Temper, temper!
While the two motions on the rigidity of masts were true or withdrawn at the AGM villas and useful discussion about the subject.
Since I have been in the State Association there have been three lots of masts ordered, two of which I have been responsible for. We buy them from Capral and in the order we specify the alloy we require, which is 6106; the temper T6; the profile which is +/-0.1mm and lineal Distortion at +/- 1mm.as we have to buy a minimum of 250 kg this equates to just over 30 mast lengths of 70 m.
I think the most interesting information was the comments that Sandor made relating to the production of the exclusions.
He pointed out that this the minimum amount of aluminium that we could order was 250 kg and this was made up of a number of the billets or pigs which go into the furnace to make up the required weight. Now aluminium is not just a pure metal but it is blended with others to enable the right kind of mix to be established, just like your gold or silver. It may not be that these billets will come from the same production batch but come from several batches and therefore have a propensity to be very slightly different.
The aluminium is forced at high-pressure through a dye to make the particular shape of the extrusion. Sandor commented that the first three or four lengths coming through this would probably be to the specified size, but as more extrusions come through it will heat & expand and there will be a very small change in the width of the product coming out. Once the product is extruded it would be put in a stack and as we are buying 30 or 40 lengths of mast these will be put in a heap to cool. Obviously the ones on the outside and particularly the exposed edge will cool faster than those right in the middle of the pile, and this means that the ones in the middle will be of more rigid nature.
What you have your mast having it and a guide will also change its temper and even putting some paint on it can affect, of course its thickness.
Generally speaking over the years that these sections have been used they have been found to be quite satisfactory and problems are only a rising if votes of being pushed beyond their design limits.
I sent this article to the General Manager of Comalco, and his reply is below. I thank him for permission to re-produce.
“Hi Barrie
Glad you found my response useful. I have an interest in your subject as I have been in extrusion and anodising for 30 years and a sailor for 45!
I am happy for you to use the letter as you see fit.
Thank you for your letter of 15 August 2007.
I have a few comments in general on extrusions and mechanical properties and then I will comment specifically on your article "Temper Temper"
Aluminium alloys chemistry, mechanical properties and extrusion tolerances are covered by specifications from the Aluminium Development Council of Australia. These specs are pretty much the same around the world.
There are tolerances or upper and lower limits on the alloying elements of magnesium, silicon, iron and manganese (main elements in 6000 series alloys). These are all present in small quantities and while the smelter supplies to much tighter limits than the spec allows, there will be some small variations in mechanical properties arising from these variations.
The 6000 series alloys are known as heat treatable alloys and therefore the rate of heating and cooling during the extrusion process and subsequent heat
treatment (known as ageing) will also introduce some variability in mechanical properties of the final product.
The mechanical properties specification for an alloy and temper are given as a minimum ultimate tensile and yield strength. There is no range of upper and lower limits, just a lower limit. So for 6106 T6 which has a minimum ultimate tensile strength of 235 MPa typical properties could be 250-280 MPa.
So it is quite possible that you may detect some variation in hardness and deflection between different production runs.
Regarding the article Without looking at the profile, I would believe that the ADC tolerances would be greater than the +/- 0.1mm that you refer to. Extrusion is a hot work process and it is just not possible to maintain tolerances as tight as you quote. A 250 kg order is a very small order! For us it’s about 3 billets of alloy and 15 minutes of extrusion time. The alloy is supplied to us in logs 203mm in diameter and 6000mm long, each weighs about 500kg. The batch size from the smelter is 30 tonnes. The log is heated to 450c and the billets are cut from the log just before extrusion. What this means is that the few billets that make up your order is likely to come from the same log and certainly from the same smelter batch.
Before extrusion the die is also heated to 450c so that from the first billet we are close to equilibrium conditions. This means that there will not be much difference in the physical dimensions of the material from the start to the finish of the run. In your case the run is very short any way.
We would prefer to make at least 1000kg when we put a die to the press!
The shape is extruded onto a cooling table in lengths of 40-50m(the product of a billet). Each length experiences the same cooling rate from forced air quenching until it is at room temperature. It is then stretched straight and cut to final length before being stacked into a skip.
The extrusions are soft at this stage so need to be heat treated to achieve final properties.
This is achieved by heating to 180-190 c for 4 hours. This is a batch process in a large forced draught oven. Heat up is quick particularly for tube shapes due to the low mass and high surface area so we would not
expect to see any significant variation in properties between those pieces on the inside of the skip vs outside.
The variations in the alloy and process will inevitably result in some variation in the final product. Paint will not impact on the strength of the extrusion
Anodising will decrease the wall thickness because part of the anodising process is etching on the surface. This would typically remove 0.1mm from the inside and outside of the tube wall ie wall thickness is lowered by
0.2mm. The anodic film that is then created on the etched surface is typically 20 microns thick and is just a nice uniform layer of aluminium oxide. By the way, the net result of the process is an over all reduction in
thickness. Aluminium oxide is very hard but has a very low tensile strength. It is also thin compared to the ductile aluminium below. So when anodised
aluminium is flexed the anodic film (anodising) will crack. You will just not see it! It does not change the temper or mechanical properties of the extrusion below the film. I believe anodising is the best surface treatment for aluminium in the marine environment but I think it is unlikely that it contributes the stiffness of your masts. You have also traded off the wall thickness
reduction to have the anodising. The best outcome would be to minimise the etch part of the anodising process to just enough to clean up the surface.
Deflection in a tube is easy to calculate so one of your members might like to work out the impact on deflection of small variations in wall thickness
and temper on the final product, I think you may find that the variation in deflection is small.
Drawing is a cold work process that is used to make rods and tubes. Typically, the manufacturing process is to first make an extrusion that is larger than the final desired drawn shape, then the extrusion is drawn or
pulled through a die to make the final shape. Because the product is cold worked it is possible to achieve strengths and tempers higher than that of extrusion eg T8. The process is generally limited to simple rods bars and tubes. Unless your mast profile is a round tube this is not an alternative to extrusion.
Hope this helps Thanks and regards
Martin Haszard