Review on Age Hardening of Aluminium Alloys for Industrial Applications

Hey, Guys! In this blog, I'll be talking about my one of the favorite topics related to metallurgy "Age Hardening"

 

Aluminum Alloys are extensively used in the world with their most important use in vehicle, aerospace and defense sector because of their lightweight and exceptional strength due to Age Hardening effect, but their use and strengthening process is still very out of specific and other heat treatment processes, new discoveries and new Al alloys are much neglected industrially and on a research-level.

Aluminum is a soft metal with a tensile strength of 8-11 MPa but its alloys have a strength of 200-400 MPa. Aluminum Alloys are extensively used industrially after Steel because of their extreme shear and tensile strength. The main reason behind this spectacular increase in strength of Al Alloys is because of Age Hardening effect. Age hardening effect was discovered in 1906 by Alfred William and its correct explanation and procedure were proposed by Merica, Waltenberg, and Scott in 1919 (Merica, 1920). Al-Si-Cu/Zn/Mg (3XXX, 6XXX, and 7XXX series) Alloys are well studied due to their alloying effect and microstructural change in the host metal (AMA, 2012). They have great industrial importance mainly because of their high strength and low weight. Al Alloys have a strength of about 200-400 MPa but after Age Hardening it increased to ≥0.7 GPa (Peter V. Liddicoat, 2010). The best ductility was found to be 79.2% and elongation to fracture ratio to 23.1% (Remøe, 2014). We can see that these alloys have great use in heavy mechanical industries as well as in daily use items. But since the discovery of age hardening effect what we have seen is the focus on Al alloys is very centered and other heat treatment processes are shadowed. Although T1-T6 (Temper Designations) for these alloys are applied other heat treatment processes are neglected. Moreover, new alloys are also manufactured having more strength and efficiency.

Age Hardening: the following figure shows the effect of age hardening in α-θ Alloy. Θ metal precipitates out after it is cooled down and it is embedded around the grain boundaries.

Age hardening is an effect that is applied to those alloys in which the alloying element has a low solubility. Enthalpy (H) and Heat Capacity (Cp) control the solidification rate while chemical equilibrium is controlled by Gibbs Free Energy (G). For Al-Cu alloy like Al2024 for age hardening effect it is heated to 450-500 Degree Celsius and then cooled down to room temperature (Natural Aging) or heated to elevated temperatures (Artificial Aging). Copper has low solubility in Al hence before heating there was a saturated solid solution between Al and Cu. When the alloy was heated at a high-temperature excess Cu was dissolved in Al and a super saturated solution formed. After heating when the alloy is cooled down excess copper precipitates out and embeds in the line of dislocations. Due to this hindrance between dislocations when a stress is applied dislocations move due to which deformation is caused, but these precipitates hinder the movement and hence the material gets strong. The same case is for Al Alloys but this small microstructural change made an evolution in the Heavy mechanical industry. This process may be of very beneficial use yet this great strength in Al alloys comes with a cost of reduction in ductility and poor conducting properties and coarser microstructure

It was thought that Al alloys were immune to H2 embrittlement because of the hindrance of the precipitates and were extensively used in high pressure and temperature areas like aerospace industry and defence industry but there have been researches which have proven this wrong and Al alloys are not immune to H2 embrittlement (Rafiq A. Siddiqui, 2007).

There have been researchers made but these were mostly made 6XXX and 7XXX series alloys although they are mostly used but since they have a lot of limitations as well. There are other Alloys as well twice as strong as normally used even they have better properties Al-Fe alloy is an example. Moreover, nanostructured hierarchy increases the strength of aluminium alloys to <1 GPa using modern techniques like Severe Plastic Deformation (SPD) techniques and much more (Peter V. Liddicoat, 2010).

Table: This table shows the used of 6xxx Al alloys in the heavy mechanical industry. F corresponds to wrought Al alloys while R Shows Cast Al Alloys (Mukhopadhyay, 2012)

We can say that age hardening has proven itself a very useful technique but a comprehensive research study of all heat treatment stages is still required. Further, the study on Al alloys has only been done on wrought alloys and cast Al alloys whose microstructural changes are more complex and important because of their use in the aerospace industry have been overlooked. Hence there is a lot to study on other aspects as well and looking only one side of a multisided technology for almost a hundred years is not a good scientific approach as well.

References:

AMA, M. (2012). Influence of Mg and Solution Heat Treatment on the occurrence of Incipient Melting in Al Cast Alloys. Journal of Material Science Engineering, A543 22-34.

Merica, W. S. (1920). Heat Treatment and Constitution of Duralumin. Heat Treatment and Constitution of Duralumin, LXIV, 41-79.

Mukhopadhyay, P. (2012). Alloy Designation, Processing, and Use of AA6XXX. International Scholarly Research Network, 16. doi:10.5402/2012/165082

Peter V. Liddicoat, X.-Z. L. (2010). Nanostructural hierarchy increases the strength of Al Alloys. Nature Communications. doi:10.1038/ncomms1062

Rafiq A. Siddiqui, S. Z.-W. (2007). HYDROGEN EMBRITTLEMENT IN 6063 ALUMINUM ALLOY. 11th International Research/Expert Conference “Trends in the Development of Machinery and Associated Technology”, (p. 4). Hammamet, Tunisia.

Remøe, M. S. (2014). The Effect of Alloying Elements on the Ductility of Al-MgSi Alloys. NTNU-Trondheim.

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