wt.%pyrite/0.1wt.%carbon composite flame retardant coatedsand laser sintering part’s early strength can reach0.6MPa. Compare to original coated sand theearly strength increased about10%, while in composite flame retardant containing boric acidcomponent. At the beginning of laser sintering strength of coated sand is on the decline, andgradually reduce with the increase of the content. Composite flame retardant strength ofpost-processing is still for adverse effects. But the composite flame retardant gas evolution are lessthan10ml/g. Some part of the carbon powder composite flame retardant hair volume less than9ml/g. Adding carbon powder in the composite flame retardant is helpful to reduce overall gasevolution.In addition, the carbon powder is obtained by the orthogonal experiment, pyrite ores and boricacid three components the optimal formula of compound flame retardant is carbon powder0.1wt.%,borate0.5wt.%, pyrite2wt.%. The early strength of the coated sand is0.58MPa, post-processingstrength is2.34MPa, gas evolution of9.77ml/g.After the contrast experiment of750℃ZM2magnesium alloy casting flame retardant foundthat without adding flame retardant of the SLS coated sand mold (core) has a serious oxidationburning. When casting magnesium alloy casting.The casting surface roughness, black and there haveobvious ablation. However, only spray coating resin coated sand mold (core) pouring ZM2also havepartial oxidation burning phenomenon, in the magnesium castings leaf area still have obvious ablation points. Using carbon powder/pyrite composite flame retardant obtained magnesium castingsurface has a large black oxide. The flame retardant effect is poor. But when coated coated sandmold (core) contain boric acid flame retardant. Its magnesium alloy casting surface is bright andclean. The flame retardant effect is better.The flame retardant mechanism of adding flame retardant is carbon powder, pyrite respectivelyreacting with the oxygen and forming CO2, SO2mixed gas. The majority of SO2reacting with NH3which decomposition by coated sand, weakening the effect of flame retardant. Meanwhile, when thetemperature is higher than700℃, the dense MgO+C protective film generated by CO2withmagnesium,which has a certain plastic lost protection effect, carbon powder, pyrite and its compositeflame retardant’s flame retardant effect is not ideal. However, composite flame retardant containingboric acid, by boron anhydride dissolved MgO developing3MgO·B2O3enamel at high temperature,the flame retardant effect is obvious.Finally, the complex-shaped shell ZM2magnesium alloy castings manufactured by addingcarbon powder, pyrite, boric acid three component composite flame retardant. By SLS we realizedthe sand mold molding sand core integration. The tubing diameter is only4mm. By750℃casted,sand mold (core) flame retardant effect is obvious. The surface of metallic magnesium alloy castingis perfect. It can improve the surface roughness, which reach Ra3.2-6.3μm. And the surface qualityis close to the metal mold casting quality. The tensile strength is172Mpa under as-castcondition,which is increase36%compared with without adding flame retardant, obtaining highquality magnesium alloy castings. |
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