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Journal of Materials Processing Technology 155156 (2004) 18391846 Lightweight, hollow-sphere-composite (HSC) materials for mechanical engineering applications E. Baumeistera, S. Klaegera, A. Kaldosb a Institute for Manufacturing Technology and Quality Management, Otto-von-Guericke-University, Magdeburg, Germany b School of Engineering, Liverpool John Moores University, Liverpool, UK Abstract Lightweight structure is a new trend in machine tool design to ensure higher speed and higher acceleration of elements. The drive and control systems in mechanical engineering requires lightweight design provided by the recently developed light materials thus resulting in economical advantages. The hollow-sphere-composites (HSCs) consist of hollow spheres up to 80 of the volume and a reactive resin systemasbinder.TherecentlydevelopedHSCmaterials,thehollowspherebodies,aremadefromceramics,silicates,plasticsormetalsand providearangeofstructuralmaterialsofdifferentchemicalcomposition,grainsizedistribution,density,bulkdensity,softeningtemperature and compression. Therefore, a vast palette of HSC-variants can be obtained with different properties for a variety of applications. The mechanical properties of HSC materials depend on the properties of the spherical hollow bodies. The mechanical and thermal behaviour of HSC materials can be characterised by using dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC) and thermomechanical analysis (TMA). The thermal and mechanical properties of selected HSC structures, e.g. machine tool components, robot arms, demonstrate the fl exibility and application feasibility of this new material. 2004 Elsevier B.V. All rights reserved. Keywords: Lightweight materials; Hollow-sphere-composites (HSCs); Mechanical properties; Mechanical engineering 1. Introduction In mechanical engineering, including automotive and air- craft manufacture, the same lightweight building principles are used to meet various and often complex demands in shape-, structure-, material coupled with the need for opti- misedproductionprocessselectionfortechnologyneedsand fi nancial considerations. The optimised design of machine tools using fi nite element methods may lead to substantial improvements in the acceleration or damping behaviours. The application of new, alternative materials in machine tool design provides dramatic improvements in mass reduc- tion through the full utilisation of material, high strength and stiffness as well as maximum functional integrity and economy 1. The requirements for the lightweight machine structures are characterised by the optimal use of material quantity. These demands can rarely be satisfi ed with mono- lithic structures. As a result, the application of cellular ma- terials, e.g. honeycomb, metal foams or syntactic foams will soon gain signifi cance. A combination of metals and fi brous Corresponding author. E-mail addresses: erika.baumeistermasch-bau.uni-magdeburg.de (E. Baumeister), a.kaldoslivjm.ac.uk (A. Kaldos). materials can be used adaptively to different conditions, sim- ilar to natural structures, like the hand bones as shown in Fig. 1. This is a foam structure connected with the sup- porting system, where muscles and sinews are utilised for movements. 2. Hollow-sphere-composites An alternative method in reducing the mass of materials is to use a mixture of high percentage volume of hollow spheres containing air or gas, and a reactive resin system 2. In this research hollow-sphere-composites consisting of corundum based (0.51mm) macro-hollow-spheres and aluminium-silicateFillite(5300?m)micro-hollow-spheres are used as shown in Fig. 2 3. In the recent research programme 12 different types of hollow spheres were used in combination with cold and warm hardener epoxy resin (EP) and with and without fi bre reinforcement, resulting in excess of 20 HSC-variants with different properties. The hollow spheres vary in diameter between 10 and 2000?m and the wall thickness is only 10% ofthediametersize.Theroundshapeofthespheresprovides a high package density and a minimal viscous drag. 0924-0136/$ see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.04.385 1840E. Baumeister et al./Journal of Materials Processing Technology 155156 (2004) 