Novel foaming agent used in preparation process of aluminum foams Novel foaming agent used in preparation process of aluminum foams

Novel foaming agent used in preparation process of aluminum foams

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Journal of Unlversity of Science and Technology BeijingMaterialsVolume 15, Number 6, December 2008, Page 735ELSEVIERNovel foaming agent used in preparation process of aluminum foamsXiangyang Zhou, Xiquan Liu, Jie Li, and Hongzhuan LiuSchool of Melurgical Science and Engnecring, Ceatral South University, Changsha 410083, China(Received 2007-10-20)Abstract: The performances of a novel foaming agent used in the preparation process of aluminum foams were investigated, and theeffects of some factors, such as addition of the foaming agent, foaming temperature on the porosity, and appearance of aluminumfoams were also discussed. Experimental results show that the novel foarning agent has a wide decomposition temperature range anda mild decomposed rate; the foaming agent has the ability to enhance the viscosity of aluminum melt, as a result, an extra viscosifiersuch as Ca or siCp is unecessary while using this foaming agent; the bubble-free zone in material decreases and the foaming efi-ciency increases with the increase of foaming agent; the bubble-fre zone disappears and the foaning eficiency is near 100% whenthe addition of foaming agent is more than 1.4wt%; the porosity of the aluminumn foam increases with the increase of foamning agentwhen the addition of foaming agent is less than 2.2wt%.。2008 University of Science and Technology Beiing. All rights reserved.Key words: aluminum foam; foaming agea; porosity; viscosity; foaming eficiency[This work was fincialy suppored by Hunan Science and Technology Program Projet (No.06SK2011).]1. Introductionrange of these powders are from 380°C to 820°C, andthey decompose rapidly in the range of 600°C toAluminum foams are finding an increasing applica-680°C, as a result that the distribution of bubbles intion in many fields, such as automobile and architec-aluminum melt is difficult to control, and the techno-ture industry, due to their special structure and per-logical conditions (e.g. foaming temperature and time)formances, such as sound absorption, electric waveare also very rigorous [7-9]; (C) these foaming agentsinsulation, impact absorption, and so on. The prepara-usually require pretreatment by thermal oxidation ortion methods of aluminum foams include penetrationcoating in order to delay gas release [8-12]; (d) visco-casting, liquid metal foaming with gas or foamingsifiers such as Ca and SiCp are necessary to enhanceagents, powder metallurgy, replication process, and sothe viscosity of aluminum melt to hold the bubbles,on[1-6].which will result in the increase of the preparationLiquid metal foaming with foaming agents is acost of aluminum foams and the presence of somepromising preparation method of aluminum foams duemechanical defects of aluminum foam products. Into simpler operation and fewer working steps. By far,addition, the bubble-free zone will appear at the bot-tom of aluminum foams due to the presence of gravi-the most commonly used foaming agents are hy-tation drainage effect, if the viscosity is not controlleddric-metal powders such as TiH2 and ZrH2. However,accurately [13-16]. So developing a novel cheapas foaming agents for production of aluminum alloyfoaming agent substituting for TiH2 and ZrH2, and in-foams, TiH2 and ZrH2 powders have some limitationsvestigating the foaming technology are necessary.that become barriers for their mass applications inevery walk of life. The main limitations of foamingCurrently, a novel cheap foaming material wastechnology using TiH2 and ZrH2 powders as foamingsynth中国煤化工and used in prepa-agents are the following: (a) hydric-metal powders areratiorhis paper, the per-very expensive; (b) the decomposition temperatureform;HCNMH Ggent used intheCorresponding uthor: Xiangyang Zhou, E-mal: xyzhou@mail.csu.edu.cnAlo available online at www.ciencedlrect.com◎2008 University of Science and Technology Beijing. AII nights reserved.736J. Univ. Sci TechnolL Bejing, VoL1S, No.6, Dec 2008preparation process of aluminum foams was investi-The foaming efficiency (m) was used as an indica-gated, and the effects of some factors such as foamingtion of foaming ability [9]; it could be calculated ac-agent content, foaming temperature on the porosity,cording to the following equation:and appearance of aluminum foams were also dis-cussed.; mo-m x100%=Yo-%x100%(1)mo2. Experimentalwhere n is the foaming efficiency, %; mo and mi arethe masses of total aluminum and bubble-free zone,2.1. Preparation of aluminum foam samplesrespectively, g; Vo and V are the volumes of total alu-Pure aluminum and a novel foaming material wereminum and bubble-free zone, respectively, cm'. Forthe main materials used in this study. The chemicalexample, the foaming efficiency 