Recycling Of GLARE Composite

Aluminum as an important metal is widely used in transportation, consumerdurables, and building and construction. Al alloys recycling plays a significant role insustainable development of Al industry. Compared with primary aluminum productionfrom bauxite, several advantages can be achieved from Al alloy recycling: energysaving, emission reduction, waste disposal reduction and capital cost reduction. Highstrength aerial Al alloys are important members of Al alloy family. The fibrereinforced metal laminates GLARE which consists of2024Al alloy and S2glass fibrehas been used as fuselage in Airbus A380, but the solution for GLARE recycling isstill lacked. Moreover, Silicon is a usual impurity element in aerial Al alloys and Sicontent always increases during recycling and exceeds the permissible Siconcentration of aerial Al alloys. However, few methods have been introduced toremove impurity silicon from Al melts in industrial scale.In this thesis, a feasible solution for fiber metal laminates GLARE recycling wasdeveloped in industrial scale. The decomposition kinetics of resins in GLARE wasstudied, as well as the recycling of obtained2024Al alloy after thermal delamination.Furthermore, a basic research based on alloying method by using Ti addition wasexecuted to investigate the Si removal efficiency, and the first-principles method wasemployed to study the purification potential of Ti addition and Si removal mechanism.Under non-isothermal conditions, the decomposition of resins consists of four steps,the first step is attributed to the decomposition of BR127, and later three steps areattributed to the decomposition of FM94. The initial decomposition temperature ofBR127and FM94is188oC and255oC respectively in both oxidative and inertatmospheres when the heating rate of1oC min^-1is employed. The final conversion ofresins in air and nitrogen is100%and89.8%respectively when the GLARE sampleof60mg were heated up to500oC in air and to600oC in nitrogen with the sameheating rate of1oC min^-1. Oxidative atmosphere is more preferred for resinsdecomposition. The conversion is respectively4.4%,48.4%,57.8%and100%whensamples were kept at230oC,310oC,350oC and450oC for3hours in nitrogen, whilethe conversion under the same conditions in air is7.2%,43.4%,69.4%and100%respectively. Two decomposition mechanisms, nth-order for decomposition at230oCand450oC and autocatalysis for decomposition at310oC and350oC, were found during isothermal analysis. The GLARE thermal delamination process is decided as480oC for2hours based on thermal analysis results and experimental optimization,the S2-glass fibres and2024Al alloys can be well separated. The tensile strengthdegradation of recycled S2-glass fibres is about45%after thermal delamination at480oC.NaCl-KCl-Na₃AlF₆flux was used for the recycling of obtained2024Al afterthermal delamination. The efficiencies of two different ratios of NaCl to KCl,70wt.%NaCl-30wt.%KCl and44wt.%NaCl-56wt.%KCl, were discussed.10wt.%additional cryolite is preferred for the two kinds of flux considering the recycled Alalloy quality together with the economic cost. But, the yield of big metal beads （>2mm） after Al alloy scrap （35mm×25mm） recycling with44wt.%NaCl-56wt.%KCl^-10wt.%Na₃AlF₆is lightly increased compared to70wt.%NaCl-30wt.%KCl^-10wt.%Na₃AlF₆, the big beads yield is97.34%and96.76%respectively. Itwas attributed to the lower melting point of equimolar NaCl-KCl system which has apositive influence on the metal coalescence due to the lower viscosity of melten flux.The concentrations of major alloying elements Cu, Mg and impurity elements inrecycled Al alloys are consistent with nomination composition of2024alloy exceptMg, and the Mg loss is caused by the reaction between Mg and cryolite.The big metal beads yield （>2mm） is obviously decreased with the amount of saltsflux. For44wt.%NaCl-56wt.%KCl^-10wt.%Na₃AlF₆flux, the obtained big metalbeads yields after Al alloy scrap （10mm×10mm） recycling is97.10%,94.83%and92.42%when the weight ratio of NaCl-KCl salts flux to Al scraps is1.5,1and0.5respectively. The big metal beads yield is decreased with decreasing of refiningtemperature. The big metal bead yield after Al alloy scrap （10mm×10mm） recyclingwith44wt.