PSI - Issue 57

C2 - Confidential

Hayder Y Ahmad et al. / Procedia Structural Integrity 57 (2024) 478–486 Ahmad et al./ Structural Integrity Procedia 00 (2023) 000 – 000

479

2

also provide a number of attractive properties such as dimensional stability, good heat dissipation and good damping effects. Furthermore, magnesium alloys provide a more cost-effective solution compared to other high strength metals. However, there are limitations for the use of these alloys in the design listed by Luo (2013), for example their low ductility & rigidity, softness, high chemical reactivity, making them prone to galvanic and pitting corrosion, reduced creep resistance and highly flammable in their pure form when molten or in powder. Despite the limitations of Mg alloys, they are still used in some common applications within the aerospace industry Pashkova et al. (2007). They can be found in gearboxes, transmissions, intermediate compressors, auxiliary gearboxes, electrical generators, canopies and various engine components. Therefore, many researchers Parkhave (2003) and Kielbus (2005) carried out studies on the microstructure, physical and mechanical specification and manufacturability (casting, forging, machining, weldability – etc) of Magnesium and its alloys. Casting is one of the common techniques used to manufacture parts out of the Magnesium alloys Dahle et al. (2000 and 2001), Lee et al.(2000) . Despite the fact that casting is a good manufacturing process for mass production and a cost effective process, it has problems. For example, the microstructures of these castings tend to be coarse with large grain sizes that will influence the mechanical properties. Furthermore, the crystal structure of the magnesium alloys are hexagonal close packed (HCP) structures which make the materials more brittle, providing fewer independent slip systems compared to other lattice structures. The second and most common problem with the sand casting of magnesium alloys is the potential reaction with the commonly used silica containing constituents of plaster or shell mold materials Avedesianet al. (1999). Therefore, the reaction of the Silica with Magnesium can produce Magnesium Silicates. This paper will present a real case study of a solid steel cylinder fitted with an interference fit to a cast Mg alloy cylinder and fixed with bolts. Due to the cast defects and interference fit load, a crack initiated and propagated to failure. In this research, the effects of the Magnesium Silicates on the crack initiation will be studied. The most fascinating part of this study is the propagation of the crack in a magnesium alloy under a static load in a similar manner to the ceramic materials due to the nature of the crystal structure of Mg alloy, which is HCP. The crack propagation will also be investigated in terms of the magnesium hydrogen embrittlement phenomenal and/or the effect of stress corrosion crackling (SCC). Detailed fractography of the fracture surfaces will also be presented in this paper. Finally, recommendations to reduce the risk of the Magnesium Silicates will be listed in this study.

Nomenclature δ r

radial deformation of the two cylinders outer radius of the outer cylinder inner radius of the inner cylinder

r o r i R ν P

interference radius

E i,o

Young’s Modulus of the inner and outer cylinder

Poisson Ratio

pressure interference fit between the two cylinders

CTE

Coefficient of thermal expansion

Mg

Magnesium Mega Pascal Giga Pascal

MPa GPa SEM

Scanning Electron microscope