Cadone,

WELCOME TO THE AWS WELDING FORUM!!

Your minimum weld size needs to be determined by a qualified Engineer.  Your mentioned method is an often used rule of thumb by shade tree welders working to the Farm Code.  Someone needs to specify process, filler, size, etc. with calculations verifying application. 

Also, no one here knows what, how many, or even if any kind of gussets are also being used.  We cannot engineer this or recommend proper procedures for you.  Including weld size.

Now, with that alum I believe you are correct that some form of treating needs to be done after the welding process.  Again, exactly how (amount of heat, how long, etc) needs to be calculated and called out by your engineer.  There may again be rules of thumb for the thickness in use, the filler and base metal combination, etc, but there is only one person who can prescribe this for your public safety project.  The public safety issue is assumed as you state they are tall light poles.  Usually people around where there are light poles.

Have a Great Day,  Brent

Hi Cadone

This is an interesting question that has a number of possible answers depending on a number of different situations. The final answer is that the weld sizes and any PWHT needs to be "engineered", so we could not give you a definitive answer. Having said that, I can give you some basic principles of working with Al alloys that will help you to grasp the issues, then I can do a little speculating, but it can not be taken as gospel. So principles are:

1) Structural Al alloys are broadly speaking of two types: Heat treatable and non-heat treatable.
2) Heat treatable alloys (The 6000 series and A356 are heat treatable.) are strengthened by precipitation hardening. This entails (as mentioned by Al) heating the material to the solutionising temperature, followed by a quench, followed by an "ageing" treatment, usually around 150 - 200 deg Celcius, depending on the particular alloy. Some will age at room temperature.)
3) The non-heat treatable Al alloys are strengthened by cold work. (Strain hardening)
4) Welding the heat treatable alloys will give a Heat Affected Zone (HAZ) that has an area of solutionised material, along with an area of approximately correctly aged material, and then a zone of over-aged material. In short, depending on the temper you start off with, the Al alloy material will lose up to 50% of its strength in the HAZ. (For your T6 temper, you will indeed lose almost 50% of strength in the HAZ.)
5) Welding cold worked alloys will cause the material to be annealed in the HAZ, also losing strength. Again, depending on the previous degree of deformation, it can typically be around 30% loss of strength.
6) The long and the short of it is that welding structural Al alloys will always result in a loss of strength in the HAZ.
7) In the case of the non-heat treatable Al alloys, there is not much that can be done about the strength loss, as it is almost never possible to subject the material to the required cold work to restore the HAZ properties.
8) In the case of the heat treatable alloys, it is theoretically possible to re-heat treat the component to restore the material properties in the HAZ. This is often done to smaller components, but in the case of most structural applications this is not practical, as larger components cannot be handled in their heated state (high temperatures for solutionizing the material, and then quenching) without deformations that would make the structure useless.
9) In case you are wondering why this PWHT can not just be applied locally, it is simply that when the heat is applied locally, there will always be a temperature distribution from the hot to the cold, that will then again result in the same over aged structure as when welded. Only now the over aged area is much bigger, so the problem is actually worse, not better.
10) Generally the weld metal also needs to be formulated so that it is crack resistant, and is rarely of a matching chemical composition to the base metal being welded. The weld metals also need to be selected based on the final application, so they need not only weld cracking resistance, but also possibly aesthetic properties, (Probably important for light poles.) corrosion resistance, or the ability to be anodised. (Possibly also to be done to an architectural application such as light poles.) These fillers are generally formulated to perform best in the as welded condition.
11) As a result, most structures are designed with the understanding that the weld will be left "as welded" without PWHT. For this reason, the components must be designed in such a way that the weld falls in a region of lower stress, or additional structural members are added to compensate for the loss of strength. (e.g. Gussets) In short, a full section weld will therefore not result in a full strength weld. Increasing the weld size (e.g. fillet weld size) may give the weld more strength, but does not change the HAZ properties, which would then become the weakest link in the chain.

Speculating on your application:

1) Welding the 6061 alloy WILL result in reduced strength in the HAZ. (Up to 50% reduction.) I will be very surprised if it is practical to perform a PWHT on the entire light pole. (Will be different if the bottom section is made as a separate component and bolted to the rest of the pole.)
2) The use of the A356 casting is interesting in as much as the welding of this component, if furnished in the solutionised condition, will generally result in a material largely still in the solutionised condition. By then applying an ageing treatment of around 160 deg Celcius for around 10 hours, the full strength of the component can be developed. This would however result in some over-ageing in the 6061 material so it may or may not be a viable option, depending on the relative stress states of the two parts being joined. I would think that the base plate would probably be highly stressed around the edges where they are bolted down (Assuming they are bolted down.) so it may be more important to get maximum strength in the base plate at the expense of the pole section, especially if the pole section could be thickened for the lower 200mm or so. But then again, the base plate casting could also be cast with a larger load bearing section in the higher stressed areas.

All the assumptions above should allow you to see that the welding needs to be engineered along with the design and materials selection process. This integrated approach is more important in Al alloys than most other metallic materials.

In terms of quality control, the best that can be said is that the design will dictate the quality requirement of the joints. Most Al structural codes have different "classes" of joint, depending on their stress state, and each one will have differing quality requirements. Mostly Al is just subjected to a rigorous qualification process, and then normal visual inspections and liquid penetrant inspections in some cases. It can also be instructive to perform hardness testing, as the strength of the material is directly proportional to the hardness. (Just a word of caution in performing hardness tests on Al. - This too needs to be well understood and applied to get meaningful results.)

Hope this gives you some appreciation of the issues involved with the welding of Al structures.

Regards
Niekie