The Effect of L pin on Joint Structure and Strength

The first series of FSLW experiments were conducted using three Lpin values (3.2, 4.2 and 5.2 mm). Considering that the workpiece material was 3 mm thick Al 6060-T5, Lpin values of 3.2, 4.2 and 5.2 mm represent slight, moderate and excessive bottom plate penetration respectively. The values of

σ

Lap plotted as function of Lpin are shown in Figure 3-2. It is clear that

σ

Lap is highly process parameters dependent. Also the trends

of the data in Figure 3-2 suggest that, for three out of four ν-ω conditions,

σ

Lap varied

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four v-ω conditions

σ

Lap did not change significantly when Lpin increased from 4.2 to 5.2 mm. Furthermore for the welds made using Lpin values of 4.2 and 5.2 mm (representing moderate and excessive penetration) higher ω or lower ν resulted in lower

σ

Lap, in a general agreement with that found in literature [15, 16, 21]. However the use

of Lpin= 3.2 mm (representing slight penetration) was very different regarding the effect of ν and ω.

Figure 3-2

σ

Lap (and one standard deviation) plotted as a function of Lpin for various v and ω combinations. Mode 1 and Mode 3 indicate modes of fracture for those samples included in the marked areas. The rest of samples fractured with Mode 2, which is not labelled

Prior to a further discussion on the effect of Lpin on

σ

Lap, the way in which the tensile

shear samples deformed and fractured needs to be discussed. A feature of interest in tested samples is local bending and rotating before fracturing. This is clearly shown in the selected tested samples in Figure 3-3, regardless of how subsequently a sample fractured. Samples fractured in three different manners. Fracturing of a joint could proceed without necking and the failure was simply by fracturing across the bottom part of the nugget and along or near the original lapping surfaces of the whole joint. This is Mode 1 and an example is given in Figure 3-3a. Mode 2 represents fracturing that is similar to a normal tensile fracture (after local bending) but the crack originated from the hook, as shown in Figure 3-3b. Mode 3 also represents a fracture that is similar to a

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normal tensile fracture (after local bending), however the failure occurred in HAZ, as shown in Figure 3-3c.

The maximum achieved

σ

Lap value, as shown in Figure 3-2, was ~ 422 N/mm. This

maximum attainable

σ

Lap value (422 N/mm) is actually slightly higher than that of FS

bead-on-plate samples (

σBoP

≈ 140 MPa  2.9 mm = 406 N/mm) using the same ω and

v, given in Figure 3-1b. The FSL weld samples with maximum

σ

Lap fractured in Mode 3

which, as explained earlier, is the same mode of deformation and fracture as that of bead-on-plate samples. This also means

σ

Lap is just under 70% of UTS of the base

metal, which is almost the same as the values obtained using the best FSLW conditions [15, 21].

Figure 3-3 Various modes of fracture, (a) Mode 1: shear fracturing along the top and bottom joint interface, (b) Mode 2: tensile fracture with the crack

propagated from the hook and (c) Mode 3: tensile fracture in HAZ

Returning to Figure 3-2 and examining the effect of Lpin on

σ

Lap, Mode I fracture

occurred in samples made using the low value of Lpin = 3.2 mm (representing slight penetration) and the higher value of v = 224 mm/min. It is generally understood that higher v should result in both lower stir zone temperature and lower volume of stir

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material per unit length. Thus, when Lpin was almost the same as the top plate thickness, the stir material did not penetrate sufficiently to disrupt the original lapping interfaces (hereafter called un-welded lap) and did not strongly bond the top and bottom plates together. The cross sectional views of a weld made using v=224 mm/min (for Lpin = 3.2 mm) is shown in Figure 3-4. It is clear that the un-welded lap was not disrupted by the material flow during FSLW, and instead was slightly pushed into the bottom plate, due to the downward material flow induced by the rotating pin. Therefore the samples readily fractured, under loading, along the insufficient bond which was the un-welded lap (Mode 1) with low

σ

Lap. It should be noted that advancing side in all of the cross-

sectioned images (in this study) is kept on the left hand side.

Figure 3-4 Cross sectional views of a Al 6060-T5 FSL weld made using Lpin=3.2 mm, ω =500 rpm and ν =224 mm/min, showing that original lapping interface were not disrupted by the material flow during FSLW and caused the subsequent fractured in Mode 1

Reducing v from 224 to 112 mm/min for Lpin = 3.2 mm had a large effect on increasing

σ

Lap (Figure 3-2) with all samples fractured in Mode 2. This agrees with the general FS

feature that with a lower v value, both stir zone temperature and stir volume increase facilitating a better bonding of the lapping plates. Cross sectional views of a weld made using v=112 mm/min (for Lpin = 3.2 mm) are shown in Figure 3-5. It is clear that un- welded lap was disrupted (at stir zone) by the material flow during FSLW and thus the top/bottom plates bonding were sufficiently high to force the fracture in Mode 2 manner with higher

σ

Lap (compared to the specimens fractured in Mode 1).

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Figure 3-5 Cross sectional view of a Al 6060-T5 FSL weld made using Lpin=3.2 mm, ω =1000 rpm and ν =112 mm/min, showing original lapping interface were disrupted by the material flow during FSLW and caused subsequent

fractured in Mode 2

When Lpin increased to 4.2 mm, a sufficient pin penetration was achieved. As already stated and shown clearly in Figure 3-2, the mode of fracture and

σ

Lap did not change

significantly from moderate (Lpin= 4.2 mm) to excessive penetration (Lpin= 5.2 mm) when v and ω were kept the same. This suggests that, once pin penetration is sufficient, the partial flow volume that causes the up-lift and thus hooking is largely the same, if other FSLW conditions are the same. Thus for the subsequent work of studying the details of hooking and how they affect

σ

Lap, as presented in the following, only Lpin = 4.2 mm was used.

In document Metallurgical studies of friction stir lap welding (Page 88-92)