Compared to most other Al alloys of the 6xxx series, 6016 contains a lot of Si. Given the low Mg content, it is much more Si than what can be used for the precipitation of beta double prime (~Mg5Si6; even when accounting for Si loss to coarse AlFeSi-type phases).
Hirth et al. (https://doi.org/10.1002/9781118787601.ch9) report that excess Si increases T4 strength, but the mechanisms are not entirely clear and the paper overall received little attention. Hirth et al. also report that excess Si appears to have little effect on yield strength after artificial ageing.
In my understanding, low T4 and high T6 strength are often desired (i.e., high hardening potential after forming). Then, adding so much excess Si appears counter-productive.
Silicon in this alloy forms particles not only with magnesium.
The iron contained in large quantities in alloy 6016 (0.5%) and manganese (0.2%) take over part of the silicon. Part of the silicon goes to the formation of Al(FeMn)Si particles (approximately 1/3 of the total iron and manganese content).
By the time of thermal hardening of the profiles at the outlet of the press, part of the silicon is already in a bound state with iron and manganese in the primary Al(FeMn) particles Si, which are formed during the casting of the extrusion billets.
*Iron reduces the tendency to cracking during casting, prevents cracking of products during heat treatment.
*Manganese increases corrosion resistance, ensures the preservation of strength under mechanical loads.
Let's look at the chemical composition (the elements we are interested in) of alloy 6016 and make calculations of the silicon we need.
Recommended values: Si - 1,0 - 1,5
Fe - 0.5
Mg - 0.6 (Take the upper bound)
Mn - 0.2
Amount of silicon by magnesium: Si= 0.6/1.7
The amount of silicon by iron and manganese:
Si = 1/3 * (0.5+0.2)
The total amount of silicon required: Si = 0,59.
The remaining 0.4 silicon in the alloy will be in excess, as well as in alloy 6016 a high content of copper (0.2%), which also forms a phase with aluminum, silicon and magnesium.
I still haven't answered your question. Thermally hardened alloy 6016 is used in the automotive industry, including for welding. The features of the weldability of such alloys are an increased tendency to form hot cracks during welding and softening in the near-seam zone. The tendency to form hot cracks is determined by the presence of a low—melting triple eutectic Al—Mg—Mg2Si, as well as double eutectic Al—Mg2Si and Al-Si, extending the interval of the solid-liquid state of the alloy.
To reduce the tendency to hot cracks, the silicon content is increased. I think the Fe/Si ratio < 1 is important here. Therefore, the silicon content in alloy 6016 of 1% with an iron content of 0.5% is acceptable.
Hello,
Please give a look at these papers.
Article Effect of alloy composition and heat treatment on mechanical...
Article The influence of Mg/Si ratio and Cu content on the stretch f...
Article Property Criteria for Automotive Al-Mg-Si Sheet Alloys
They do consider very similar compositions. It looks like there is a low Mg/Si to keep high work hardening capacity and resulting formability.
Indeed it is amusing to have so many compositions developed many decades ago and left for us to dig into their nature. Especially it applies to 6xxx big family.
Hope it helps
Best wishes,
Pavel
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