Compound casting is used as a process to join various similar and dissimilar metallic couples. The ratio of melt-to-solid volume is one of the main factors that can affect the contact time between melt and the solid insert. In this investigation, magnesium and aluminum metals (magnesium as the cast metal and aluminum as the solid insert) having melt-to-solid volume ratios (V m /V s ) of 1.25, 3, and 5.25 were successfully bonded via compound casting. Results demonstrated that by increasing the ratio of V m /V s from 1.25 to 5.25, the thickness of the reaction interface between Al and Mg varies within the range of 200 to 1800 lm. X-ray diffraction, scanning electron microscopy, and Vickers microhardness study of the bonding of these two metals showed that the interface consisted of three separate sub-layers within reaction layer. These sublayers had higher hardness than those of the Al and Mg bulk metals. In all specimens, composition of the sub-layer adjacent to Al (layer I) was Al 3 Mg 2 and that adjacent to Mg (layer III) was Al 12 Mg 17 /(Mg) eutectic structure. The intermediate layer composition (layer II) in specimens with volume ratio of 1.25 and 3 was a single-phase Al 12 Mg 17 , while for the case of volume ratio 5.25 this sub-layer consisted of Al 12 Mg 17 / (Mg) eutectic dispersed in Al 12 Mg 17 intermetallic. The results of this research showed that in low melt/solid volume ratios, diffusion-reaction was the dominant mechanism for formation of Al-Mg intermetallic. However, when V m /V s and the melt/solid insert contact time increased, the dominant mechanism of Al-Mg intermetallics changed to fusion-solidification due to increase in surface melting of the solid insert. Also the results of push-out tests showed that shear strengths of the interface decrease from 27.1 to 15.1 and 8.3 MPa for the Al/Mg couples prepared at 1.25, 3, and 5.25 V m /V s respectively.
