Papers by Author: Gonasagren Govender

Abstract: Al-Si-Mg casting alloys, such as Al-7Si-0.3Mg alloy A356, are heat treatable and can be precipitation hardened to the T6 temper condition. However, Al-Mg-Si casting alloys (5xx series) are generally not considered to be heat treatable. These 5xx series castings are known for good castability and good resistance to corrosion, especially in marine environments. This paper investigates the extent to which 5xx series alloys could possibly be artificially aged. The influences of artificial ageing time on the overageing characteristics of both Al-Mg-Si and A356 casting alloys have been studied. A356 aluminium alloy castings were produced using the CSIR rheo-high pressure die casting process (R-HPDC). Al-Mg-Si alloys were cast using permanent mould casting. The rate of overageing of these alloys is of importance for potential higher temperature applications. The overageing characteristics of Al-Mg-Si and A356 aluminium alloys have been investigated at an artificial ageing temperature of 190°C for ageing times up to 128 hours. It is shown that the rate of overageing of Al-Mg-Si casting alloys is lower than for alloy A356. This could possibly result in the use of these alloys in applications at temperatures that are higher than where alloy A356 can be employed. It also allows the possibility of using the 5xx series alloys as an alternative to other Al-alloys for R-HPDC applications.

Abstract: A powder metallurgical process was used to fabricate Metal Matrix Composites (MMCs). A 2124 aluminium alloy was reinforced with 5 and 10 vol.% of Al₂O₃ (40-70nm) to form Metal Matrix Nano Composites (MMNCs) as well as 10 and 15 vol.% of SiC (1-10µm) to fabricate low micron MMCs. It was observed that the nano-sized Al₂O₃ particles were evenly dispersed in the aluminium matrix while a lot of loose SiC particles settled on the grain boundaries in the low micron MMCs. The relative density of all the composites increased due to sintering, however full densification was not achieved. This result was attributed to the hindered motion of dislocations, grains and grain boundaries by reinforcing particles. The 2124-Al/10%-SiC composite was cold extruded and the extruded part fractured. A metallographic evaluation was carried out and it was deduced that the mode of failure was intergranular cracking. Hardness tests performed after sintering indicated that hardness increased with an increase in volume fraction of reinforcement in the matrix. Annealing of the extruded part resulted in a decrease in hardness.

Abstract: This paper investigates the optimization of the solution heat treatment parameters of the rheo-high pressure die cast (R-HPDC) 2139 aluminium alloy. Differential Scanning Calorimetry (DSC) and optical microscopy were used to investigate the incidence of incipient melting and therefore determine suitable solution heat treatment temperatures. A three-step solution heat treatment where the alloy was heat treated from 400°C to 513°C using controlled heating conditions and held at 513°C for 2 hours and finally heated up from 513°C to 525°C and held there for 16 hours was done. R-HPDC is known to produce surface liquid segregation and when processing the alloys these areas are most prone to incipient melting. The applicability of a single (525°C for 16h) and three-step solution heat treatments on the R-HPDC 2139 aluminium alloy was also investigated. A single-step solution heat treatment results in incipient melting, whereas this is mostly eliminated using the three-step solution heat treatment. However, a high volume fraction of undissolved phases remain in the liquid segregated areas, even after the three-step solution heat treatment.

Abstract: The functional requirements of lower extremity paediatric prostheses in developing regions and the materials used in the manufacture thereof is presented in this paper. Specific advantages and disadvantages of materials used in relation to the African paediatric amputee are highlighted. The paper examines the critical issues for which further research and development is needed into material and manufacture tailoring to meet the functional requirements of a paediatric prosthetic. The applicability of light metal alloys in meeting these requirements is emphasised.

Abstract: Near-net shape casting of wrought aluminium alloys has proven to be difficult due to hot tearing. The Council for Scientific and Industrial Research (CSIR) has successfully processed wrought aluminium alloy 2139 into plate castings using the Rheo-high pressure die casting process (R-HPDC). Alloy 2139 is a Ag-containing aluminium alloy from the Al-Cu-Mg 2xxx series family. The addition of Ag enhances the age hardening response through the formation of co-clusters that act as precursors to the formation of plate-like Ω precipitates. These co-clusters typically form during natural ageing and 12-24 h of natural pre-ageing is normally specified before artificial ageing in Ag-containing Al-Cu-Mg alloys. The T6 hardness and tensile properties of R-HPDC 2139 alloy were investigated with and without natural pre-ageing. It is shown that there is no significant difference in both peak hardness and tensile properties in R-HPDC alloy 2139 with and without natural pre-ageing. The possible precipitation phenomena in both cases are discussed.

