In all constructional applications where welded connections are used either a stress analysis is carried out or ample experience of acceptable performance exists for the specific joint design. The principles of stress analysis are outside the scope of this chapter - it deals instead with those shop-floor fabrication activities that the designer directly influences. There are many national and international specifications dealing with the design aspects of specific structures. For instance, BS 8118 deals with the structural use of aluminium, as does the US specification D 1.2. Pressure vessel design is covered by BS PD 5500 and ASME VIII. For advice on the design of such structures the designer can do no better than consult the relevant specifications. For a list of relevant specifications see Appendix A at the end of this book.
The objective of the designer is to provide an assembly with adequate strength for the specific application with the least amount of weld metal and the minimum number of joints. This requires the designer to plan for a smooth flow of stresses through the joint, to compensate for any strength loss due to welding, to design the component such that there is sufficient access for welding and to select the metal to be welded with optimum weld - ability in mind. As mentioned in Chapter 2 there is little that can be done to improve the strength of the weldment to match that of the cold worked or precipitation-hardened alloy. All that the designer can do to compensate for the loss is to thicken the component, either overall or locally, or to move the weld to an area of low stress. For advice consult British Standard BS 8118 or the AWS Structural Design Code D 1.2, as mentioned above.
There are a number of factors that the designer needs to take into account that are specific to designs in aluminium. Some of these have been mentioned in earlier chapters and include such physical properties as the high coefficients of thermal conductivity and expansion, the major loss of strength of certain alloys in the HAZ and the low Young’s modulus. In addition, the designer must consider access for both welding and inspection, joint design to enable high-quality welds to be made, the effects and minimisation of distortion and the effect of welding on stress concentrations and fatigue.
The ease with which a weld can be made is crucially dependent on joint design and this will have a direct effect on fabrication costs. It is thus essential that the designer is aware of certain fundamentals of welding practice in order to achieve the objectives of the lightest structure capable of performing its desired function at the lowest cost.
There are a number of ‘golden rules’ that the designer should keep in mind when detailing the drawings:
• Remember that weld metal is very expensive. Do not over-specify fillet weld throat thickness and specify the narrowest weld preparation angle that is consistent with quality. Specify these sizes clearly.
• Keep welding to a minimum - use formed sections instead of welded
plate, keep stiffeners to a minimum. The cheapest weld of all is the one
you do not make!
• Specify welds to be made in the flat position.
• Allow adequate access for the welder - see below.