Inﬂuence of vibration on the solidiﬁcation behaviour and tensile properties of an Al–18 wt%Si alloy G. Chirita a , I. Stefanescu b , D. Soares a , F.S. Silva a, * a Mechanical Engineering Department, School of Engineering, Minho University, Portugal b Faculty of Mechanical Engineering, Dunarea de Jos University Galati, Romania article info Article history: Received 22 May 2008 Accepted 22 July 2008 Available online 31 July 2008 Keywords: Vibration Acceleration Mechanisms abstract This paper is concerned with the inﬂuence of vibration on the mechanical properties of castings. The main vibration effects include promotion of nucleation and thus reducing as-cast grain size; reduction of shrinkage porosities due to improved metal feeding; and production of a more homogenous metal struc- ture. In this study, mechanical mould vibration was applied to an Al–Si hypereutectic alloy at ﬁxed ampli- tude and different frequencies. Tensile tests were done on specimens obtained with the different vibrating frequency levels. Experimental results show that mechanical properties were inﬂuenced by the level of applied frequency. The tensile strength was improved for low vibration frequencies but decreased for high frequencies, as compared with gravity castings without vibration. A microstructure analysis along with a solidiﬁcation behavior study was performed in order to understand the mechanism responsible for the previous behavior. A heat-transfer mechanism, that is acceleration dependent, seems to be responsible for the shift in mechanical properties response to the vibration effect. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Due to many advantages such as good thermal conductivity, excellent castability, high strength-to-weight ratio, wear and cor- rosion resistance, pressure tightness and good weldability, alumin- ium–silicon alloys are considered one of the most commonly used foundry alloys. Controlling the microstructure that results from the casting process is considered one of the main challenges faced by today’s foundry industry. Fine equiaxed microstructures generally exhibit favourable mechanical properties of strength and ductility with low susceptibility to microporosity and cracks. The use of mechanical, sonic or ultrasonic vibration may have the advantage of promoting grain reﬁnement, increased density, degassing, low shrinkage porosities, and changing the shape, size and distribution of the second phase [1–9]. Regarding the vibration effect in microstructure it is docu- mented [1] that applying mechanical vibration to a mould during solidiﬁcation may have an effect on the mechanical properties of the casting. The microstructure responsible is where the lamellar spacing tends to reduce and silicon morphology becomes ﬁbrous with the increasing of the vibration amplitude as compared to gravity casting. However, it is also reported that by exceeding a critical value of vibration amplitude, the silicon tends to coarsen [1]. Fragmented primary dendrites with thicker dendrite arm thickness and reduced solidiﬁcation time were obtained on Al– 8% Si with rectilinear vibration by transforming rotary motion of a DC motor, 100 cycles/min (2 Hz). The same level of vibration was applied to Al–12% Si and a reduction of the eutectic cell size from 5 to 1.6 mm and a tendency of coarsening of eutectic Si [2] was reported. Signiﬁcant reduction in gas content was obtained with low frequency melt agitation in Al–20Si [3]. With an applied vibration at a constant frequency of 100 Hz and different ampli- tudes from 18 to 199 lm, an increase between 19% and 68%, in per- cent elongation was reported while ultimate stress had a slight change, about 3% [1]. The increase in elongation was correlated with the increase in the amount of eutectic volume fraction com- pared to the non-vibrated case [1]. In another study [9], the amount and size of pores were increased in LM25 [Al–Si 7.15%] and LM6 [Al–Si 12.30%] alloys with increasing frequencies between 15 and 41.7 Hz and amplitudes between 0.125 and 0.5 mm. Thus, it is clear that vibration may promote changes in micro- structure and consequently in mechanical properties, either increasing or decreasing it. However the mechanisms under which those changes occur are still unclear. This work proposes a mech- anism that is able to explain the reason for the shift on metallurgi- cal and mechanical properties with vibration acceleration. 2. Experimental methods and materials 2.1. Materials The material used for castings is a commercial AlSi18 alloy with the following chemical composition (wt%): 18 6 Si 6 22; Fe  0.75; 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.07.045 * Corresponding author. Tel.: +351 253510254; fax: +351 253516007. E-mail address: fsamuel@dem.uminho.pt (F.S. Silva). Materials and Design 30 (2009) 1575–1580 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes