Heat treatment of aluminium

The most important of these are the refractory cements formed by the heat treatment of aluminium acid phosphate solutions. This subject has been well reviewed by Kingery (1950a), Morris et al. (1977), Cassidy (1977) and O Hara, Duga Sheets (1972). The chemistry of these binders is extremely complex as the action of heat on acid phosphates gives rise to polymeric phosphates, with P-O-P linkages, and these are very complex systems (Ray, 1979). 

Aluminium-base alloys resist the action of many molten salts which are nearly neutral in reaction. Molten sodium nitrate or mixtures of sodium nitrate and potassium nitrate are used for salt bath heat treatment of some aluminium alloys. 

In nature aluminium oxide is mostly mined as the minerals bauxite and laterite, but these as extremely impure. Most bauxite is purified according to the Bayer process which removes the oxides of iron(III), silica and titanium. This takes place by autoclaving the bauxite with sodium hydroxide and sodium carbonate. The precipitated aluminium hydroxide is subsequently heated, or calcined. Calcination involves a heat treatment of a powder as a result of which the latter breaks down ... 

The original synthesis of diethyl teUnride by Wohler was carried out by treating potassium telluride with ethyl sulfate. The preparation of potassium telluride was rather inconvenient and involved the reduction of elemental tellurium by heating the element at red heat with potassium-hydrogen D-tartrate. An early report of Natta demonstrated that the treatment of aluminium telluride by alcohols or ethers at 250-300 °C is also a promising route to diorganyl tellurides. 

Using Al MAS and Al MQ MAS NMR, dealumination of zeolite H- 3 has been observed at specific T-positions in the framework. " Al MQ MAS NMR was able to resolve aluminium in the Ti and Tj positions from the other (T3-T9) positions in the framework. A quantitative analysis of the Al MQ MAS NMR spectra has shown that aluminium atoms in positions Ti and T3 resist dealumination and do not adopt an octahedral coordination. It has also been shown that a heat treatment of NH4-P gives a single type of fairly symmetric framework octahedral aluminium, which can be reconverted into framework tetrahedral aluminium by ammonia treatment. 

Fig. 20.52 Some of the microstnictures observed during the heat treatment of precipitationhardening aluminium alloys...

Salt baths that are used for certain thermal treatments of aluminium alloys such as homogenisation and solution heat treatment are mixtures of sodium nitrate (melting point, 307 °C) and potassium nitrate (melting point, 334 °C)." The mass loss of aluminium in a nitrate bath is 20 mg-m -h of immersion, which corresponds to a decrease in thickness of less than 0.01 p,m h. This is very acceptable for thermal treatments that do not exceed a few hours. 

Of the generic aluminium alloys (see Chapter 1, Table 1.4), the 5000 series derives most of its strength from solution hardening. The Al-Mg phase diagram (Fig. 10.1) shows why at room temperature aluminium can dissolve up to 1.8 wt% magnesium at equilibrium. In practice, Al-Mg alloys can contain as much as 5.5 wt% Mg in solid solution at room temperature - a supersaturation of 5.5 - 1.8 = 3.7 wt%. In order to get this supersaturation the alloy is given the following schedule of heat treatments. 

In the case of the aluminium-magnesium system, most commercial alloys are usually supersaturated, so that elevated service temperatures and inexpert heat treatment are inadvisable, since any resultant grain boundary precipitation may induce susceptibility to intercrystalline attack. The extent of this susceptibility may be approximately deduced from the continuity of... 

In addition to examining pre-exposure effects, the slow strain-rate testing technique has been used increasingly to examine and compare the stress-corrosion susceptibility of aluminium alloys of various compositions, heat treatments and forms. A recent extensive review draws attention to differences in response to the various groups of commonly employed alloys which are summarised in Fig. 8.57. The most effective test environment was found to be 3 Vo NaCl -F 0.3 Vo HjOj. The most useful strain rate depends upon the alloy classification. 

Heat treatment after plating Heat treatment may be necessary after plating to improve the adhesion of coatings on aluminium and its alloys when certain processes, e.g. the Vogt process, are used, or to minimise hydrogen embrittlement of steel parts. Care is needed since heating may distort the part and impair the mechanical properties of the substrate. 

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