Welding in Aluminium Heat Affected Zones (H.A.Z)

08 Nov

Transcript

Hello and welcome Back to the Drawing Board with me Vicky Mastoridou today we'll be talking about welding and aluminium and the heat affected zones. Aluminium is a really interesting material it is low in density therefore it's quite lightweight and its mechanical properties can be significantly increased after alloying , from 90 Mega Pascal up to 512 Mega Pascal meetting the properties of a really good quality steel. However when we ‘ve got weld all these mechanical properties are going away. Let's see what is happening: When welding, heat is being generated and this heat softens the material in the close proximity to the weld. These are the heat affected zones.

Eurocode 9 suggests two ways to approach the heat affected zones and this softening of the material. One is to reduce the cross-sectional area by a factor the value of which depends on the class in which the cross-section belongs to. The other way is to keep the cross-section of the parent material and reduce the mechanical properties using this time the 0.2 proof strength in the heat affected zone or the ultimate tensile strength in the heat affected zone. Again, which one we are going to use depends on which class our cross-section belongs to.
 

The width of the heat affected zone is been affected by multiple factors one of them is a type of welding. In the tungsten inert gas more heat is being generated therefore the area affected is bigger. I'll give you an example if we had two pieces that are less than six millimetres thick welded together with a tungsten inert gas method the affected zone would be 30 millimetres. If we used the metal inert gas welding process that would be limited down to 20 millimetres. Another factor that is affecting the width of the heat affected zone is the proximity of other welds. When we've got two welds close to each other, the width of the heat affected zone is the width of the group weld because it may overlap. Sometimes in order to achieve the thickness of the weld we may need to lay down multipass welds. In this case we expect that the temperature will be increased. Eurocode 9 suggests that in cases we have temperatures above 60 degrees Celsius we should increase the area that we will consider with reduce properties. Last but not least, the thickness of the two elements welded together is also affecting the width of the heat affected zone. If the thickness of the material is different then we will account for the average thickness provided that this average thickness is less than 1.5 times the thickness of the smaller material. However if this is not the case then we will have to test to assess the heat affected zone.

Now that we have seen what the heat affected zones are and how Eurocode suggests an approach for the reduced strength properties let's see what is happening with the resistance. I'll give you an example: Let's assume that we've got one meter long square hollow section 50 x 50 x 4 that is in pure compression. The compression resistance of this material will be determined by the cross section of the area multiplied by the ultimate tensile strength divided by γM2 factor. Now if we didn't have a whole one metre piece and we had two half meters and we wanted to weld all the way around to create the desired length then in this case we should account for the heat affected zone created by this weld. So as Eurocode suggests we will use the cross-section area in the heat affected zone therefore it's been calculated to be half of the initial. This means that the resistance will be half of the parent material in the heat affected zone.

This is just a simple example to show you how the resistance of material can be affected in the areas close to the weld. I hope you enjoyed this video if you liked it please subscribe. Hope to see you again back to the drawing board.

Tony Gould
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