IMPACT OF COOLING TIME ON THE STRUCTURE AND TRIBOLOGICAL PROPERTIES OF METAL MATRIX COMPOSITE CASTINGS

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1 KOMISJA BUDOWY MASZYN PAN ODDZIAŁ W POZNANIU Vol. 28 nr 3 Archiwum Technologii Maszyn i Automatyzacji 2008 KATARZYNA GAWDZIŃSKA, JAKUB HAJKOWSKI, BARTOSZ GŁOWACKI IMPACT OF COOLING TIME ON THE STRUCTURE AND TRIBOLOGICAL PROPERTIES OF METAL MATRIX COMPOSITE CASTINGS The article presents the influence of cooling time on the structure of metal matrix composites (MMC) castings. The examination covered two types of composites manufactured by mechanical mixing. Composite reinforcement consisted of SiC particles, and the matrix consisted of AlSi9 alloy in the first case and AlSi11 in the second. Comparison covered samples cooled in sand moulds and gravity dies by presenting their cooling curves and their abrasive resistance. Key words: metal matrix composites, casting, cooling curves 1. INTRODUCTION In recent years, metal and non-metal composites have been rapidly developing as construction materials of various applications in different industries. One of major composite advantages is the possibility of obtaining desired usable properties. This involves, among others, the use of relevant fabrication methods. Composites with hard particles reinforcement (SiC, Al 2 O 3, and BC) are frequently used as elements comprising tribological pairs [4 5]. Methods used to test their wear enable assessing relative resilience of the materials. Tribological phenomena depend mainly on the status of surface layers at interfaces between interacting elements of machines [3 4]. For this reason, it is important to examine the impact of the composite structure, which comprises at least two different materials [9], on the wear and tear. The wear and tear of such elements involves a number of processes that accompany friction and apply to the surface layer and change in mass, surface geometry, and shape. The objective of the paper is to present findings of the research focusing on resistance to wear and Dr inż. Institute of Basic Technical Sciences, Maritime University of Szczecin. Mgr inż. Instytut Technologii Materiałów Politechniki Poznańskiej. Mgr inż. Institute of Basic Technical Sciences, Maritime University of Szczecin.

2 42 K. Gawdzińska, J. Hajkowski, B. Głowacki tear of composites which structures were determined by different cooling conditions (sand mould and gravity die). 2. METHODOLOGY The examination covered MMCs fabricated using mechanical mixing. It is an intermediary method also known as ex-situ [2, 3]. The method involves introducing reinforcement particles (or fibers) to liquid metal (alloy) and their mechanical spreading in the matrix as shown in Fig. 1 (suspension composites) [9]. The research covered two types of materials which differed as regards their matrix: composite of AlSi11 matrix and 15% SiC particle reinforcement. The material was fabricated in the Chair of Metal Alloy and Composites Technologies at the Silesian University of Technology in Katowice, Poland, composite of AlSi9 matrix and 15% SiC particle reinforcement fabricated by ALCAN Canada. particle gas GAZ liquid h = 30 mm a = 100 mm b = 50 mm Fig. 1. Fabrication of composites using mechanical mixing [3] Rys. 1. Wytwarzanie kompozytów metodą mechanicznego mieszania [3] Fig. 2. Dimensions of trial castings Rys. 2. Wymiary próbki użytej do badań The composite input was prepared by cutting pig sows. It was then melted in a graphite crucible, moulds were filled and temperature changes registered during liquid metal cooling, solidification and cooling of casting. Trial castings were made of each of the materials. Their shape and dimensions are presented in Fig. 2.

3 Impact of cooling time on the structure and tribological properties COOLING CURVES Solidification conditions were based on solidification time [7 8], determined according to cooling curves using a set of K type thermocouples and Eurotherm 5100 V. The hot end of the thermocouple was placed in the geometric center of perpendicular samples. Cooling curves and their first derivatives for the sand mould and gravity die are shown in Fig. 3a for the AlSi9 + 15% SiC alloy and in Fig. 3b for AlSi % SiC alloy. Solidification times for trial castings are shown in Table 1. Temperature, C A A dt k /dt B p B k T p T p B C a) dt p /dt -0,2-0,4-0,6-0,8-1, ,4 D k D p 500-1, Time, s Sand mould Gravity die C 0,2 0-1 dt/dt dt k /dt b) dt p /dt 0,2 0 Temperature, C 600-0,2 580 A A T p -0,4 560 B p C B C -0, ,8 D k D p 520 T k , Time, s Sand mould Gravity die dt/dt Fig. 3. Cooling curves and theirs first derivatives: a) AlSi9 + 15% SiC, b) AlSi % SiC Rys. 3. Krzywe stygnięcia i ich pierwsze pochodne: a) AlSi9 + 15% SiC, b) AlSi % SiC

