Solidification behaviour of Al-Sn-Cu immiscible alloys and Al-Si cast alloys processed under intensive shearing

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Alloy castings are usually solidified with a coarse columnar grain structure under normal casting conditions unless the mode of the solidification is carefully controlled. It is desirable for the grain structure to be fine and equiaxed to improve their mechanical performance as finished castings. It is possible to develop a fine and equiaxed grain structure either by increasing the number of nucleation sites or by grain multiplication. Immiscible alloys with a microstructure in which a soft phase is dispersed homogeneously in a hard matrix have significant potential applications in advanced bearing systems, especially for the automotive industry. Despite considerable efforts made worldwide, including extensive space experiments, no casting techniques so far can produce the desired immiscible microstructure of alloys. Experimental results on Al-Sn-Cu immiscible alloys have confirmed that intensive shearing using melt conditioning by an advanced shearing technology (MCAST) unit, is an effective way to achieve a fine and uniform dispersion of the soft phase without macro-demixing, and that such a dispersed microstructure can be further refined in alloys with precipitation of the primary Al phase prior to the demixing reaction. In addition, it was found that melt shearing at 200 rpm for 60 s will be adequate to produce a fine and uniform dispersion of the Sn phase, and that a higher shearing speed and prolonged shearing time can only achieve further minor refinement. A study of Al-Si hypoeutectic and hypereutectic alloys presents the effects of the processing temperature and intensive shearing on the microstructural and mechanical properties which have been investigated systematically. Attempts have been made to explain the solidification mechanism with intensive melt shearing. The sheared melt was cast into tensile test samples by high pressure die caster (HPDC) to examine the microstructures and mechanical properties. The experimental results reveal that significant grain refinement and uniformity of grains was achieved by the intensive shearing and also a considerable increase in mechanical properties with pouring temperature by changing intermetallic particles morphology, the position of defect band and reduced microscopic defects

Direct observation via in situ heated stage EBSD analysis of recrystallization of phosphorous deoxidised copper in unstrained and strained conditions

Recrystallization of phosphorous deoxidised copper used for strength critical applications at elevated temperatures was investigated by means of in situ heated stage EBSD analysis using a Gatan Murano heated stage mounted within a Carl Zeiss Sigma FEGSEM electron microscope. The influence of applied strain as the result of deformation within a Nakajima test as an analogue for industrial forming on the recrystallization temperature was investigated, the impact of increased heating rates on microstructural evolution was also investigated. Inverse pole figure plots combined with regions of reduction in local misorientations and variations in geometrically necessary dislocations were used to establish the point of recrystallization and the recrystallized fraction of the material. Recrystallization was observed to occur at temperatures as low as 130 °C in highly strained samples compared to around 300 °C within the annealed samples dependent upon heating rate. Increased heating rates were observed to produce a finer final grain structure but had little effect on presence of 60° grain twins, which was influenced more by initial material condition

Microstructural modification of recycled aluminium alloys by high-intensity ultrasonication: Observations from custom Al–2Si–2Mg–1.2Fe–(0.5,1.0)Mn alloys

The effect of ultrasonication on the solidification microstructure of recycled Al-alloys is investigated using custom Al–2Si–2Mg-1.2Fe-xMn alloys (x = 0.5 and 1%, in wt.%) through cooling curve measurement, optical and electron microscopy, X-ray diffraction, differential scanning calorimetry and computational thermodynamic calculations. Applying ultrasonication throughout the primary-Al nucleation stage resulted in refined non-dendritic grain structure. Cooling curves indicate a noticeable reduction in primary-Al nucleation undercooling and reduction of the recalescence peak under ultrasonication. However, terminating ultrasonication prior to the nucleation of primary-Al led to dendritic grains with marginal refinement. Without ultrasonication, coarse Chinese-script α−Al15(Fe,Mn)3Si2 intermetallics developed from initially polygonal particles due to interface growth instability under thermo-solutal undercooling. In contrast, ultrasonication produced refined and polygonal α−Al15(Fe,Mn)3Si2 particles by promoting nucleation and growth stabilisation under strong fluid flow. The enhanced nucleation from ultrasonication is presumably due to the pressure-induced shift of freezing point along with improved wetting of insoluble inclusions under cavitation. The present results show that ultrasonication can effectively modify the Fe-intermetallics and refine the grain structure in recycled Al-alloys

Microstructural evolution of 316L austenitic stainless steel during in-situ biaxial deformation and annealing