Abstract: The automotive manufacturing industry, worldwide, has been engaged in a race to produce lightweight vehicles. Consequently, the industry continues to deploy significant resources in developing and utilising advanced lightweight materials and cutting-edge technologies in the manufacture of new vehicle models that are energy efficient, more reliable, safer, more user-friendly and less polluting; without compromising the other important vehicle attributes such as, size, cargo space and payload, structural integrity, power and acceleration. Mass reduction is one consistent and cost-effective strategy that can be combined with other efficiency improvement strategies and technologies to meet the requirements of fuel economy and emission reduction. The materials used in automotive light-weighting must fulfil several criteria imposed by regulation and legislation with the environment in addition to satisfying customer requirements. The choice for light, high strength automotive materials is between advanced high-strength steel (AHSS) on one hand, and composites of aluminium (aluminium metal matrix composites (AlMMCs)), magnesium and polymers, on the other. In this paper, the potential of AlMMCs as a replacement for most steels and aluminium alloys in the manufacture of automotive parts and components is discussed as well as their current status and future trends of utilisation in automotive light-weighting.

Abstract:

Semi-solid metal forming of aluminium alloys has demonstrated the capability to produce near net shaped high integrity components. Anodising of these components for aesthetic and/or improved corrosion resistance is specified by some designers or users of this technology. The corrosion behaviour of fully anodised and partially anodised A356 aluminium alloy plates produced using the CSIR Rheo-High Pressure Die Casting (CSIR-RHPDC) process was investigated using immersion testing in a 3.5% NaCl solution with pH = 7. Optical microscope equipped with image analysis software and scanning electron microscope (SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS) were used to evaluate the behaviour of the corroded samples. The fully anodised sample showed that the anodised surface displayed some surface degradation. This degradation was more severe on the anodised surface with surface liquid segregation (SLS), but provided sufficient protection to prevent corrosion of the base metal. The partially anodised sample showed severe corrosion of the based metal with the corrosion concentrated in the silicon rich eutectic and SLS regions.

Abstract:

This paper investigates the selection process of Al-Mg-Si-(Cu) 6xxx series alloys when used specifically for rheo-high pressure die casting (R-HPDC). The 6xxx series alloys have been developed as wrought alloys and certain factors must be taken into consideration when utilising them for semi-solid metal processing. It is shown that chemical composition has a significant effect on the solution treatment parameters that should be employed i.e. high Cu and excess Si levels necessitate the use of a two-step solution treatment to reduce incipient melting. This incipient melting is especially severe in areas within the component where liquid segregation occurs, which is a common phenomenon in R-HPDC. However, high Cu and excess Si levels also have advantages: it results in higher T6 strength and Cu-additions have been shown to minimise the negative effects of natural pre-ageing. Therefore, the composition of the alloy must be selected in such a way as to achieve acceptable strength without the dangers of incipient melting in liquid segregated areas. Another important modification of 6xxx series alloys used for R-HPDC that is presented is the addition of Ti to minimise hot tearing.

Abstract:

This paper investigates the influence of natural pre-ageing time on T6 peak hardness of rheo-high pressure die cast (R-HPDC) 6xxx series aluminium alloys. Natural pre-ageing has a negative effect on the 6xxx series Al-Mg-Si alloys that contain higher quantities of Mg₂Si (typically > 0.90%). However, a positive effect is observed in alloys with lower quantities of Mg₂Si. The negative effect can be attributed to the clustering of solute atoms that form at room temperature because the precipitates which develop directly from clusters formed at room temperature are coarser than those developed in alloys artificially aged immediately after quenching. The opposite occurs in alloys where a positive effect of natural pre-ageing is found. The effects of the duration of natural pre-ageing on the subsequent T6 properties are of importance. In this study the T6 (190^oC-4h) hardness values of various R-HPDC 6xxx series alloys were determined after natural pre-ageing times ranging from 0h to 3240 hours. Alloys that show either the positive or the negative effect of natural pre-ageing are used. This paper also compares the influence of natural pre-ageing time on Cu-containing and Cu-free alloys. The addition of copper to the 6xxx series aluminium alloys lessens the negative effect of natural pre-ageing in the higher strength alloys.

Abstract: Anodising of aluminium alloy components can be used for decorative purposes and to enhance the corrosion and wear resistance. In this research, four different Al-alloy series namely Al-Cu-Mg (2xx/2xxx), Al-Si-Mg (3xx), Al-Mg-Si (6xxx) and Al-Zn-Mg-Cu (7xxx) were used to produce plates by rheo-high pressure die casting (R-HPDC). Castings produced by R-HPDC suffer from a surface liquid segregation (SLS) phenomena. This surface layer is enriched with alloying elements and it is shown to have different characteristics as compared to the bulk material. The study focused on the anodisability of plates from the four series produced by R-HPDC with and without the SLS. The samples were anodised at a commercial facility. The oxide layer thickness and homogeneity were studied using optical microscopy.