18391846 Fig. 1. Cellular structure of human hand. 3. Properties of hollow-sphere-composites In order to establish the application areas of HSC in me- chanical engineering, it is extremely important to charac- terise the thermal and mechanical behaviour of the material and to determine the characteristic values, which are neces- sary for the FE-calculations of the machine elements. Due to lackofstandardsonHSCmaterials,thethermalandmechan- ical tests must be governed by the appropriate standards for polymer concrete and plastic materials. The German Stan- dards DIN 51290 prescribe that the minimum dimensions of Fig. 2. (a) Bulk material of corundum 0.51mm; (b) interior of Fillite (SEM); (c) hollow-sphere-composite (corundum and Fillite); (d) interior of hollow-sphere-composite (SEM). the sample shouldnt be smaller than three times the maxi- mum grain size of the used fi ller 4. The result is that the preferred sample geometry based on plastic standards must be modifi ed to apply to HSC. 3.1. Thermal properties Investigations were carried out to obtain the thermal be- haviour and hardening of epoxy resins 5 and HSC using differential scanning calorimeters (DSC 200). The obtained typical temperatures are: glass transition temperature (Tg), cured temperature (Tcure), temperature at the beginning of thermal degradation (Tox). The obtained temperatures and their effects on residual reaction heat of the remaining re- actants (?Hr) are shown in Fig. 3. It can be stated that the thermal behaviour of HSC is mainly governed by the epoxy resin used. The linear thermal expansion coeffi cient for Tg(1) and linear thermal expansion coeffi cient over Tg(2) can be measured using thermomechanical analysis (TMA). Fig. 4 demonstrates, that with increased percentage volume of fi llers from 65% (Sample 2) to 78% (Sample 3) the 1- and 2-values will be smaller, which is attributable to the smaller thermal expansion values of the 78% HSC material used in the research. In order to minimise the thermal distortion of machine tool elements it is important to know the 1-values. Table 1 includes the 1-, Tgand 2-values for some HSC variants. E. Baumeister et al./Journal of Materials Processing Technology 155156 (2004) 183918461841 Fig. 3. DSC-scan of 11.1mg epoxy resin Ebalta (1) and 12.3mg HSC consist of corundum and Fillite (2) with heating rate of 20K/min in air. Fig. 4. TMA-curves of epoxy resin and HSC-variants with different resin volume fractions. These values depend on the base materials used and can be determined from the following equations 6: = ? vii(1) where viis the volumetric percentage, ithe thermal expan- sion coeffi cient. The viand ivalues of the components are normally available, but in this case the thermal expansion coeffi cient of the fi llers and the infl uences of the encapsulated gas in HSC on 1are unknown. However, the -value of the corundum (?-Al2O3) is 9.5 106K1according to 7. Table 1 1-Values of epoxy resin and HSC SampleComposition1(106K1)Tg(C)2(106K1)Curing time (days) 1Only epoxy resin Ebalta70.960105.330 265vol.% Fillite + corundum33.151.564.128 378vol.% Fillite + corundum22.352.451.930 478vol.% Fillite34.562.649.121 578vol.% corundum 02mm23.451.330.819 The calculated -value of epoxy resin is 70 106K1. The calculated 1-value of Sample 5 is 22.8 106K1, which agrees well with the experimentally obtained value of 23.4106K1. The dynamic mechanical analysis (DMA) investigationsofthree-point-bending-samplesofepoxyresin (a) and of HSC-Sample 3 (b) are shown in Fig. 5. At higher frequencies the Tgmoves to higher temperature values 8 andduetothesensitivityoftheDMA-methodstwoTgpoints are found for the semi-cured samples. At the start of the Tg area the microbrown movements takes place followed by an entropy elastic state, where the dependence of the elastic 1842E. Baumeister et al./Journal of Materials Processing Technology 155156 (2004) 18391846 Fig. 5. Elastic bending modulus (E?), loss modulus (E?) and log decrement (D) of epoxy resin (Sample 1) (a) and HSC (Sample 3) of (b). modulus on the temperature is less signifi cant. It is notable that the fi llers improve the stiffness (E?) of Sample 3 (HSC) in comparison to Sample 1 (epoxy resin). 