T.B. Sercombe

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

Heat treatment of Ti‐6Al‐7Nb components produced by selective laser melting

Purpose – The purpose of this paper is to describe a preliminary investigation into the heat treatment of Ti‐6Al‐7Nb components that had been produced via selective laser melting (SLM).Design/methodology/approach – Bars of Ti‐6Al‐7Nb were produced using SLM by MCP‐HEK Tooling GmbH in Lubeck, Germany. These bars were then subjected to a range of heat treatments and the resultant microstructure evaluated with respect to its likely effect on fatigue.Findings – It was found that the as received material consisted of an α′ martensitic structure in a metastable β matrix. Evidence of the layer‐wise thermal history was present, as were large (up to ∼500 μm) pores. Solution treatment at 955°C (below the β transus) did not completely disrupt this layered structure and is therefore not recommended. When solution treatment was performed at 1,055°C (above the β transus) a homogeneous structure was produced, with a morphology that depended on the post‐solution treatment cooling rate. It was concluded that the most prom...

The effect of trace elements on the sintering of Al-Cu alloys

Trace additions of Sn, In, Bi, Sb and Pb have been used to activate the liquid phase sintering of an Al-4Cu-0.15Mg alloy. Additions of as little as 0.05 wt% (similar to 0.01 at.%) increases the sintered density from 88 to 92% of the theoretical density. The elements which aid sintering have both high vacancy binding energies and high diffusivities in Al. It is suggested that the trace element diffuses into the Al, and forms trace element-vacancy clusters. This reduces the diffusivity of the Cu in the Al matrix, delaying Cu dissolution therefore causing the liquid to persist for longer times. This enhances sintering and therefore densification

On the use of trace additions of Sn to enhance sintered 2xxx series Al powder alloys

The mechanical properties of a typical sintered aluminium alloy (Al-4.4Cu-0.8Si-0.5Mg) have been improved by the simultaneous use of trace additions of Sn, high sintering temperatures and modified heat treatments. Tin increases densification, but the Sn concentration is limited to less than or equal to 0.1wt% because incipient melting occurs during solution treatment at higher Sn levels. A sintering temperature of 620 degrees C increases the liquid volume over that formed at the conventional 590 degrees C sintering temperature. However, the higher sintering temperature results in the formation of an embrittling phase which can be eliminated if solution treatment is incorporated into the sintering cycle (a modified TS heat treatment). These conditions produce a tensile strength of 375 MPa, an increase of nearly 20% over the unmodified alloy

Sintering of freeformed maraging steel with boron additions

This paper describes the sintering of an 18Ni(350) maraging steel with additions of boron, with the aim of producing high hardness rapid tooling. Reaction of the boron with the alloying elements in the maraging steel resulted in the formation of a Mo- and Ti-rich borides. The former melted at similar to1220degreesC, providing a liquid phase for enhanced sintering. Although densification could occur regardless of the boron content, especially at high temperature, 0.4% B was required to produce a near full density component. The formation of the various borides depleted the matrix of critical age hardening elements. However, by altering the starting powder composition to compensate for this, hardness close to the wrought alloy has been achieved. This hardness was comparable to a common die casting tool steel. Examples of dies produced using selective laser sintering (SLS) are also shown

Selective laser melting of aluminium and aluminium metal matrix composites: review

Selective laser melting (SLM) is gaining importance as companies begin to exploit its advantages to produce parts that will enable them to enter the market sooner, at a lower cost and/or with parts having an increased geometric complexity. Since aluminium is the second most popular engineering material after steel, its use in SLM was inevitable. In this review, we look at the SLM of aluminium and aluminium matrix composites. We explore some of the inherent difficulties in working with aluminium including the presence of a stable oxide layer, high reflectivity and thermal conductivity and poor flowability of the powder. We also review the unique microstructures that are produced during the SLM process and its effect on the mechanical properties. Included in this is the effect of heat treatment on the structure and properties. Finally, we look at the benefits and problems of producing aluminium metal matrix composites using SLM.

On the sintering of uncompacted, pre-alloyed Al powder alloys

The effect of the addition of elemental Mg, Sri and Pb on the sintering of impressed prealloyed 2124 and 6061 powder has been investigated. Despite being sintered at a temperature that resulted in similar to20 vol.% liquid phase, high density was possible only upon the addition of similar to1% elemental Mg as well as trace amounts of Sri or Pb. Additions of Mg facilitated oxide disruption, while Sri and Pb segregated to the liquid phase, where they reduced the liquids surface tension, thus improving sintering

Process repeatability and sources of error in indirect SLS of aluminium

Purpose – The purpose of this paper is to investigate the accuracy and repeatability of the indirect selective laser sintering of aluminium process.Design/methodology/approach – This work characterised the shrinkage of indirect SLS aluminium parts during the various stages of production. Standard scale parts were measured using a Giddings and Lewis co‐ordinate measuring machine in both the green and infiltrated condition.Findings – The experiments conducted show that most accuracy is lost during the furnace cycle and that the greatest loss of accuracy occurred in the Z dimension. Additionally the position of parts within the part bed in both X, Y and Z is shown to influence accuracy, with smaller parts being built closer to the edge of the bed later in the build. These results have been interpreted as being a result of the phenomenon of “Z‐growth”. Finally, the research shows that the overall accuracy of the indirect selective laser sintering of aluminium process is comparable with many existing processes...

Selective Laser Melting of Low-Modulus Biomedical Ti-24Nb-4Zr-8Sn Alloy: Effect of Laser Point Distance

As many complex processing parameters are involved in Selective Laser Melting (SLM), an understanding of the scientific and technical aspects of the production route on the microstructural evolution during SLM process is required in order to obtain parts with near full density and desirable surface finish. Although the effects of the various processing parameters on the density of parts have been well documented, the effect of laser point distance on density and mechanical properties of the SLM-produced parts has not been widely studied. In this paper, we present the results of using SLM to produce biomedical beta Ti-24Nb-4Zr-8Sn components. Both the density and hardness of the material increases with increasing incident laser energy and reaches a near full density value of >99% without any post-processing. When the laser energy density input is high enough to fully melt powder, the laser point distance has no influence on the density or hardness of the samples. In contrast, at low energy densities, large point distances have been shown to be detrimental.

Metal injection moulding of aluminium alloy 6061 with tin

Abstract A metal injection moulding technique for the production of aluminium alloy components is described. A part is formed by injection moulding a mixture of alloy 6061 and 2 wt-%Sn with a resin consisting of stearic acid, palm oil wax and high density polyethylene. The resin is removed by a combination of solvent and thermal processing. The parts are then sintered in a nitrogen atmosphere to a density of 97%. Sacrificial magnesium blocks which act as an oxygen and moisture getter are placed in the vicinity of the parts during sintering. This ensures surface integrity. Aluminium nitride forms throughout the part, which provides structural rigidity and dimensional stability and limits grain growth. After artificial aging, the tensile strength is 300 MPa. The technique allows the production of small complicated shapes and provides an additional means to manufacture aluminium components for a wide variety of applications.