Roger Neil Lumley

Liquid phase sintering of aluminium alloys

The principle that alloys are designed to accommodate the manufacture of goods made from them as much as the properties required of them in service has not been widely applied to pressed and sintered P/M aluminium alloys. Most commercial alloys made from mixed elemental blends are identical to standard wrought alloys. Alternatively, alloys can be designed systematically using the phase diagram characteristics of ideal liquid phase sintering systems. This requires consideration of the solubilities of the alloying elements in aluminium, the melting points of the elements, the eutectics they form with aluminium and the nature of the liquid phase. The relative diffusivities are also important. Here we show that Al-Sn, which closely follows these ideal characteristics, has a much stronger sintering response than either Al-Cu or Al-Zn, both of which have at least one non-ideal characteristic

Enhanced creep performance in an Al-Cu-Mg-Ag alloy through underageing

Abstract Tests at 130 °C and 150 °C have shown that the creep resistance of an Al–Cu–Mg–Ag alloy is significantly increased if it is heat-treated at an elevated temperature to an underaged condition rather than the fully hardened, T6 temper. This beneficial effect of underageing is manifest in reduced rates of secondary creep. Similar results have been obtained for the commercial alloy 2024. Delays at ambient temperature after underageing and before testing lead to secondary precipitation and a progressive decrease in creep performance that eventually reverts to close to that for the T6 condition. This detrimental effect may be overcome by slow cooling from the underageing temperature, which arrests or impedes subsequent secondary precipitation. Microstructural observations suggest that the enhanced creep resistance in the underaged condition is a consequence of the presence of “free” solute in solid solution that is not yet involved in precipitation.

Interrupted aging and secondary precipitation in aluminium alloys

Abstract It is shown that, if elevated temperature aging of aluminium alloys is interrupted with a dwell period at a low temperature (65 °C), age hardening continues due to so called secondary precipitation. If elevated temperature aging is then resumed, significant improvements can be obtained in mechanical properties compared with those available using a conventional T6 temper. Average increases in 0.2% proof stress and tensile strength of 10%, combined with improved fracture toughness, have been achieved in a wide range of alloys. These effects arise primarily because interrupted aging promotes the formation of more finely dispersed precipitates in the final microstructures. The concept of interrupted aging is described in some detail with respect to the model system, Al-4Cu, and examples are then given of the effects of the treatment on the microstructures and mechanical properties of several wrought and cast aluminium alloys.

The effect of solubility and particle size on liquid phase sintering

Three binary systems were examined: Al-Sn, Al-Zn, and Al-Cu. Starting additive powders were either <45 {micro}m in size or 125--150 {micro}m. The aluminium powder was air atomized, passed through a 100 mesh screen and had an average particle size (d{sub 50}) of 60{micro}m. The use of fine additive powders in systems having a substantial transient aspect results in a greatly reduced quantity of liquid phase formed during sintering. Conversely, the use of coarse powders increases the amount of liquid phase that forms and prolongs its existence during sintering. This occurs in systems having appreciable solid solubility of the additive in the base and/or exhibiting preferential diffusive flow from the additive to the base. Coarser powders enhance sintering in such systems. Where there is no mutual solid solubility, particle size is unimportant in liquid development.

Development of mechanical properties during secondary aging in aluminium alloys

Abstract Earlier work has shown that, if the artificial aging of aluminium alloys is interrupted by a dwell period at lower temperature, higher values of tensile properties and fracture toughness may be achieved than are possible with single stage T6 tempers. A second interrupted aging cycle has now been developed that involves underaging at the elevated temperature, quenching, and then allowing secondary precipitation to occur at, or just above, room temperature. Designated a T6I4 (I=interrupted) temper by the authors, this simpler aging cycle may reduce heat treatment costs. Tests on some 30 cast and wrought alloys have resulted in tensile properties close to those for a T6 temper, with higher values of fracture toughness being recorded for some cases. Such an aging treatment can be incorporated into a paint bake cycle to simplify the heat treatment of coated automotive components.