Austenitic stainless steel 316L was investigated by a combination of in-situ biaxial straining and subsequently by in-situ annealing within a Carl Zeiss Sigma FEG-SEM. A Micromecha Proxima stage was used to impart biaxial strain on the sample, results were compared with macro scale testing to validate the method, and physical properties were established in close correlation to macro scale tests. Samples were subsequently subjected to annealing cycles using a Gatan Murano 525 heated stage to assess the influence of imparted strain on the recrystallization kinetics of the material. Following annealing trials the material was observed to initiate recrystallization around 100 °C earlier within heavily strained samples at 750 °C in comparison to 850 °C within unstrained samples

Challenges and Opportunities in Remote Laser Welding of Steel to Aluminium

In the last two decades, the automotive industry has been facing demands to reduce fuel consumption and to meet CO2 emissions through applications of lightweight materials. Therefore, aluminium alloys have replaced substantial amounts of steel; and they are receiving significant attention to achieve greenhouse emission targets. However, a critical factor in applications of advanced aluminium in automotive Body in White (BIW) designs depends on availability of cost effective and high performance joining processes. Currently, a Self-Pierce Riveting (SPR) process is extensively used for aluminium BIW sheet metal parts joining which is expensive, additionally increase the weight of the vehicle and cause inefficiency in manufacturing operations. As aluminium alloys are difficult to weld by conventional technologies such as electrical resistance spot welding, MIG arc welding etc., various joining technologies had proposed to weld aluminium alloys and dissimilar alloys over the years. Often, these technologies restrict design flexibility and are expensive for mass production. In this context, Remote Laser Welding (RLW) has gained popularity because of its distinct advantages such as design flexibility, production speed, material and cost savings. This paper provides a critical review of challenges and opportunities for application of RLW to dissimilar metal welding of steel to aluminium. Next steps of research and development are also highlighted

Influence of Zn Concentration on Interfacial Intermetallics During Liquid and Solid State Reaction of Hypo and Hypereutectic Sn-Zn Solder Alloys

In this study, Sn-Zn solder samples containing 2 to 12 wt.% Zn were fabricated and reflowed into a Cu substrate. The microstructure of solder samples was observed after reflow and aging for up to 1000 h at 150°C. Thermodynamically stable intermetallics (IMCs) Cu-Zn and Cu-Sn formed at the interface depending on the solder composition. Formation of different interfacial IMCs during soldering and after prolonged aging is explained by the spalling mechanism that resulted from the depletion of Zn from the solder matrix

Novel approach to copper sintering using surface enhanced brass micro flakes for microelectronics packaging

Copper pastes suitable for low temperature and low pressure die-attach bonding were developed to enable sintering at 275 °C under N2 atmosphere. First, brass flakes were treated with HCl to selectively etch Zn and to realize enhanced surface modifications on the flakes. Then, polyethylene glycol was added as binder to the modified flakes due to its reducing effects on copper oxides and its property to prevent agglomeration. Shear strength of ca. 50 MPa was achieved while sintering with 10 MPa bonding pressure thereby providing suitable, easy and low-cost sintering pastes for microelectronics packaging applications

A study on interfacial activities between Nb and liquid Al in static and dynamic condition

In the present study, an effort has been made to investigate the interfacial activities between the solid Niobium and liquid Aluminium. Aluminium was melted at 750 °C temperature and subsequently, solid niobium was immersed into the liquid aluminium and held for different time intervals. The study was conducted in static as well as dynamic conditions. Temperature and time were similar for both static and dynamic conditions. After solidification, the Nb-Al interface was studied under optical microscopy, scanning electron microscopy, EDS and X-Ray diffraction analyzer. The analysis revealed that there was no change in the dimension of Nb sample and no intermetallic compound formed on the surface or subsurface of solid Niobium under present experimental condition

Challenges and opportunities in remote laser welding of steel to aluminium

In the last two decades, the automotive industry has been facing demands to reduce fuel consumption and to meet CO2 emissions through applications of lightweight materials. Therefore, aluminium alloys have replaced substantial amounts of steel; and they are receiving significant attention to achieve greenhouse emission targets. However, a critical factor in applications of advanced aluminium in automotive Body in White (BIW) designs depends on availability of cost effective and high performance joining processes. Currently, a Self-Pierce Riveting (SPR) process is extensively used for aluminium BIW sheet metal parts joining which is expensive, additionally increase the weight of the vehicle and cause inefficiency in manufacturing operations. As aluminium alloys are difficult to weld by conventional technologies such as electrical resistance spot welding, MIG arc welding etc., various joining technologies had proposed to weld aluminium alloys and dissimilar alloys over the years. Often, these technologies restrict design flexibility and are expensive for mass production. In this context, Remote Laser Welding (RLW) has gained popularity because of its distinct advantages such as design flexibility, production speed, material and cost savings. This paper provides a critical review of challenges and opportunities for application of RLW to dissimilar metal welding of steel to aluminium. Next steps of research and development are also highlighted