100% means no bub-analysis results of pure aluminum are given in Table 1.ble-free zone in the foam samples.Table 1. Chemical composition of commercial pure alu-The thermal decomposition behavior of the novelminumfoaming agent was measured using an SDT Q600 (Si-ElementContent 1 wt%multaneous DSC-TGA Instrument) analyzer. TheBalancedsample was heated to 800°C or 680°C in a flowingFe0.300(100 mL/min) air atmosphere and at heating rate of 50.080K/min.Cu0.00In addition, digital camera (SONY, DSC-P10) andGa0.014a JSM-6360 scanning electron microscope (SEM)Mg0.003 .equipped with energy dispersive spectrometer (EDS)The preparation approach of aluminum foam sam-were used to observe the apparent appearance and mi-ples using pure aluminum and the novel foamingcrostructure of samples. The porosity of samples wasagent as materials in this study mainly included thetested according to the usual method described else-following steps: (a) melting the pure aluminum underwhere [18].the temperature range from 700°C to 780°C, and thenholding at one temperature point (i.e. foaming tem-3. Results and discussionperature); (b) dispersing a given amount of foaming3.1. Thermodynamics and kinetics of the novelagent powder into the molten metal by conventionalfoaming agentmechanical stiring (about 1500 rpm), and stiring forFig. 2 reveals the curves of decomposition rate and3 min after introducing foaming agent into the melt; (C)the differential thermal analysis (DTA) of the foamingsoaking at the foaming temperature for 5 min; (d)agent in the range of 500-800C. Apparently, the de-cooling the melt in air so as to be samples.composition rate changes a lttle before 650°C, and2.2. Testing methodsthe starting decomposition temperature point of theA typical model of aluminum foams based on thefoaming agent reaches to 608.31°C; the decomposi-method of blowing foaming agent is shown in Fig. 1,tion rate gradually increases when the elevation ofwhere the upside of the model is foamed part and theheating temperature is fom 650°C to 750°C; the big-bottom of the sample is bubble-free zone.gest heat absorption peak position is 738.82°C, anddecomposition reaction ends at 755.869C. At the heat-ing rate of 5 K/min, the entire decomposition reactionprocess lasts 29.5 min.The curves of decomposition rate and DTA of thefoaming agent at 680°C are shown in Fig. 3. Obvi-ously, the shape of decomposition rate curve is similarFoamed partto a straight line, and the time of complete decomposi-tion is near 30 min at 680°C. These results reveal thedeco中国煤化工ing agent at 680°Cis cha_ition rate and longdeconYHCNMHGBubble-free zoneThe melting point of pure aluminum is about 660°C,Fig. 1. Typlcal model of aluminum foams.and the foaming temperature of pure aluminum isXY. Zhou et al, Novel foaming agent used in preparation process of aluminum foams737usually in the range from 680°C to 740°C. Fromciency increases to 100%, namely there is no exis-above analysis, the novel foaming agent can be usedtence of bubble-free zone when the addition of novelto prepare pure aluminum foam or aluminum alloyfoaming agent is more than 1.4wt%. However, thfoam due to its favorable thermal decompositionprocess of introduction and dispersion of foamingcharacteristics. In addition, compared with hy-agent become difficult to operate because the viscositydric-metal foaming agents, the novel foaming agentof aluminum melt is too high when the content ofcan make the preparation technological conditions offoaming agent is above 2.2wt%. That is to say, the ad-aluminum foams (such as foaming temperature,dition of the novel foaming agent leads to the increasefoaming time, and stirring intensity) more easily con-in viscosity of aluminum melt; as a result, the extratrolled due to its even decomposition rate and longviscosifiers such as Ca or SiCp are unecessary duringdecomposition time.the aluminum foam preparation process when usingthis foaming agent. The main reason why the novel00 tfoaming agent increases the viscosity of aluminummelt may be caused by some reactions between the80foaming agent and aluminum melt, which generate a755.8650608.31lot of fine particles that play an important role in in--10creasing the viscosity of the melt.20-150t000| 738.82~-205t50060070800Temperature/C70 t+95Fig. 2. Decomposition rate and DTA curves of the foaming.g65agent.260 t-9051.6121416182022Foaming agent addition/ wt%-1260Fig. 4. Effects of the addition of foaming agent on the po-rosity and foaming efficiency.40-143.3. Apparent appearance and microstructure ofaluminum foam samples-1The apparent appearances of vertical and CrOSS sec-103tions of aluminum foam samples are shown in Fig. 5.Time 1 minSamples used in Figs. 5(a)-(d) are prepared by usingFig. 