Tuty Asma Abu Bakar

Role of bismuth on solidification, microstructure and mechanical properties of a near eutectic Al-Si alloys

Computer aided thermal analysis and microstructural observation showed that addition of bismuth (Bi) within the range of 0.25 and 2 wt% produced a greater effect on the Al-Si eutectic phase than on primary aluminium and Al2Cu phases. Results showed that with addition of 1 wt% Bi the eutectic silicon structure was refined from flake-like morphology into lamellar. Bi refines rather than modifies the Si structure and increases the Al-Si eutectic fraction solid and more significantly there was no fading even up to 180 min of melt holding. Transmission electron microscopy study showed that the Si twin spacing decreased from 160 to 75 nm which is likely attributed to the refining effect of Bi. It was also found that addition of 1 wt% Bi increased the tensile strength, elongation and the absorbed energy for fracture due to the refined eutectic silicon structure.

Refinement of Mg2Si Particulate Reinforced Al-20%Mg2Si In Situ Composite with Addition of Antimony

Al-Mg2Si in-situ composite have received wider attention due to their improved properties which mark them as attractive candidate to manufacture most of automotive and aerospace components. Result show that the melt treatment of the in-situ composite by addition of antimony caused to refining the morphology of primary Mg2Si reinforcement which would expect to improve the mechanical properties. Effect of Sb on the characteristics parameters of Mg2Si particles have been investigated by thermal and microstructural analysis. The result has revealed that addition of 0.8 wt.% Sb produced optimum refinement effect on the morphology as coarse structure has been changed to polygon one. The result also showed depression with nucleation temperature, TN and growth temperature, TG which also correspond to the refinement effect.

Effect of the formation of CuO flowers and SnO2 on the growth of tin whiskers on immersion tin surface finish

The effect of the formation of CuO flowers and SnO2 on tin whiskers formation and growth in 30°C/60%RH environmental condition for tin surface finish had been studied. Immersion plating method was used to coat a layer of tin onto a copper, Cu substrates. The coated surface was subjected to external stress by micro hardness indenter with 2N load in order to simulate an external stress in coating layer and to promote the formation of tin whiskers. FESEM and EDX was used to study the type and chemical composition of whiskers and oxides formed. Image analyser was used to measure the whiskers length using JEDEC Standard No 22-A121A. After 1 week of the exposure under 30°C/60%RH environmental condition, kinked-type whiskers were formed and the whiskers growth were discontinued at week 4 due to the distruction at the end-tip of kinked-type tin whisker to grow further when it touched the coating surface. An obervation until 52 weeks of exposure time found that the formation and growth of CuO flowers and the oxidation of non-kinked-type of tin whiskers to form SnO2 promoted by external stress also contributed to the discontinuity of whiskers growth.

Characterization of Phase Formation in Al-20%Mg2Si-2%Cu Metal Matrix Composite

Advantage of Al-Mg2Si metal matrix composite (MMC) is due to the particulate reinforced Mg2Si in the Al matrix that has improved mechanical properties of the in situ composite. In particular, the composite has been chosen as the structural material for automotive and aerospace components. The hypereutectic Al-Mg2Si composite should be comprised of two morphologies, namely primary Mg2Si and pseudo-eutectic Al-Mg2Si phase. However, as-received commercial Al-20Mg2Si-2Cu in situ composite contained a wide range of elements that affect the resultant microstructure of the in situ composite. In fact, four different morphologies have been identified in the in situ composites. The first phase is dark facet primary Mg2Si particles, surrounded by pseudo-eutectic Al-Mg2Si phase in lamellar structure. Along the eutectic boundary is the formation of needle-like Al5FeSi intermetallic and the phase is surrounded by Al5Mg8Si6Cu2+Al2Cu as last phase nucleated. Optical micrograph, SEM imaging, EDX analysis and elemental mapping have revealed these phases correspond to shape of morphologies and respective molecular compound.

The Effects of Nickel Underlayer and Solder Dipping as Tin Whisker Mitigations in Tin Surface Finishes

Driven by environmental concerns and the enforcement of Restriction of Hazardous Substances Directive (RoHS) to ban the use of lead in electronics, the global electronics industry has migrated toward lead-free electronics. However, the adoption of lead-free tin (Sn) surface finish is known to form whiskers. These whiskers grow spontaneously from the Sn finish layer as a stress-relief over time causing device failures. In the present research, whisker growth is investigated via immersion Sn finishes on Cu substrate. The effects of Sn layer thickness, addition of Ni under-layer and solder dipping on whisker growth are investigated by storing the samples under ambient temperature for up to 24 weeks. The effects of external stresses were also studied using bending test. The results showed that whisker length on immersion Sn increases with time for all the samples either with or without Ni under-layer. Thicker Sn coating showed more whisker growth compared with thinner Sn coating. The longest whisker length of 23μm was observed for Sn coating with 2μm thickness. The addition of Ni as under-layer was found to be more effective in mitigating the whisker growth by extending the incubation time for whisker formation. Compared to immersion Sn, solder dipping in pure Sn showed no whisker growth. However, alloying Sn with 0.4%wtCu resulted in whisker growth indicating the role of Cu in promoting whiskers formation.