CHAPTER Heat-affected zone softening at welds 6.1 GENERAL DESCRIPTION An annoying feature in aluminium construction is the weakening of the metal around welds, known as HAZ (heat-affected zone) softening (Figure 6.1) With the 6xxx-series alloys, the heat of welding can locally reduce the parent metal strength by nearly half With 7xxx alloys, the weakening is less severe, but extends further out from the weld For work-hardened material (5xxx, 3xxx series), the metal in the HAZ becomes locally annealed, with properties falling to the O-condition level Only for parent metal supplied in the annealed or T4 condition can HAZ effects be ignored The metal in the HAZ may be weaker than the actual weld metal, or it may be stronger, depending on the combination of parent and filler materials used It often pays to locate welds in regions of low stress, i.e away from the extreme fibres or at a section of low moment in a beam A full-width transverse weld, as used for the attachment of a web stiffener, brings a high penalty as it causes softening right across the section A designer needs data on two aspects of HAZ softening: its severity and its extent The severity is largely a function of the parent material used, while the extent depends on various factors The ratio of the area of affected parent metal (transverse to the weld) to the weld deposit area can vary from or for a large multi-pass weld to 20 or more for single-pass It must be emphasized that the subject of HAZ softening is far from being an exact science A well-known method for estimating the extent of Figure 6.1 Zone of HAZ softening at aluminium welds Copyright 1999 by Taylor & Francis Group All Rights Reserved the softening is the famous ‘one-inch rule’ which often proves adequate, but not always [18] This simple method is explained in Section 6.5.3, after which we go on to present a more scientific treatment (RD method), which may be used to replace the one-inch rule in situations that demand a more accurate estimate of the HAZ extent (Sections 6.5.4–6.5.11) Many people think that HAZ softening is such a minor effect that a very rough estimate of its extent is all that is needed It is true that with large multi-pass welds, as used in massive members, the softened area only extends a short distance in relation to the size of the weld For these, almost any extent-rule will usually But with smaller welds, as used in thin members, the extent of the HAZ is relatively much greater and a better approach is desirable For these, the one-inch rule can lead to an estimate of member resistance that is unacceptably low The BS.8118 procedure for predicting HAZ extent gets the worst of both worlds, since it is more awkward to apply than the one-inch rule and often inaccurate Our proposed method is more realistic, although still fairly approximate compared to most structural calculations In recent years, special computer programs have become available, which accurately model the temperature changes during welding and the resulting metallurgical effect [19] For a mass-produced component, it may be sensible to employ one of these Alternatively, the HAZ pattern can be found experimentally by making a hardness survey on a prototype Designers should also be aware of the locked-in (‘residual’) stresses in welded components, even though these are not directly considered in the design process As with steel, there is a region of locked-in longitudinal tensile stress at any weld, balanced by compressive stresses elsewhere in the section But compared to steel, where the tensile stress at the weld is invariably up to yield, the stress levels in aluminium are relatively low The zone of locked-in longitudinal tension at an aluminium weld is generally narrower than the HAZ Mazzolani provides interesting plots of residual stress in welded aluminium members [26] Most of this chapter specifically covers HAZ softening at welds made by the MIG process TIG welds, for which the HAZ effects are much less predictable, are considered in Section 6.8 The friction-stir process is still very new, but some preliminary results suggest that the softening at FS welds will tend to be less extensive than that at arc welds (Section 6.9) 6.2 THERMAL CONTROL The extent of the HAZ can be critically affected by the control of temperature during fabrication In a large multi-pass joint, if no such control were exercised, the temperature of the surrounding metal would just keep on rising as more passes were laid, leading to a vastly enlarged Copyright 1999 by Taylor & Francis Group All Rights Reserved area of softening With 7xxx-type material, it would also increase the severity of the softening What matters is the temperature T° of the adjacent parent metal when any new weld metal is about to be deposited, known as the initial or interpass temperature The following effects tend to increase T°: The metal is still hot from the welding of a nearby joint Insufficient cooling time has been allowed since the laying of previous passes in the same joint Preheat is used The ambient temperature is high, as in the tropics In order to limit the adverse effects of overheating, an aluminium fabricator is required to exercise thermal control, namely to ensure that T° never exceeds a specified maximum value British Standard BS.8118 recognizes two levels of thermal control, normal and strict, as follows: All fabrication should satisfy normal control and this is what a designer would usually specify With 6xxx or 5xxx-series material, there is often little advantage in going to strict control, since this affects the area of softening rather than the severity There is a stronger case for strict control with 7xxx, as it also reduces the severity When in any doubt, design calculations should be based on normal control The assumption of strict control can be justified only in the following cases: a MIG-welded joint for which strict control is specified, with the maximum permitted value of T° stated to the fabricator; an isolated joint containing one single-pass MIG weld laid without preheat It is obviously advantageous to be able to use the more favourable HAZ parameters corresponding to strict control, when possible, and it is necessary to be specific as to which joints can count as case (2) In our treatment we assume welds to be single-pass up to a size (w) of mm (Section 6.5.5) The definition of an isolated weld is discussed in Section 6.5.10 6.3 PATTERNS OF SOFTENING 6.3.1 Heat-treated material Figure 6.2 