3. Decomposition rate and DTA temperature curvesthe novel foaming agent of 1.6wt% at different foam-of the foaming ageot at 680°C.ing temperatures. The apparent appearances of an3.2. Effects of the addition of foaming agent on thealuminum foam sample prepared by using AISi12 alloyporosity and foaming efficiencyas material and TiH2 powder as foaming agent are alsoThe foaming temperature is 740°C in this part ofshown in Fig. 5(e) and Fig. 5(1) for comparison pur-experiments. The effect of the addition of foamingose.agent on the porosity is shown in Fig.4. The resultsFrom Fig. 5 and the testing results of porosity, itindicate that the porosity of the aluminum foam in-can be easily observed that the porosties of samplescreases with the increase of foaming agent when theprepared by using the novel foaming agent increaseaddition of foaming agent is less than 2.2wt%.from 66% to 75% with the foaming temperature fromFig. 4 also shows the influence of the addition of700°C up to 740°C, but the influence of foamingfoaming agent on the foaming efficiency. When thetemp中国煤化工so obvious as thataddition of foaming agent increases from 1.1wt% tooffocosity; and the alu-1.4wt%,the foaming efficiency increases fromminuYHC N M H Gone at the bottomn,89.41% to 91.73% accordingly. When the addition ofuniform distribution of bubbles, and pore sizes of 2-5foaming agent further increases, the foaming effi- mm can be obtained while using the novel foaming738J Univ. Sci Technol Beijing, VoL1S, No.6, Dec 2008agent; for an aluminum foam sample prepared by us-80%, the appearance of bubl-free zone at the bottoming TiH2 powder, although the porosity reached tois unavoidable.bottom of saumple50780101112bubble-free zone30 1001210141510 191011 1213-145FIg. 5. Apparent appearances of aluminum foam samples: (2) vertical section; (b) cross section, 700°C, porosity>66%; (C)cross section, 720°C, porosity=70%; (d) cross section, 740°C, porosity~75%; (e) vertical setion; 0 cross section, 650°C, po-rosity~80%.SEM images of the inner surface of cells in differ-EDS analysis on the inner surface of the cells, andent aluminum foam samples are shown in Fig. 6. TheAl3Fe phase was also detected in the latter sample. Insample used in Fig. 6(a) was prepared from AlSi12 al-addition, compared with the latter sample, there areloy and TiH2 powder as foaming agent, whereas themuch more cavities (see arrows in Fig. 6(a)) in the in-sample used in Fig. 6(b) was manufactured fromner surface of the cells of the former sample. This maycommercial pure aluminum and the novel foamingbe another reason why the porosity of the former isagent. The presence of TiH, (in Fig. 6(a)) and thehigher than that of the latter.novel foaming agent (in Fig. 6(b)) were detected by(a)(bFoaming agent20LUFig. 6. SEM images of the inner surface of ce中国煤化工4. ConclusionsscopTYH:omposition above(1) Studies on thermal decomposition behavior of600CNMHG6s0°C)andamildthe novel foaming agent powder indicate that it hasdecomposition rate (the time of complete decomposi-the characteristic of a wide decomposition temperaturetion approaches to 30 min at 680°C). Those make itXY. Zhou et al, Novel foaming agent used in preparation process of aluminum foams739possible for the novel foaming agent to subtitute for47(2005), p.26.hydric-metal foaming agents such as TiH2 and ZrH27] Z.J. Wu, Y.M. Wang, and D.P. HResearch advances inthe foamed process of cellular aluminum, Foundry (induring the preparation process of aluminum foams.Chinese), 1999, No.4, p.1.(2) This foarming agent has the ability to enhance8] Z.H. Wang and P.M. Cben, Fabrication of foamed alumi-the viscosity of aluminum melt. It may be due to thenum by foaming Technique, Dev. Appl. Mater. (in Chi-formation of some fine particles by certain reactionsnese), 13(1998), No.3, p.30.9] G. Dai, D.P. He, and J.T. Shang, Change of viscosity inbetween the foaming agent and aluminum melt. As athe foamed process of Al alloy metal, Chin. J. Mater. Res.result, an extra viscosifier such as Ca or SiCp is un-(in Chinese), 19(2005), No.1, p.35.necessary during the aluminum foam preparation[10] V. Gergely and T.W. Clyne, The FORMGRIP process:process by using the novel foaming agent.foaming of reinforced metals by gas release in precursors,Adv. Eng. Mater, 2(2002), p.175.(3) The free-bubble zone in the foam decreases andthe foaming efficiency increases with an increase in[11] JX. Fang, K. Zhao, and C.Q. Gu, Preparation ofAl2O3-coated TiH2 foaming reagent by chemical metbod,the addition of the novel foaming agent, and when theChin. J. Nonferrous Met. (in Chinese), 12(2002), No.6,addition of foaming agent is more than 1 .4wt%, thep.1205.free-bubble zone disappears and the foaming effi-12] YH. Wang, J. Zhao, W. Song, et al, Effect of SiO, gelciency is near 100%; the porosity of aluminum foamcoating of TiH2 on is hydrogen liberation, J. Southeastincreases with the increase of foaming agent when theUniv. (in Chinese), 29199), No.6, p.145.addition of foaming agent is less than 2.2wt%.[13] C.C. Yang and H. Nakae, Foaming characteristic controlduring production of aluminum alloy foam, J. 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