Lecture 17 Aluminium and its alloys

Transcription

1 Lecture 17 Aluminium and its alloys

2 Aluminium and its alloys Reference Text Higgins RA & Bolton, Materials for Engineers and Technicians, 5th ed, Butterworth Heinemann Section Ch 17 Additional Readings Section

3 Aluminium and its alloys Note: This lecture closely follows text (Higgins Ch17)

4 Aluminium (Higgins 17.1) READ HIGGINS 17.1 Aluminium is very reactive strongly electropositive and readily combines with all the non-metal (electronegative) ions. So it wasn t produced until 1825 (Oersted) when it was more expensive than gold. Not any more! Aluminium Wikipedia

5 Uses of aluminium READ HIGGINS 17.1 Demand for Aluminium was driven by aircraft. Today it is widely spread though most industries. Lightweight Durable Easy to form, extrude, diecast Good finishes High strength to weight

6 Worldwide Aluminium Production Worldwide Aluminium production Aluminium needs electricity. It is actually easier to ship the ore to the electrical energy source rather than bring the electricity to the ore.

7 The extraction of aluminium (Higgins 17.2) READ HIGGINS 17.2 The modern electrolytic process turns bauxite (Al2O3) into aluminium metal. Unlike a blast furnace that removes oxygen (reduction) with coke, aluminium requires electricity to do this: All 91 megajoules of the stuff per kg! So aluminium product suits countries with ample hydroelectricity. (e.g. Norway)

8 The extraction of aluminium (VIDEO) VIDEO The making of aluminium [videorecording] Advanced version. Russell, Geoff Video Education Australia. DVD (30 min.) Explains the process of smelting aluminium using the smelter at Portland smelter in Victoria as an example. Detailed analysis of refining and smelting of aluminium. Covers production, economic, environmental issues. Excellent quality. DVD /RUSS

9 Properties of aluminium (Higgins 17.3) READ HIGGINS 17.3 Conductivity (electrical and thermal) Corrosion resistance Ductility for forming, extrudion Low MP for diecasting Highly machinable Lightweight Good finishes - anodising High strength to weight Lots of alloys and heat treatments

10 Aluminium alloys (Higgins 17.4) READ HIGGINS main classes of aluminium alloys Non-heat Treatable Heat Treatable Wrought Cast Traps to watch out for 1. A common mistake is to call aluminium components alloy simply because it starts with al. Brass and even steel is alloy 2. In USA, Aluminium is called Aluminum. Same stuff, different name.

11 Aluminium alloys (Higgins 17.4) READ HIGGINS Wrought alloys Cast alloys READ HIGGINS Wrought alloys which are not heat-treated 17.6 Cast alloys which are not heat-treated

12 Some common aluminium alloys

13 Aluminum alloys They are normally identified by a four figure system which originated in the USA and is now universally accepted (with variations usually extra digits). Note: Higgins is based on British Standards (BS) from which most Australian standards are based. Where it is written 1473: 5083 It simply means BS 1473 and grade 5083 (e.g. table 17.1)

14 Higgins

15 READ HIGGINS Figure 17.2 The aluminium-silicon thermal equilibrium diagram. The effects of 'modification' on both the position of the eutectic point and the structure are also shown. Higgins

16 Higgins

17 (i) 12 per cent silicon in aluminium - unmodified, as cast. Since this alloy contains more than the eutectic amount (11.6 per cent) of silicon (see Figure 17.2), primary silicon (angular crystals) are present. The eutectic is coarse and brittle and consists of 'needles' of silicon in a matrix of a solid solution because the layers of a in the eutectic have fused together to form a continuous mass (the amount of silicon being only 11.6 per cent of the eutectic so that the layers of a would be roughly ten times the thickness of those in silicon), Higgins

18 (ii) The same alloy as (i) but modified by the addition of 0.01 per cent sodium. This has the effect of displacing the eutectic point to 14 per cent silicon so that the structure now consists of primary crystals of a (light) in a background of extremely fine-grained eutectic (dark). The alloy is now stronger and tougher, Higgins

19 (iii) A duralumin- type alloy in the 'as extruded' condition (unetched). The particles consist mainly of CuAl2 (see Figure 17.4) elongated in the direction of extrusion. Most of this CuAl2 would be absorbed during subsequent solution treatment. Higgins

20 Wrought alloys which are heat-treated (Higgins 17.7) Figure 17.4 Structural changes which take place during the heattreatment of a duralumintype of alloy. Higgins

21 Wrought alloys which are heat-treated (Higgins 17.7) Figure 17.5 The effects of time and temperature of precipitation treatment on the strength of duralumin. Higgins

22 Wrought alloys which are heat-treated (Higgins 17.7) CAREFULLY READ Heat-treatment Age Hardening

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25 Cast alloys which are heat-treated (Higgins 17.8)

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27 Online Resources. Aluminium Non Ferrous Metals

28 GLOSSARY Bauxite Electrolysis Precipitation hardening Age hardening Wrought Cast Anodising Oxide layer As quenched Alumina Glossary

29 QUESTIONS Moodle XML: Some questions in Non-Ferrous 1. Define all the glossary terms. 2. Aluminium has been dubbed solid electricity. Producing 1 kg of Aluminium uses 91 MJ. Calculate the cost to produce 1 kg of aluminium based on current domestic electricity charges. What rate would you expect a smelter to pay? 3. It has been stated that aluminium is the most economically viable material for recycling. Comment on this statement using a comparison of current recycling values for other common scrap materials like metals, plastics and paper products. Ref: List the proportions of Aluminium that are lost during recycling. 4. Why have car radiators switched from copper to aluminium? Explain why these two metals dominate other areas like evaporators and condensers in air conditioning and heat exchangers for heat reclamation systems for reducing energy losses in manufacturing and process plants. 5. Explain the age hardening process and mechanism for an aluminium alloy such as Duralumin. 6. Explain why Aluminium is highly reactive yet is used for its corrosion resistance.