shows patterns of softening at a single-pass MIG weld, as might be obtained with 6082-T6 and 7020-T6 material Such plots are determined Copyright 1999 by Taylor & Francis Group All Rights Reserved Figure 6.2 Typical hardness plots at a weld in heat-treated aluminium experimentally by conducting a hardness survey, and crudely indicate the variation in fu (ultimate stress) rather than fo (proof stress), since indentation hardness relates primarily to fu The plots in the figure have not been continued into the actual weld metal, because the strength of this varies with the filler used Two curves are shown for each alloy type, corresponding to welds made with normal and with strict thermal control The HAZ can be divided into two regions (1, 2) as indicated on the plots for normal control In region 1, the metal attains solution-treatment temperature and is thus able to re-age to some extent on cooling In region 2, this temperature is not reached, and the metal is over-aged The hardness is at a minimum at the boundary between the two regions (point A), and then rises steadily as we move out to point B Beyond B, the heat of welding has negligible effect and full parent properties are assumed to apply In region 1, the hardness increases as we move in towards the weld, although only slightly so for 6xxx material It is seen that the use of strict thermal control considerably reduces the extent of the softening for both alloy types With 7xxx material, it also improves the properties in region and reduces the amount of drop at A With 6xxx material, the effect of thermal control on severity of softening is only slight Very roughly, the relative widths of the two regions (1, 2) in Figure 6.2 satisfy the expressions below, where xA and xB are the distances of A and B from the middle of the weld (applicable to MIG welds): 0.5xB (6.1a) » 7xxx series xA » 6xxx series xA 0.8xB (6.1b) 6.3.2 Work-hardened material With the non-heat-treatable alloys, HAZ softening need only be considered for work-hardened material; it is not a factor with extrusions or Copyright 1999 by Taylor & Francis Group All Rights Reserved Figure 6.3 Typical hardness plots at a weld in work-hardened aluminium annealed plate Figure 6.3 shows the pattern of softening that might be obtained at a single-pass MIG weld on 5083-H22 material Again, regions and can be identified In region 1, the hardness is now uniform and corresponds to the properties of the alloy in the annealed condition As with the 6xxx series, the use of strict thermal control reduces the extent of the softening, but does not improve the strength in the HAZ The relative widths of the two regions at a MIG weld are roughly given by: » 5xxx series xA 0.3xB (6.1c) A generally similar pattern would be obtained for 3xxx-series material, but possibly with a different width of softened area Data on 3xxx materials are not generally available 6.3.3 Stress-strain curve of HAZ material Figure 6.4 compares typical stress-strain curves that might be obtained using coupons from the HAZ and from the parent metal It is seen that the Figure 6.4 Parent and HAZ stress-strain curves compared (6082-T6) Copyright 1999 by Taylor & Francis Group All Rights Reserved Figure 6.5 Pattern of softening at multi-pass weld on thick material HAZ curve has a more rounded knee, with a lower proof/ultimate ratio Plots such as those in Figures 6.2 and 6.3, based on hardness surveys, give a visual picture of how the ultimate stress (fu) is reduced in the HAZ The drop in proof stress will be more marked This is especially so for non-heat-treatable material (5xxx series) supplied in a hard temper 6.3.4 Multi-pass welds Figure 6.5 shows the typical softened zone at a large multi-pass weld Regions and can again be identified, analogous to those shown in Figures 6.2 and 6.3 for a single-pass weld, now extending uniformly around the edge of the deposit As we move away from the weld, the strength varies in the same general way as before, the lines A and B being metallurgically equivalent to points A and B in Figure 6.2 or 6.3 6.3.5 Recovery time With work-hardened alloys, the final HAZ properties are reached as soon as the metal has cooled after welding But, with heat-treated material, the immediate strength in the HAZ is low, the final HAZ properties only being developed after enough time has elapsed to allow natural ageing to occur Providing the component is held at a temperature of at least 10°C after fabrication, this time (the recovery time) may be roughly taken as: 6xxx-series alloys 7xxx-series alloys days 30 days If heat-treated material is held significantly below 10°C, the recovery time will be longer On the other hand, quicker recovery can be achieved by post-weld artificial ageing This involves holding the welded component at a temperature between 100 and 180°C for up to 24 hours, the exact procedure depending on the alloy Such treatment also has a strengthening effect Copyright 1999 by Taylor & Francis Group All Rights Reserved 6.4 SEVERITY OF HAZ SOFTENING 6.4.1 Softening factor The severity of softening in the HAZ is expressed in terms of a softening factor kz which is intended to represent the ratio of HAZ strength to parent metal strength At the current state of the art, it is only possible to suggest approximate values for this factor Typically HAZ experiments employ hardness surveys, and although these give a good indication of the extent of the softened zone, they say much less about the actual tensile properties of the softened metal, because the hardness number correlates only crudely with tensile strength and hardly at all with proof stress Also, there tends to be a lot of scatter between specimens In fact, for any given material we recognize three different values for kz (as in BS.8118), and Section 6.6 explains which value to use when k z1 This value is used for calculations involving the limiting stress pa (Table 5.2) Because p a=0.5(f o+f u), the factor k z1 is notionally intended to represent the ratio of the HAZ value of this quantity to that for the parent metal, averaged over the width of region (Figure 6.2) kz2 This value is employed for resistance calculations that involve the limiting stress po Because po is normally equal to the proof stress fo, we (notionally) take kz2 as the ratio of HAZ proof to parent metal proof, again averaged over the width of region The fact that the HAZ material has a lower proof/ultimate ratio than that for the parent metal (Figure 6.4) means that kz2