3.2. Mechanical properties The elasticity modulus (E) of epoxy resin Ebalta 120/TL (EP) and HSC were obtained from mechanical tests and are shown in Table 2. The mechanical properties of epoxy resin andHSC-samplesareshowninTable2,alongwithsteel(St), glass fi bre (GF) and carbon fi bre (CF) materials for purpose of comparison. The density () of materials indicates that HSC are lightweight materials. The ratio of stiffness (E) to density is an important parameter for material selection. To compare the compression strength of two bars of equal dimension but different materials the equation is simplifi ed to 3 E/g 9. It is clear from the table that HSC-Samples 24 have higher compression modulus than either steel or glass fi bre 10. If GF or CF is manufactured as laminate, then its mechanical properties becomes much smaller. A clear disadvantage of CF is its anisotropy, whereas HSC is isotropic in all directions. E. Baumeister et al./Journal of Materials Processing Technology 155156 (2004) 183918461843 Table 2 Density and Youngs modulus (E) of epoxy resin and HSC in comparison to steel (St), glass fi bre (GF) or carbon fi bre (CF) ValueEP, Sample 1Hollow-sphere-compositesStGFCF Sample 2Sample 3Sample 4Sample 5 (g/cm3)1.150.950.90.61.78 E (GPa)4.18.721073235 3 E/g( 3 MPacm3/g) 13.221.42124.618.77.61634.5 Fig. 6 shows the tensile strength (t ) and specifi c strength of epoxy resin and HSC-Samples 25. The tensile test speci- menwas250mminlength,10mminthicknessand25mmin width.Thetensilestrengthtestswerecarriedoutwithaspeed of 5mm/min according to DIN EN ISO 527-3. The specifi c strength of Sample 3 (Fillite and corundum 0.51mm) and Sample 4 (Fillite) are higher than that of epoxy resin. The result is, than using the same mass of material, a higher vol- ume of component can be made when using Samples 24, and it withstands the same tensile strength as a component made from Sample 1. Compression tests were conducted with test pieces hav- ing a length of 100mm, a thickness of 30mm and a width of 30mm. The speed of compression tests was 1mm/min. The compressive stressstrain curves of selected HSC-variants are presented in Fig. 7. The symbols of circle, square, etc. mark the mean values of the compressive strength (c) and the corresponding mean values of compression-strain of Samples 510 of each variant. The c-values in Fig. 7 are greater than that of tin Fig. 6 because in compression tests the pores will be closed and they stop the propagation of the cracks.Samples4and5inFig.7showthattwotypicalstages occur during deformation in the course of compression test of cellular solids such as polymer foams or metal foams 11,12. Following an almost linear-elastic behaviour at low strains the curve shows a long plateau with almost constant load,butincomparisontotheanothercellularsolidstheHSC material is superior in withstanding compression. Sample 4 fi lled with the smaller fi ller type Fillite behaves better under Fig. 6. Tensile strength and specifi c strength of EP (Sample 1) and HSC (Samples 25). compression than the fi lled with corundum, because Sample 5 has a higher porosity. Samples 2 and 3 have high packing density thus providing higher compressive strength values. The increase in the volumetric percentage of resin in Sam- ple 2 improves the c-values 13. The smaller the size of the spheres the more marked the plateau areas are, as in this case the crack propagation can be rapidly stopped by imped- iments (spheres or pores). This explains why the samples fi lled with smaller particles cracks appear to be diagonal, while samples fi lled with greater fi llers develop transversal cracking develops. It has to be noted that adhesion bonds between fi llers and binders are of paramount importance. If the stiffness of the spheres is higher than the stiffness of the resin then cracking starts in the resin and vice-versa. The damage propagation can be explained using the scan- ning electron micrograph (SEM) images of the fracture sur- faces of Samples 3, 4 and 5 in Fig. 8. The Fillite spheres of Sample 4 in Fig. 8a are broken. Due to the different wall thickness of the ceramic hollow spheres of Sample 5 in Fig. 8b, the spheres are broken at different levels. The space between the greater corundum spheres of Sample 5 are greater than the space between the smaller Fillite spheres of Sample 5. A better packing density of the fi llers is shown in Fig. 8c, where Sample 3 is fi lled with different grain size of spheres of known volumetric percentage fraction, thus causing to improve mechanical properties of Sample 3 in comparison to Samples 4 or 5. The bending stress in Fig. 9 was determined using three-point-bending samples with following dimensions: 1844E. Baumeister et al./Journal of Materials Processing Technology 155156 (2004) 18391846 Fig. 7. Typical compressive stressstrain curves of HSC variants and test samples after compression test. Fig. 8. SEM images of fracture surfaces among bending HSC-samples. 