Plates of 3-5 mm in thickness were extracted from an AZ91D ingot and then butt joints of the plates were produced using tungsten inert gas (TIG) welding method. The TIG arc was also used to deposit welding beads on some of the thin plates. No cracking was found in the butt joints. However, hot cracking was always observed to propagate from the heat affected zone (HAZ) under the welding bead into the weld metal right after a welding bead was deposited on the thin plate. Metallographic and fractographic evidence was obtained to show that the hot cracking is 'liquation cracking' in the partially melted HAZ under the high thermal stresses. In the butt joints, the weld metal has the finest grains, highest strength and best ductility, and the HAZ was found to be the 'weakest link'.

Mg alloys are ultralight but their structural applications are often limited by their poor wear and corrosion resistance. The research aimed to address the problem by laser-cladding. Cladding with SiC powder onto surface of AZ91D was carried out using Nd :YAG laser. The laser-clad surface was analyzed using the optical microscope, SEM equipped with EDS, and XRD and found to contain SiC and other Si compounds such as Mg 2 Si and Al 3.21 Si 0.47 as well as much refined α- Mg grains and β- Mg 17 Al 12 intermetallics. The laser-clad surface possesses considerably higher hardness but its corrosion resistance is not improved, indicating that the laser-cladding technique can only be adopted for applications in noncorrosive environments where wear is the predominant problem.

Considerable research has been carried out to study the laser welding of magnesium alloys. However, the studies are mainly devoted to butt welding, and there has been limited information in the published literature concerning the bead-on-plate laser welding of AZ91D, even though bead-on-plate welding is required for the repair of cast AZ91D parts with surface defects. In the present investigation, surface cracking of the weld metal was observed when an AZ91D magnesium alloy was bead-on-plate welded using the laser welding method. This paper presents the experimental results and analyses to show that the cracking is “solidification cracking” initiated from the weld surface under high thermal stresses. This is in contrast to the “liquation cracking” observed in heat affected zones in tungsten inert gas welding of the same magnesium alloy. Laser power was found to be one of the main factors affecting the distance of the crack propagation. The higher laser power resulted in longer crack propagation distance into the weld metal. It is demonstrated that hot cracking could be avoided by lowering the laser power and welding speed.

Failure is always a potential problem to engineers whenever materials are used. Much effort has been directed towards the development of satisfactory fracture criteria to enable structural engineers to design confidently against such possible failures. However, so far understanding of failures in many materials is quite poor, although considerable knowledge has been obtained on failure mechanisms of steels. The project, therefore, aimed to gain better understanding of deformation and fracture processes in those engineering materials whose fracture mechanisms are poorly understood. Two ultralight magnesium alloys AZ91D and AMSOA were chosen for the study due to their importance to engineering applications. In this project, a technique was developed to combine the capabilities of optical microscope or SEM and mechanical testing device. A microtensile tester was installed under the optical microscope or inside an SEM to make it possible to carry out in-situ observation of deformation and fkacture processes when the tensile test is going on. The testing results for notched specimens of AZ91D magnesium alloy with different notch angles show that the smaller the notch angle, the higher the fracture stress level. This is quite unexpected, but the phenomenon observed can be explained in terms of sampling process. Small notch angle leads to localized plastic deformation zone and therefore makes it less likely to "sample" large defects in the magnesium alloy. The in-situ observation revealed clearly the deformation and fracture process in as-cast AZBlD, die-cast AZ91D and AMSOA. First, plastic deformation occurred at the notch root, as can be seen fiom the slip lines. In the 2"' stage, the plastic deformation and the progressively higher loading led to cracking of the brittle intermetallic particles MgI7All2 (especially those located at the grain boundaries), because they were strong barriers to dislocation movement and thus caused build-up of local stress. In the final stage, when both applied stress increased and the crack length increased, brittle fracture propagation suddenly occurred when the Griffith equation was satisfied.