This paper discusses the joining of AZ91 magnesium alloy with AlSi17 aluminium alloy by compound casting. Molten AZ91 was cast at
650oC onto a solid AlSi17 insert placed in a steel mould under normal atmospheric conditions. Before casting, the mould with the insert
inside was heated up to about 370oC. The bonding zone forming between the two alloys because of diffusion had a multiphase structure
and a thickness of about 200 µm. The microstructure and composition of the bonding zone were analysed using optical microscopy,
scanning electron microscopy and energy dispersive X-ray spectroscopy. The results indicate that the bonding zone adjacent to the AlSi17
alloy was composed of an Al3Mg2 intermetallic phase with not fully consumed primary Si particles, surrounded by a rim of an Mg2Si
intermetallic phase and fine Mg2Si particles. The bonding zone near the AZ91 alloy was composed of a eutectic (an Mg17Al12 intermetallic
phase and a solid solution of Al and Si in Mg). It was also found that the compound casting process slightly affected the AZ91alloy
microstructure; a thin layer adjacent to the bonding zone of the alloy was enriched with aluminium.
Al-enriched layer was formed on a magnesium substrate with use of casting. The magnesium melt was cast into a steel mould with an
aluminium insert placed inside. Different conditions of the casting process were applied. The reaction between the molten magnesium and
the aluminium piece during casting led to the formation of an Al-enriched surface layer on the magnesium substrate. The thickness of the
layer was dependent on the casting conditions. In all fabricated layers the following phases were detected: a solid solution of Mg in Al,
Al3Mg2, Mg17Al12 and a solid solution of Mg in Al. When the temperature of the melt and the mould was lower (variant 1 – 670o
C and 310 o
; variant 2 – 680o
C and 310o
C, respectively) the unreacted thin layer of aluminium was observed in the outer zone. Applying higher
temperatures of the melt (685o
C) and the mould (325o
C) resulted in deep penetration of aluminium into the magnesium substrate. Areas
enriched in aluminium were locally observed. The Al-enriched layers composed mainly of Mg-Al intermetallic phases have hardness from
187-256 HV0.1.
Plates of AZ91 cast magnesium alloy with a thickness of 3.5 mm were butt-welded using a laser power of 2000 W and helium as the shielding gas. The effect of the welding speed on the weld cross-sectional geometry and porosity was determined by microscopic analysis. It was found that to avoid the formation of macropores, welding should be carried out at a speed of 3.4 m/min or higher. Non-equilibrium solidification of the laser-melted metal causes fragmentation of the weld microstructure. Joints that were welded at optimal laser processing parameters were subjected to structural observations using optical and scanning microscopy and to mechanical tests. The mechanical properties were determined through Vickers hardness measurements in the joint cross-section and through tensile testing. The results indicate that the hardness in the fusion zone was about 20 HV (30%) higher than that of the base material. The weld proved to be a mechanically stable part of the joint; all the tensile-tested specimens fractured outside the fusion zone.