240mm length, 20mm width and 12mm height, according to the DIN EN ISO 178, with a proof-speed of 4.8mm/min. The bending strength values of HSC are smaller than that of epoxy resin. Some HSC variants at the opposite side of the applied force were reinforced with carbon or glass fi bre to improve tensile properties. Sample 3 is a mixture of ceramic and aluminium sili- cate hollow spheres and presents better mechanical proper- ties than Samples 4 or 5, which were fi lled with a single fi ller type. The thermal expansion coeffi cient of Sample 3 is smaller in comparison to Sample 2 or 4. Sample 3 was se- Fig. 9. Bending strength values for epoxy resin, HSC with and without carbon fi bre or glass fi bre. lected as construction material for machine tool components and other engineering parts. 4. Application of HSC in mechanical engineering On the research programme a number of machine ele- ments, such as jigs of milling tables and robot arms for SCARA Adept robots were developed. These components were successfully tested and the application of HSC materi- als in mechanical engineering was demonstrated. The fi nite E. Baumeister et al./Journal of Materials Processing Technology 155156 (2004) 183918461845 Fig. 10. Finite element models and robot arms made from aluminium alloy (a) and HSC (b). Fig. 11. Table of a milling machine made from HSC, steel plate and carbon laminates. element program COSAR provided indications for the need of design changes regarding the direction of carbon fi bre re- inforcements and the aluminium connection elements. The models in Fig. 10 were loaded with 1000MPa bending force and the developed stresses remained below acceptable limit 14. Based on the results obtained, two robot arms were made from HSC reinforced with carbon fi bre or aluminium alloys bars. These robot arms were 10 and 25% lighter in weight than as the original aluminium alloy arms. A milling machine table was successfully developed from HSC to replace a steel table. The developed HSC table was designed with reinforcing steel elements and carbon fi - bre laminates to withstand the typical tensile strengths. The achieved mass reduction is between 30 and 80%, thus en- hancing dynamic characteristics. The damping properties of the HSC table are superior to that of cast iron table, which is partly attributed to the ply structure as shown in Fig. 11 15. 5. Conclusion ItcanbestatedthatHSCmaterialscombinedwithmetalor fi bre reinforcements promise a successful alternative to light metals or metal foams. In this research a number of machine building parts with good dimensional accuracy have been produced and tested with good results. The spherical form of the hollow materials provided a considerably smoother surface than that of fi brous or irregular fi llers and the resin consumption was signifi cantly reduced. The application of HSC materials is advantageous for the user because of the low material and production costs. The excellent vibration and damping properties coupled with very low heat con- ductivity and resultant heat distortion predestines the HSC materials to be used successfully in a variety of engineering areas. The chemical resistance and the ease of recycling are further advantages of this material by changing the compo- sition of the matrix material and the volume. Acknowledgements The authors wish to thank the Ministry of Culture, Land Sachsen-Anhalt in Germany. Many thanks go to the col- leagues in the Institute for Manufacturing and Quality Man- agement and the Institute of Material and Material testing at the Otto-von-Guericke-University, Magdeburg. The fi nan- 1846E. Baumeister et al./Journal of Materials Processing Technology 155156 (2004) 18391846 cial suppo