Precipitation and solute distribution in an interrupted-aged Al–Mg–Si–Cu alloy

Quantitative analysis of the precipitate species and solute distribution was carried out on Al–Mg–Si–Cu alloy 6061 aged to peak hardness using a conventional T6 heat treatment and the so-called T6I6 heat treatments. In this latter, a dwell period at reduced temperature (65°C) is introduced into the T6 ageing cycle (at 177°C or 150°C) which modifies the microstructure and results in the simultaneous improvement of both tensile properties and fracture toughness. Analysis of three-dimensional atom probe data reveals that the superior mechanical properties of the T6I6/177 temper are achieved by a combined effect of a greater consumption of solute atoms by precipitates, an increased number density of fine precipitates and the presence of greater fractions of the effective strengthening precipitates in the final microstructure. Three types of precipitates were found to be characteristic of the peak aged conditions: β′′ precipitates, Guinier–Preston zones and Mg–Si(–Cu) co-clusters. The composition of the strengthening precipitates was found to vary over a wide range for the different heat treatment schedules, corresponding to a variation in the number density of stable nuclei, without any accompanying change in their morphology. All precipitates were found to contain substantial quantities of aluminium. The results also indicate that the strengthening precipitates are preferentially formed from Si-rich nuclei that contain Cu atoms, as opposed to Cu-free nuclei.

The effect of additive particle size on the mechanical properties of sintered aluminium-copper alloys

Fine additive particle sized often have advantageous sintering characteristics because the high surface area to volume ratio increases the driving force for sintering and because the powders are better distributed throughout the compact. Fine additive particles also leave smaller secondary pores in transient liquid phase sintering systems. Transient systems are sensitive to process variables such as heating rate because they control the volume of liquid which forms and the duration for which it exists. Additive particle sizes are therefore also likely to influence sintering in such systems. Indeed, it has recently been shown by microstructural examination that coarse additive particles can promote liquid formation and therefore enhance sintering in transient liquid phase systems. Here, the authors extend this work by determining the tensile properties as a function of additive particle size and heating rate in an aluminum alloy.

Blister free heat treatment of high pressure die-casting alloys

Conventionally produced high pressure die-cast (HPDC) components are not considered to be heat treatable because gases entrapped during the die-casting process expand during solution treatment causing unacceptable surface blistering. Components may also become dimensionally unstable. Both these effects prevent the heat treatment of die-castings as these phenomena are detrimental to the visual appearance, mechanical properties and utilisation of the component. Recent work has revealed a process window in which HPDC aluminium alloys that are capable of responding to age hardening may be successfully heat treated without encountering these problems. As a result, improvements of greater than 100% in the tensile properties are possible, when compared with the as-cast condition. The new heat treatment schedules are described for HPDC parts of different size and shape, the role of chemistry on ageing is discussed and microstructural development during heat treatment examined†.

The Role of Alloy Composition and T7 Heat Treatment in Enhancing Thermal Conductivity of Aluminum High Pressure Diecastings

The thermal conductivity of some common and experimental high pressure diecasting (HPDC) Al-Si-Cu alloys is evaluated. It is shown that the thermal conductivity of some compositions may be increased by more than 60 pct by utilizing T7 heat treatments. This may have substantial performance and cost benefits for applications where thermal management is a key design parameter.

An Evaluation of Quality Parameters for High Pressure Die Castings

Several techniques for examining casting “quality” as it relates to high pressure diecast alloy A380 have been evaluated in the as-cast condition. The roles of three simple parameters were considered: a) metal velocity at the gate, b) the effect of increased Cu or Zn content, and c) the effect of rotary degassing on a recycled melt. It was shown that tensile failure in high pressure die casting (HPDC) specimens is influenced by complex defect clusters and the interaction of a variety of casting defects. The two major defect cluster types identified in the current work were comprised of a dispersed foam-like shrinkage defect, and/or large oxide films present on the fracture surfaces. The removal of hydrogen had little effect on average tensile properties which was a surprising result, but rotary degassing did appear to remove a portion of the oxides present in the melt, thereby improving casting quality. It is shown that of the different analyses conducted, all could differentiate a degree of casting quality, but some techniques (i.e., Weibull statistics combined with flow curve derivations based on the Ludwik-Holloman equation) are particularly useful. It is proposed that complex strain localization and failure occurs in HPDC specimens, which results in a proportionately large fraction of defects appearing on the fracture surface.