Continuing the previous post on the environmental impact of constructing aluminium and steel, in the following paragraphs we will be reviewing their forming and welding methods.
We are at the stage that the plates and rolled profiles, bulb bars, angle bars etc, need to be curved. Some have 2d curved shapes, like the frames in the middle part, while other have more complicated 3d curvatures, like the bulbous bow or the mast. There are three main methods for forming plates and profiles, namely the hydraulic press, the rollers and the heat line bending. The rollers can support the simpler formations and are quite fast, while the heat line bending method is mostly just for non-developable, thick plates and profiles.
Looking at their environmental footprint, two parameters are under the scope, energy that is always present and material waste. It is estimated that the overall power demand of metal forming from hydraulic presses, not specifically for shipbuilding, counts to more than 280 million kWh annually, which equal to the total energy consumption in Spain in 2014. Regarding material waste, hydraulic press bends, for example, a plate to a specific shape that after releasing the pressure, is slightly straightened through what is called the elastic spring-back. This phenomenon has to be factored in when setting the pressure, to ensure that the end result will be the desired one. Also in the case of heat line bending, the area where heat is applied is further away from the point of bend, which only an experienced craftsman can identify. Presently there are finite element models (FEM) simulating the effect and helping to better determine the lines, but it is still heavily depended on the individual performing the task.
Therefore if the elastic spring-back is miscalculated or the heat is applied at the wrong spot or with the wrong pattern, the plate goes to waste. In the case of steel, it might be possible to further form it and make corrections, but aluminium reaches plasticity a lot sooner and is less forgiving.
Needless to say that all aforementioned are addressing the CNC bending machines, not manual procedures, templates made of wood, created by 1:1 scale lines from the mould loft. I bet some of you don’t know what I am talking about, it is something from the history books. Yet 4 years ago, I was asked, as an owner’s representative, to supervise a yacht built in Egypt that they construct traditionally. This approach is considerably adding to the waste of wood material as well.
Next to forming comes welding, which is primarily done with Metal Inert Gas (MIG) for steel components, although stick welding is still practised, and Tungsten Inert Gas (TIG) welding for aluminium. The welding machines and processes are quite similar, with one main difference that the MIG is always using a metal filler. A small note is added for the advanced skills required for TIG welding, which laterally results to some extra scrap during the learning process of a novice welder.
In respect of the environmental impact, it is estimated that 99million US dollars are spent annually worldwide for the powering welding machines, a figure accountants best relate to. The amount includes the necessary idle time of the welding machine, which percentage wise is quite high. Regarding the employed gases, both argon and helium have no known negative footprint on the environment. Yet helium can’t be artificially made, is finite in our atmosphere and has the tendency to escape to space. It is also the only medium that can support some medical equipment like MRIs, therefore it would be beneficial to use the existing alternatives not helium. Another note for argon is that in closed, unventilated spaces, it can cause suffocation.
The last paragraph is dedicated to the connection between steel hull and aluminium superstructure, which can’t be done with arc welding. In most cases bimetallic strips are employed, created through explosion welding. As their name denotes, they are formed with the detonation of one of the two metals. It is a cold, solid state welding, you can touch the bond with naked hand immediately after the welding, but it is dangerous and sometimes unsuccessful. Even when it is successful, it produces a lot of metal waste. New developments have introduced magnetic pulse welding, that has the same effect, without the same risks.
Rounding off points presented in the previous paragraphs.
- Once again we are obliged to stress the necessity of clean energy;
- FEM analysis focused on the heat line bending can reduce energy consumption taking the load from hydraulic presses and material waste;
- Using inverters for the welding machines improve their efficiency up to 85% and reduce consumption during down time. Old welding machines need to be retired at the first opportune time;
- Helium need not be used for TIG;
- Need further investment and development in magnetic pulse welding, as it can have so many applications;