%NaCl-56wt.%KCl^-10wt.%Na₃AlF₆system is96.06%and96.82%when the re-melting temperature is720oC and760oC respectively. But the big sizebead yield is reduced to92.14%under720oC and94.40%under760oC when70wt.%NaCl-30wt.%KCl^-10wt.%Na₃AlF₆flux was employed. The experimental resultsindicated that the scrap size of bigger than10mm×10mm should be preferred.For44wt.%NaCl-56wt.%KCl-MgF2flux, the big beads yield after Al alloy scrap（35mm×25mm） recycling is97.74%,97.65%,97.21%and96.84%when theadditional MgF2in salts flux is5wt.%,10wt.%,15wt.%and20wt.%respectively.Mg and other alloying elements are well controlled during recycling, andconcentrations of impurity elements in recycled Al alloy also meet the requirements of2024Al nominal concentration. Big metal beads yield is decreased with MgF₂ addition amount. The big beads yields after recycling with MgF2addition are biggerthan those with cryolite addition when the fluoride addition amount is lower than12wt.%, but the situation is reversed when the fluoride addition amount is above12wt.%.The effect of Ti-6Al-4V shear Hi lock in fuselage on the GLARE End of Liferecycling was discussed. The experimental results indicated that the Ti-6Al-4V Hilock did not dissolve into Al melt while settle down to the bottom of crucible due tothe higher melting point and bigger density of Ti-6Al-4V alloy compared to2024Al.Thus, the Ti concentration in the recycled Al alloy within the range of2024Al alloynominal composition can be obtained after re-melting.After the evaluation of Al-Si-X ternary phase diagrams, titanium is the possibleelement which can react with low concentration Si in Al melt and form binary orternary phases with high melting point which can be easily removed to decreaseimpurity Si concentration. Si can replace Al in Al₃Ti phase and form high meltingpoint (Al_1-x,Si_x)₃Ti particles, but no TiSi or other Si-rich binary phases were found. Sipurification efficiency is heavily related to the initial Si concentration in Al melt. Withthe addition of1wt.%Ti, the decrement of Si concentration is0.01wt.%when theinitial Si concentration is0.14wt.%but the decrement is enhanced to0.17wt.%forAl melt with initial Si concentration of1.04wt.%. The calculation results based on1wt.%Ti addition amount and Si concentration decrement indicate the Si concentrationin Al₃Ti was increased from0.57at.%to9.12at.%when initial Si concentration in Almelt is enhanced from0.14wt.%to1.04wt.%. Though the increase of Ti additionamount can slightly improve Si purification efficiency in Al-0.2Si melt but willconsume more Al. Hence, the Si purification efficiency by alloying method with Tiaddition is poor for Al melt with low Si initial concentration.The first-principles calculations indicated that Si prefers to occupy Al site in Al₃Ti,both doped Si atoms in Al₃Ti on Al1site or Al2site prefer to diffuse via the Al1vacancy. But the most probable vacancy in Al₃Ti under Al-rich condition is Ti vacancy,resulting in difficulty of Si diffusion in Al₃Ti. This difficulty is unfavorable for Siremoval efficiency when alloying method of Ti addition is used.Therefore, the quality of Al2O3is important for the production of industrial pure Alwith low impurity Si concentration. Al2O3with low concentration of impurity SiO₂ispreferred for the production of electrolytic aluminum, which benefits the compositionof generated industrial pure Al. Considring that the upper limit of impurity Siconcentration is strictly controlled in aerospace Al alloys, e.g.2xxx Al alloys and 7xxx Al alloys, a strict classification should be conducted during the recycling ofaerospace Al alloys. The aerospace Al scrap should be pre-sorted by alloy type, mostimportantly by the separation of2xxx and7xxx series alloys. The classification issignificant to obtain preferred compositions of secondary aerospace Al alloys whichbenefit the quality as well as the market value of secondary aerospace Al alloys.