Advances in the Production of Infiltrated Aluminium Parts Using Selective Laser Sintering

Recent advances in a rapid freeform fabrication process for the production of aluminium parts are considered. The methodology involves the formation of an unconstrained, resin bonded aluminium powder part by Selective Laser Sintering, the burnout of the resin, the partial transformation of the aluminium into a rigid aluminium nitride skeleton by reaction with the atmosphere under a magnesium/alumina blanket and the subsequent infiltration with a second aluminium alloy. Here we describe the process and consider potential applications. Strategies for controlling the growth of the aluminium nitride are also to be discussed.Mechanical Engineerin

The effects of sample position and gas flow pattern on the sintering of a 7xxx aluminum alloy

The effects of sample position and gas flow pattern on the sintering of a 7xxx aluminum alloy Al-7Zn-2.5Mg-1Cu in flowing nitrogen have been investigated both experimentally and numerically. The near-surface pore distribution and sintered density of the samples show a strong dependency on the sample separation distance over the range from 2 mm to 40 mm. The open porosity in each sample increases with increasing separation distance while the closed porosity remains essentially unchanged. A two-dimensional computational fluid dynamics (CFD) model has been developed to analyze the gas flow behavior near the sample surfaces during isothermal sintering. The streamlines, velocity profile, and volume flow rate in the cavity between each two samples are presented as a function of the sample separation distance at a fixed nitrogen flow rate of 6 L/min. The CFD modeling results provide essential details for understanding the near-surface pore distribution and density of the sintered samples. It is proposed that the different gas flow patterns near the sample surfaces result in variations of the oxygen content from the incoming nitrogen flow in the local sintering atmosphere, which affects the self-gettering process of the aluminum compacts during sintering. This leads to the development of different near-surface pore distributions and sintered densities

Pressureless infiltration and resulting mechanical properties of Al-AlN preforms fabricated by selective laser sintering and partial nitridation

A novel manufacturing process has recently been developed for the fabrication of intricate Al-AlN composite parts. The process involves green shape formation by selective laser sintering, preform development by nitridation, and net shape forming by pressureless infiltration. The infiltration atmosphere has an important influence on the final fabrication and mechanical properties. This work presents a detailed investigation on the infiltration of Al-AlN preforms with AA 6061 at various temperatures above its liquidus under nitrogen, vacuum, and argon. The green shapes are formed by selective laser sintering of a premix of AA 6061-2Mg-1Sn-3Nylon (wt pct) powders. They are then partially nitrided to create a rigid, 2- to 3-μm-thick AlN skeleton for subsequent infiltration. Nitrogen infiltration results in the highest density (2.4 gcm) and best tensile properties (UTS: 214 MPa; elongation: 2.5 pct), while argon infiltration gives the lowest density. Fractographs confirmed the difference in density arising from the use of different atmospheres where small pores are evident on the fracture surfaces of both argon and vacuum-infiltrated samples. The molten AA 6061 infiltrant reacts with nitrogen during infiltration leading to a 5-μm-thick AlN skeleton compared to the original 2- to 3-μm-thick skeleton in both argon and vacuum-infiltrated samples. Transmission electron microscope (TEM) examination revealed inclusions of MgSi and MgSi Sn in both nitrogen- and argon-infiltrated samples but not in vacuum-infiltrated samples. Vacuum infiltration is slower than nitrogen and argon infiltration. The mechanisms that affect each infiltration process are discussed. Infiltration under nitrogen is preferred

Development of a high strength aluminium MIM alloy

Aluminium MIM has the potential to combine the light weight advantages of aluminium with the cost benefits of metal injection moulding for the manufacture of small, intricately shaped parts. The key issue in the development of an Al MIM system is the sinterability of aluminium. Here, we describe the development of an aluminium MIM system that is based on prealloyed AA6061 powder mixed with elemental tin. It was the aim of this work to develop an alloy and a MIM processing route that produced a material with mechanical properties that approached those of conventionally processed aluminium alloys so that it could be used in a wide variety of applications

Thermal analysis and dilatometer studies of the liquid-phase sintering of Al-Mg-Si-Cu alloys

An understanding of the expansion and shrinkage behaviour during the liquid phase sintering of admixed aluminium-magnesium-silicon alloys is required to control dimensional tolerances. The expansion and shrinkage behaviour in this system have been characterized using dilatometry and calorimetry. The formation of liquid during sintering causes expansion due to the non-wetting behaviour of the liquid. There is a transition from expansion to shrinkage which is influenced by the atmosphere and alloying

Effect of substrate temperature on the interface bond between support and substrate during selective laser melting of Al-Ni-Y-Co-La metallic glass

An Al-Ni-Y-Co-La metallic glass was laser-melted onto an Al substrate which was at two different temperatures: 25 degrees C and 250 degrees C. It was found that the substrate temperature played a critical role in determining the interface bonding between substrate and support and final solidification microstructures. The higher substrate temperature resulted in the formation of a stronger interface bond between metallic glass and substrate while lower substrate temperature resulted in the formation of a weaker interface bond. This has been attributed to different cooling rates and thermal histories present in the two cases. A multi-physics-based computational model based on the heat transfer theory in heat transient mode of COMSOL(TM) was introduced to explain the underlying mechanism. (C) 2014 Elsevier Ltd. All rights reserved

On the study of tailorable interface structure in a diamond/Al12Si composite processed by selective laser melting

International audienceAluminum-diamond composites were produced using selective laser melting (SLM) from mechanically mixed diamond and Al12Si powders for use in thermal management applications. The feasibility of SLM for both interface tailoring and simultaneously densification, which are the key for determining the overall thermal conductivity (TC) of the 2 composites, has been investigated. The results indicate that the interface structure of the as-built composites can be controlled, despite the complex physical and chemical nature of SLM. A 'clean' diffusion-bonded interface revealed at the micron scale, which is desirable for enhancing TC, was obtained at a laser energy density of 95.2 J/mm 3. This demonstrates a possible new potential of SLM in creating metal matrix composites for functional applications. However, despite manipulation of the processing parameters, it was not possible to achieve a relative density above 90 %. Thus, how to realize near full density, while maintaining such a 'clean' interface during SLM remains a key technical problem for the future. The complex metallurgical mechanisms responsible for interface evolution and microstructure formation are discussed in detail with the aid of finite element analysis, and further densification strategies are proposed

A correlation between failure angle and constituent for Al-AlN composites under uniaxial tensile conditions

Al-AlN composites with different AlN reinforcement fractions and porosity were fabricated through nitridation of laser-sintered AA 6061 powder followed by infiltration with AA 6061. Their failure behaviors were investigated under uniaxial tensile loading conditions. Tensile testing and fractography indicate that the fracture mode changes gradually from ductile to brittle fracture with increasing AlN reinforcement or porosity. An analysis of the fractured Al-AlN tensile samples reveals that the failure surface angle, h, is dictated by the volume fraction of the matrix, fm, in a form of tan h = fm 5.5