Foaming characteristics of Al–Si–Mg (LM25) alloy prepared by liquid metal processing R. Nadella*, S. N. Sahu and A. A. Gokhale LM25 (Al–Si–Mg) foams were made by melt processing route using Al foam scrap turnings as thickening agent in place of aluminium powder or Ca metal. The effects of foaming temperature and excess Mg content on melt expansion and cell structure were studied. The selection of foaming temperature significantly influenced foam decay. While severe collapse occurred at 670uC, foam decay is hardly observed at 640uC. Moreover, presence of excess Mg (3 wt-%) in this alloy enhanced collapse at 670uC, presumably due to its surface tension lowering effect. Despite foam collapse, a well defined and uniform cell size could be obtained at 670uC as compared to the foams obtained at 640uC. Uniaxial compression tests showed serrated plateau stress–strain behaviour with extensive localised but non-catastrophic damage leading to progressive breaking away of foam pieces. Overall, these experiments showed the potential to manufacture good quality LM25 foam using inexpensive Al foam turnings. Keywords: Al–Si alloy, Al foam, Foam collapse, Compression Introduction Aluminium foams are ultra light weight materials, which have a high potential for structural and functional applications involving sound and high energy absorp- tion. Among various manufacturing techniques, liquid metal processing, involving the addition of foaming agent to the thickened aluminium melt, has gained an attention due to its simplicity and low cost. Although large amount of literature is available on Al foams, very limited work 1–3 exists on foams of one of the most prominent cast alloys, LM25 (A356). Yang and Nakae have carried out foaming experiments by varying TiH 2 content (0?5–2?5%), and foaming temperature (620– 660uC) on A356 alloy with aluminium powder (5 wt-% and 60 mm size) as the thickening agent. 1 They suggested an optimum combination of temperatures (640–650uC) and TiH 2 levels (1–1?5%) for good foamability. A uniform cell structure with high void content (86%) could be obtained with 1% TiH 2 foamed at 640uC. They have also reported that rise in viscosity (by varying the stirring time) resulted in generation of more particles which aided in obtaining higher porosity levels in foams. 2 Yang et al. 3 showed that in an Al–6?7Si–2?8Cu alloy with 2 wt-%Ca and 1 wt-%TiH 2 foams with different pore structures could be produced by varying the stirring time at a foaming temperature of 667uC (940 K). In spite of its potential cost savings, no attempts to utilise foam scrap for Al foam production were reported. Also, the role of surface tension reducing elements on the quality of Al foams has not been investigated. The aim of the present work is twofold. First, to check the feasibility of preparing Al alloy foam with Al–Ca foam scrap turnings as the source of oxides. In this way, in addition to cost reduction, major presence of Ca, which is not an alloying element in aluminium alloys, can be avoided. The other aim is to compare the foam quality with and without Mg, which oxidises readily and is a surface tension reducing element in Al. The work also tries to examine the interplay between the Mg content and the foaming temperature. Materials and experimental method The materials used for the foam synthesis are LM25 alloy ingots (Al–6?9Si–0?28Mg–0?12Fe), Al foam turn- ings (Al–2Ca–0?72Ti) and TiH 2 powder (all elemental compositions are described in wt-%). The foaming experiments were carried out in a cylindrical clay graphite crucible (inner diameter about 90 and 410 mm height) with 1?25 kg of alloy melt, following the general principles of Alporas technique described elsewhere. 4 Instead of Ca, pure aluminium foam turn- ings of size range from 75 to 105 mm (4 wt-%) were added to the melt for thickening at 680uC and stirring was carried out at 1200 rev min 21 for 10–13 min. Upon cooling to foaming temperature, 1?25 wt-%TiH 2 was added to liquid metal and the temperature was main- tained in order to facilitate the blowing agent (TiH 2 ) decomposition and foaming process. Once the foaming activity ceased, which was confirmed visually, the crucible was removed from the furnace and allowed to cool in air for solidification of the foam. The holding time at the foaming temperature varied between 3 and 7 min. Keeping all other experimental conditions Light Alloy Casting Group, Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500 058, India *Corresponding author, email nvravi_in@yahoo.co.uk 908 ß 2010 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 27 February 2009; accepted 17 March 2009 DOI 10.1179/174328409X430500 Materials Science and Technology 2010 VOL 26 NO 8