Precipitation Hardening of a

Heat Treatable Alloy

MPB014

Abstract

          When looking at Aluminium alloys, the process of Precipitation Hardening is a relatively fast and cheap way of increasing the strength and hardness. This comes hand in hand with reduced ductility and toughness, and if aged for too long, the alloy can be subject to a process called over-aging whereby a visible precipitation appears in the microstructure and the hardness levels start to decrease.[1]

          As the samples were left in the furnace for longer and longer amounts of time, precipitates of Mg2si appear within the structure of the Al. This precipitate is much harder and more brittle than the Aluminium which explains the rise in hardness figures.

Introduction

          Precipitation hardening is a widely used process to increase the hardness in a usually non-ferrous metal that have high level of malleability. It was first introduced in the early 20^th century on a material called Duralumin by a professor in America. In essence, the process produces a uniform dispersion of a very fine coherent precipitate in a softer, more ductile base matrix.

The process has three stages, Solution treatment, Quenching, and Ageing. In the Solution treatment stage, the alloy is heated and held above the solvus temperature to create a solid solution. This stage dissolves the precipitate and creates a homogeneous un-segregated solution that is the starting point of the process.[2]

The quenching stage is just cooling the alloy very rapidly. This forces the particles in solution to solidify as they are as they don’t have time to diffuse to potential nucleation sites. After the quench, the structure contains Aluminium particles with Magnesium silicate in solid solution. The Ageing process is where the supersaturated solid solution is heated below the solvus temperature, this allows some magnesium silicate atoms to diffuse to nucleation sites and grow.[2] The best properties can be obtained by stopping this process before it gets to the stage of Over-ageing.

The industrial process of making aluminium drinks cans utilises this process extensively. To obtain the perfect can, the compromise between the amount of hardness needed to keep the can stable and the amount of ductility needed in order that the can form properly is vitally important.

Cans are produced by a method called Impact-Extrusion, whereby a punch is driven into a circular slug of Aluminium creating the base and sides all in one piece.[1] If the Aluminium alloy used is too hard, there will be cracks appearing in the furthest corners of the base, but if the aluminium is too soft, a phenomenon called ‘earing’ occurs, where metal is forces out the top and over the edge, forming a lip that needs to be trimmed off before the can can be used. 

Experimental Procedure

        Eight identical pieces of Aluminium alloy HS30, with composition within the limits of 0.4-1.5% Magnesium, 0.6-1.3% Silicon, 0.6% maximum Iron, 0.4-1.0% Manganese (the balance being Aluminium) were placed in a holding furnace at 525°C.

          These were solution treated in the furnace for 15 minutes, whereupon seven of the samples were removed from the furnace and quenched in cold water. The furnace was then switched off and the eighth specimen allowed to cool inside the furnace very slowly. Care was taken to ensure that the temperature did not rise above the maximum solution temperature for the alloys as it is very close to the solidus of the Aluminium alloy.

          One specimen was tested as a freshly quenched sample, the others were put in another furnace at 200°C and individually taken out to be tested at intervals of 2.5, 5, 10, 20, 40, and 80 minutes. These samples were cooled in water before each test.

          The samples were tested using a Vickers Hardness machine with a 5kg load. Three hardness impressions were made on the outside of each sample, taking the average as our usable result.

          The ductility of each sample was obtained using an Erichson cupping tester. Each sample was tested by forcing a ball bearing to deform the surface, stopping when the first cracks appeared. The height of the deformation was measured, thus giving a ductility result.

Results

Discussion

          The process of precipitation hardening is a solution treatment process. In this process, the alloys is first heated above the solvus temperature and held until a homogeneous solid solution is produced. This step reduces the segregation within the original alloy.

When the aluminium alloy is heated to 525°C in this process, the Magnesium and silicon molecules react and combine to form Magnesium silicate or Mg2Si. At this point, the Mg2Si is in solution with the Aluminium, much like salt in water. When the first sample is quenched in water, the magnesium silicate particles do not have time to diffuse to potential nucleation sites so are trapped as a solid solution. This mean the Aluminium is at its purest form in the freshly quenched sample as the impurities are trapped outside the aluminium grains. This sample is different to the others as putting the others in an oven at 200°C artificially age-hardens them.

As can be seen in the graph, at the 2.5 minute stage, the hardness increases, this is due to very fine particles of Mg2Si coming out of solution and appearing within the Aluminium grains. These extremely fine grains are much harder and more brittle than Aluminium, and as these are now within the grains of Aluminium, the properties of the alloy start to change.

As the time continues, more and more Mg2Si appears creating a harder and harder alloy, with ever decreasing ductility. This continues until a point on both of the graphs at around the 50 minute mark where the hardness starts to decrease and ductility starts to increase. This is the point where the alloy moves into the stage of Over-ageing. This is where the alloy has been in the furnace for too long, and the particles of Mg2Si come together and form course particles.[4] The particles are larger than they were in previous stages, but the process of clumping together means that there is less surface area exposed. This in turn means that there is more Aluminium particles exposed, so the alloy starts to return to the original properties of Aluminium, hence the higher ductility and lower hardness.

Conclusion

• As the process continues, the alloy releases Mg2Si
• Mg2Si is harder and more brittle, so as more is produced, hardness goes up, ductility goes down.
• This happens until over-Ageing occurs at roughly the 50 minute stage.
• After this, more Al is exposed aiding ductility but hampering hardness.

References

1. The Metallurgy of Aluminium and Aluminium Alloys  -  Robert John Anderson,
2. The Science and Engineering of Materials  -  Donald Askeland
3. An Introduction to Materials Engineering and Science for Chemical and Materials Engineers - Brian S. Mitchell
4. Foundations of Materials Science and Engineering - William Fortune Smith