Page 86 - Welding of Aluminium and its Alloys
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Welding design     75



                                45° equal leg  Convex fillet
              Mitre             length, regular                   convex face
              fillet            isosceles triangle                excess weld
                                                                  metal does
                                                                  not contribute to
                                                                  strength. Poor
                                                                  toe blend
                                              Unequal leg
                                  concave face                        L > L
              Concave                         length fillet            2  1
              fillet              reduced throat
                                  thickness if           L
                                  judged on leg           1
                                  length                      L 2
                    5.7 Mitre, convex, concave and unequal leg length fillet.



               For the fillet weld the shape of the weld and the amount of penetration
             into the root will affect the throat thickness. The effective throat thickness
             is t 1 in Fig. 5.6.The size of the fillet weld must be determined by the designer
             and should be of sufficient size to carry the load. The throat may therefore
             be completely different from the material thickness.
               The fillet weld may also be described as a mitre,a concave or a convex
             fillet. In addition the fillet weld may have unequal leg lengths. These four
             types of fillet weld are illustrated in Fig. 5.7.

             5.5.1 The butt weld

             The butt weld, typical forms of which are illustrated in Fig. 5.8, is a simple
             and easily designed joint which uses the minimum amount of material.
             Figure 5.8 also includes definitions of some of the features of a weld prepa-
             ration such as ‘root face’, ‘angle of bevel’ and ‘included angle’. Butt welds,
             as illustrated in Fig. 5.5, may also be classified as full penetration or partial
             penetration.
               With the conventional fusion welding processes of TIG and MIG pene-
             tration of weld metal into the surface of a flat plate from a bead-on-plate
             run is typically 3mm and 6mm respectively. To achieve a full penetration
             butt weld at thicknesses over these it is necessary for the two close square-
             butted edges to be bevelled, although leaving a small gap between the edges
             will increase penetration. Typical weld preparations for the various
             processes will be found in the relevant process chapter. Butt joints may be
             single or double sided – if double sided it is often necessary to back-gouge
             or back-grind the first side to be welded to achieve a joint that is free of
             any lack of penetration.
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