4 44 K. Gawdzińska, J. Hajkowski, B. Głowacki Solidification time Czasy krzepnięcia Table 1 Alloy AlSi9+15%SiC AlSi11+15%SiC Mould/die Solidification time [s] Sand mould 370 Gravity die 205 Sand mould 640 Gravity die 225 t p /t k In the case of AlSi9+15%SiC composite casting, α phase nucleation in sand mould and gravity die did not show visible recalescence of temperature on the cooling curve (A p A k Fig. 3a). The α phase nucleation in the AlSi11+15%SiC casting involved recalescence of temperature in both forms (A p, A k Fig. 3b). The α + Si eutectic nucleation occurred at about 567 C in the casting which solidified in a sand mould without temperature recalescence (B p Fig. 3a), and in a gravity die with temperature recalescence (B k Fig. 3a). Crystallizing eutectic α+si surrounded SiC particles (Fig. 4a, magnification 500). Triple eutectic α+mg 2 Si+Si in the shape of Chinese writing crystallized at C (Fig. 4b, magnification 500). The ratio of solidification time for composites in the sand mould and in the gravity die is 1.80 and 2.84 respectively for AlSi9+15%SiC and AlSi11+15%SiC. a) b) Fig. 4. Composite structure: a) SiC particles surrounded by α+si eutectic, b) triple eutectic α+mg 2 Si+Si Rys. 4. Struktura kompozytu: a) cząstki SiC otoczone eutektyką α + Si, b) eutektyka potrójna α + Mg 2 Si + Si SiC particles are unevenly distributed in the structure of castings examined. The uneven distribution is higher in castings which solidified in sand moulds (Fig. 5).

5 Impact of cooling time on the structure and tribological properties α SiC α SiC α+si α+si a) b) α α SiC SiC α+si c) d) Fig. 5. SiC distribution in microstructure of castings: a) AlSi9+15%SiC sand mould, b) AlSi11+15%SiC sand mould, c) AlSi9+15%SiC gravity die, d) AlSi11+15%SiC gravity die Rys. 5. Rozłożenie SiC w mikrostrukturze odlewów: a) kompozyt AlSi9+15%SiC krzepnący w formie piaskowej, b) kompozyt AlSi11+15%SiC krzepnący w formie piaskowej, c) kompozyt AlSi9+15%SiC krzepnący w kokili, d) kompozyt AlSi11+15%SiC krzepnący w kokili Systematic scanning was used to determine homogeneity of dispersed phase distribution in the composite. According to the method, the binary picture of a microstructure of the material examined is divided into identical interosculant square fields [3, 6]. The surface content of analyzed AAij phase is measured for each field and results are used to calculate the following: average value of surface content: AA = AAij i j (1) n2 (n2 number of measurement frame uses), standard deviation: ( AAij AA i j s ( AA ) = n2 1 ) 0,5, (2)

6 46 K. Gawdzińska, J. Hajkowski, B. Głowacki particle distribution coefficient: s( AA) v ( AA) = 100. (3) A Results have been shown in Table 2. A Table 2 Quantitative analysis of particle distribution coefficient for SiC in AlSi9 and AlSi11 matrix Wyniki ilościowej analizy jednorodności rozmieszczenia cząstek SiC w osnowie AlSi9 i AlSi11 kompozytu Alloy AlSi9 +15%SiC AlSi11 +15%SiC Mould/die Average SiC surface share A A (equation 1) Standard deviation s(a A ) (equation 2) Particle distribution coefficient v(a A ) [%] (equation 3) Sand mould Gravity die Sand mould Gravity die ABRASIVE RESISTANCE OF METAL COMPOSITES Abrasive tests on composites have been performed in the Chair of Ship Material Engineering, Institute of Basic Sciences at the Maritime University in Szczecin, Poland. The tests used a weight method described in greater detail in [1, 4]. Tests involved rectangular prism samples of mm and counter samples made of grey ductile ferritic-pearlitic cast iron of mm. Weight measurements were performed every 1800 s, whereas the total test time was s. Each time three samples made of the same material were tested. Average results are presented in Fig. 6 (figures show the loss of weight in time in materials tested excluding the preliminary phase of placing a mould). 5. CONCLUSIONS It has been established that: different mass loss appears between materials cooled in a sand mould and in a gravity die (Fig. 6a and 6b); in both cases (composites based on AlSi9 and AlSi11 matrixes), mass loss was larger in the case of composites cooled in a sand mould; mass loss during analysis was higher in the case of a AlSi9 composite.

