Authors: Roger N. Lumley, Maya Gershenzon, Dayalan R. Gunasegaram
Abstract: Recently, heat treatment technologies have been developed by the CSIRO Light Metals Flagship in Australia that allow the 0.2% proof stress of conventional aluminum alloy high pressure diecastings (HPDC’s) to be more than doubled without encountering problems with blistering or dimensional instability [1,2]. A range of other properties may also be improved such as fatigue resistance, thermal conductivity and fracture resistance. However, the current commercial HPDC Al-Si-Cu alloys have not been developed to exploit heat treatment or to optimize these specific mechanical properties, and one potential limitation of heat treating HPDC’s is that fracture resistance may be reduced as strength is increased. The current paper presents the outcomes of a program aimed at developing highly castable, secondary Al-Si-Cu HPDC alloys which display significantly enhanced ductility and fracture resistance in both the as-cast and heat treated conditions. Kahn-type tear tests were conducted to compare the fracture resistance of the conventional A380 alloy with a selection of the newly developed compositions. A comparison has also been made with the current permanent mold cast aluminium alloys and it is shown that the new HPDC compositions typically display higher levels of both tensile properties and fracture resistance.
954
Authors: Ian J. Polmear, Roger N. Lumley
Abstract: Deliberate partitioning of solute elements between the matrix solid solution and dispersed precipitates in aged aluminium alloys can be facilitated by underageing during heat treatment. Although this practice may cause some reduction in tensile properties, it has been shown that significant improvements may be achieved in creep resistance, fatigue strength and fracture toughness. Exploitation of secondary precipitation can allow simultaneous increases to be obtained in tensile and fracture properties.
487
Authors: Roger N. Lumley, Maya Gershenzon, Dayalan R. Gunasegaram
Abstract: Recently, heat treatment technologies have been developed by the CSIRO Light Metals Flagship in Australia that allow the yield stress in conventional aluminium HPDC’s to be more than doubled without encountering problems with blistering or dimensional instability. These procedures involve a severely truncated solution treatment step conducted at lower than normal temperatures followed by quenching and artificial ageing. Typically, heat treated HPDC’s may display increases to the yield stress of around 80 to 100%, but a range of other properties may also be improved such as fatigue resistance, thermal conductivity and fracture resistance for some tempers.
However, the HPDC alloys currently used worldwide have not been developed specifically for heat treatment or the optimization of specific properties. In particular, recent work in Al-Si-Cu HPDC alloys has identified ranges of alloys specifically for achieving yield strengths exceeding 400 MPa, or for high strength combined with elevated ductility levels. The role of alloying elements, composition limits and effects on microstructure development are discussed.
331
Abstract: Until recently, the solution heat treatment of conventional aluminum high pressure die
cast (HPDC) parts has been considered impractical because the high temperatures involved cause
surface blistering and dimensional instability. Now, a new heat treatment procedure has been
developed by the CSIRO Light Metals Flagship in Australia which avoids these problems and, in
many cases, allows tensile properties such as 0.2% proof stress to be doubled with little change to
ductility. This development has the potential to reduce costs by allowing existing HPDC parts to be
re-designed to use less metal and still achieve performance requirements.
One issue, however, is the possibility that heat treating die castings to increase tensile properties
may have an adverse effect on fracture toughness. This paper reports preliminary results of Kahntype
tear tests conducted to assess the fracture resistance of as-cast and heat treated HPDCs.
Studies of the alloys A360, A380 and C380 have shown that T4 and underaged (UA) T6 tempers
produce an optimal combination of fracture resistance and tear strength. Furthermore, the fracture
properties compare well with permanent mold and sand cast aluminium alloys that have similar
tensile properties.
141
Authors: Roger N. Lumley, J.R. Griffiths
Abstract: High pressure die-casting (HPDC) is widely used as a cost-effective way to massproduce
metal components that are required to have close dimensional tolerances and smooth
surface finishes. Approximately 50%, by mass, of the aluminium castings produced worldwide are
made by this manufacturing route. However, HPDC components are relatively porous compared
with other types of castings and so cannot usually be conventionally heat treated to improve
mechanical properties. This follows because during solution treatment (e.g. at 540°C for 8h), the
pores expand, resulting in unacceptable surface blisters, distortion and poor mechanical properties.
Recent work within the CSIRO Light Metals Flagship has revealed a heat treatment procedure by
which the problems of blistering and distortion can be avoided [1]. As a result, large improvements
in strength have been achieved, as compared with the as-cast condition. One uncertainty is the
behaviour of heat treated HPDCs under cyclic stress and this paper investigates the fatigue
properties of a common high pressure die-casting alloy, A380 (Al-8.5Si-3.5Cu). Comparisons are
made between as-cast, T4 and T6 conditions. Fatigue strength is highest for the alloy aged to a T6
temper and ratios of fatigue strength to tensile strength for the as-cast, T4 and T6 conditions are
constant at a value of approximately 0.6, which is particularly high for aluminium alloys.
99
Authors: Roger N. Lumley, Robert G. O'Donnell, Dayalan R. Gunasegaram, Michel Givord
Abstract: 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†.
351
Authors: Roger N. Lumley, J. Buha, Ian J. Polmear, A.J. Morton, A. Crosky
Abstract: This paper reviews some of the practical outcomes of exploiting secondary precipitation
during the heat treatment of aluminium alloys and discusses current understanding of this
phenomenon. Recent studies have utilised the techniques of positron annihilation spectroscopy
(PAS), 3D atom probe (3DAP) as well as the more traditional transmission electron microscopy
(TEM) and differential scanning calorimetry (DSC) to investigate early precipitation events that
occur during secondary ageing at low temperatures (~20-65°C). This work has confirmed that
clustering and GP zone formation can modify the nature and distribution of precipitates that form
when ageing is subsequently resumed at more elevated temperatures. Prospects for achieving
further improvements in heat treatment schedules and alloy compositions to take greater advantage
of secondary precipitation are also considered.
283
Authors: Roger N. Lumley, Ian J. Polmear, A.J. Morton
303
Authors: Roger N. Lumley, Ian J. Polmear, A.J. Morton
893
Authors: Roger N. Lumley, A.J. Morton, Ian J. Polmear
1495