Production of Aluminum Foams with Ni‐coated TiH₂ Powder

Abstract

A major drawback in producing aluminum foams by the powder metallurgy technique is the temperature mismatch between the aluminum melting point and the decomposition temperature of the blowing agent. This mismatch is believed to lead to poor foam expansion and to cause a lack of pore uniformity resulting in non-isotropic and non-reproducible properties of the foams. [1] The reduction of the above mentioned mismatch is thought to improve aluminum foam process control leading to uniform closed-cell foams.Oxide passivation has been used in the past in order to find a way to reduce the temperature mismatch. [2,3] The higher the oxidation temperature the thicker the surface oxide layer and the higher the decomposition temperature of TiH 2 powders, nevertheless the thicker the oxide layer the less hydrogen that remains in TiH 2 powders to produce the foam expansion. Moreover, for actual foaming conditions, the oxide layer, if not created carefully, is prone to breakage due to the stress caused by the mixing, compaction or extrusion steps required to produce the foamable compact. [4] A different hydrogen diffusion layer is the creation of a SiO 2 coating by the sol-gel method which avoids hydrogen loss, but is not suitable in the powder metallurgy route due to the inherent brittleness of oxides. [5] During this research an alternative diffusion layer is deposited over TiH 2 particle surfaces to avoid loosing hydrogen by creating a diffusion barrier but, more importantly, to delay the dehydrogenation onset under actual foaming conditions. An electroless coating technique was used to produce the nickel coatings on TiH 2 powders. This technique produces uniform coatings disregarding particle size and shape. [6] The coating process when performed on non-conductive surfaces involves an activation step prior to deposition in which the powders surface is first chemically cleaned and micro-roughened to be catalytically activated with Pd. The minimum reagents required in the coating bath are a source of nickel ions and an appropriate reducing agent. Deposition rates are a function of the reducing agent concentration at a given pH and temperature. [7] The deposit thickness can be controlled by modifying the reducing agent concentration, the time, the amount of powders to be plated and coating cycles. Experimental ProcedureElectroless Ni-coating on TiH 2 and Loose Powder Characterization: As-received and oxidized TiH 2 powders were catalytically activated and Ni-coated using an electroless Ni-bath of which the formulation and plating conditions are described elsewhere. [8] Prior to the activation process, some TiH 2 powders were heat treated in air inside a horizontal furnace with a soaking time of two hours at two different temperatures of 500 and 550°C to produce an oxide passivation layer.The hydrogen desorption behavior of TiH 2 powders in the as-received, oxide passivated, catalytically activated and Ni-coated condition were characterized by TG and DSC techniques (Setaram Evolution 24 thermo-gravimetric analyser), experiments ...

Current Status and Recent Developments in Porous Magnesium Fabrication

“…This causes premature hydrogen release and an actual porosity lower than that calculated. [24] Some research try to prevent this phenomenon by surface oxidation, covering the particles with a thin layer of nickel [24,25] or aluminum. [26] For foaming a Mg alloy (AZ31), initially an Al-10TiH 2 [26] composite, which contains TiH 2 particles entrapped in the aluminum matrix, is prepared.…”

Porous Metals and Metallic Foams: Current Status and Recent Developments

“…This can be done by oxidation [26,27] or by coating the particles with a thin layer of nickel. [28] Another option is to work without blowing agent (see Fig. 2) and rely on gas residues contained in powder or scrap that is added to the melt.…”

A kinetic study on the mechanism of hydrogen evolution in Ni–P coated titanium hydride powder

“…But the major drawback to the utilization of TiH 2 powder in aluminium foaming industry is that the thermal decomposition temperature range of titanium hydride (%400°C) is noticeably lower than the solidification temperature of most commercial aluminium alloys (600-650°C) [3]. This mismatch is believed to lead to poor foam expansion and a heterogeneous pore structure which can result in some non-isotropic and non-reproducible properties of metallic foams [5,6]. On the other hand, the pore size, pores texture and volume fraction of porosities in Al alloy foams are in close relationship with the thermal decomposition behavior of TiH 2 .…”