7 Impact of cooling time on the structure and tribological properties ,1800 0,1600 weight loss [g] 0,1400 0,1200 0,1000 0,0800 0,0600 0,0400 0,0200 0, Time [s 10 2 ] 1Time [s x 10] 2 a) weight loss [g] 0,1800 0,1600 0,1400 0,1200 0,1000 0,0800 0,0600 0,0400 0,0200 0, Time [s 10 2 ] 1 2 b) Fig. 6. Total weight loss in materials tested against time: a) AlSi9+15%SiC (1 sand mould, 2 gravity die), b) AlSi11+15%SiC (1 sand mould, 2 gravity die) Rys. 6. Suma ubytków masy badanych materiałów w zależności od czasu: a) AlSi9+15%SiC (1 forma piaskowa, 2 kokila), b) AlSi11+15%SiC (1 forma piaskowa, 2 kokila) This resulted from a lower content of silica in the matrix and uneven distribution of SiC particles in the cast structure. The latter is confirmed by the particle distribution coefficient (Table 2). REFERENCES [1] Biało D., Rola przeciwpróbek w procesie zużywania węzłów tarcia z kompozytami aluminiowymi, Kompozyty, 2006, nr 1, s [2] Gawdzińska B., Grabian J., Dolata-Grosz A., Selected usable properties of Al11/1H18N9T suspension composite, Archiwum Odlewnictwa, 2006, vol. 6, nr 22, p

8 48 K. Gawdzińska, J. Hajkowski, B. Głowacki [3] Gawdzińska K., Structure Defects Classification of Casts from Saturated Metal Composites, Doctor Thesis, Technical University of Szczecin [4] Grabian J., Gawdzińska K., Głowacki B., Właściwości tribologiczne odlewanych kompozytów metalowych o zróżnicowanej budowie zbrojenia, Archiwum Technologii Maszyn i Automatyzacji, 2004, vol. 24, nr 1, p [5] Ottmuller M., Korner C., Singer R.F., Influence of the fiber-matrix interface on the strength of unidirectional carbon fiber reinforced magnesium composites, w: Metal-Matrix Composites and Metallic Foams, 1999, Euromat 99, Vol. 5. [6] Piątkowski J., The analysis of solidification and microstructure of Al-Si alloys, Archiwum Odlewnictwa, 2006, vol. 6, nr 18(1/2), p [7] Pietrowski S., Characteristic features of silumin alloys crystallization, Materials & Design, 1997, vol. 18, nos. 4/6, p [8] Sakwa J., Zur kristallisation der aluminiumguslegierung AlSi7Mg, Giessereiforschung, 1988, 4, 134. [9] Wong S., Neussl E., Fettweis D., Sahm O.R., Flower H.M.L., High strength Al-Zn-Mg matrix-alloy for continuous fibre reinforcement, in: Metal matrix composites and metallic foams, 2000, Euromat 2000, Vol. 5, Wiley 2000, p Praca wpłynęła do Redakcji Recenzent: prof. dr hab. inż. Ferdynand Romankiewicz WPŁYW SZYBKOŚCI CHŁODZENIA NA STRUKTURĘ I WŁAŚCIWOŚCI TRIBOLOGICZNE METALOWYCH ODLEWÓW KOMPOZYTOWYCH S t r e s z c z e n i e W prezentowanym artykule przedstawiono wpływ szybkości chłodzenia na strukturę odlewów z metalowych materiałów kompozytowych. Badano dwa rodzaje kompozytów wytworzonych metodą mechanicznego mieszania. Zbrojenie kompozytu stanowiły cząstki SiC, osnowę w pierwszym przypadku stop AlSi9, w drugim AlSi11. Porównano materiały chłodzone w formie piaskowej i kokili i przedstawiono ich krzywe krzepnięcia oraz odporność na ścieranie. Słowa kluczowe: metalowe materiały kompozytowe, odlewy, krzywe krzepnięcia