Tensile strength of dissimilar materials TIG-FSW

-4.2.3 Tensile strength of dissimilar materials TIG-FSW hybrid

Table 4.7 Tensile strength of dissimilar materials TIG-FSW hybrid welded joints

Rotation speed (rpm)

Travel speed (mm/s)

Fractured specimen Cross section

T.S (MPa)

300

0.8 174.7

1.0 211.9

1.2 226.2

1.4 222.1

1.6 199.2

350

0.8 231.3

1.0 261.9

1.2 301.5

1.4 243.5

1.6 182

400

0.8 212.4

1.0 222.3

1.2 260.5

Rotation speed (rpm)

Travel speed (mm/s)

Fractured specimen Cross section

T.S (MPa)

400

1.4 211.0

1.6 187.6

500

0.8 253.0

1.0 256.5

1.2 278.8

1.4 220.3

1.6 217.6

Table 4.8 Stress-strain curve for dissimilar materials TIG-FSW hybrid welding joints

Rotation speed (rpm)

Travel speed(mm/s)

0.8 1.0 1.2

300 1.4 1.6

-350

0.8 1.0 1.2

1.4 1.6

-Rotation speed (rpm)

Travel speed(mm/s)

0.8 1.0 1.2

400 1.4 1.6

-500

0.8 1.0 1.2

1.4 1.6

-(a) FSW (b) TIG-FSW hybrid welding

Fig. 4.2 Comparison of tensile strength with welding process

Comparing the results of FSW and TIG-FSW hybrid welding, tensile strength of TIG-FSW hybrid welding shows better than FSW(Fig. 4.2).

Tensile strength of above 80% of base metal is obtained 400 and 500RPM in case of FSW joint whereas TIG-FSW hybrid weld joint made at 350RPM and 500RPM at welding speeds 1.0~1.2mm/sec gives better tensile strength. The tensile strength of TIG-FSW hybrid was 80~90% of base metal and best tensile strength value (301.5MPa) is obtained at a welding speed of 350RPM and 1.2mm/sec welding speed.

Therefore, weld joint with better tensile strength can be made at higher welding speed by TIG-FSW hybrid than by FSW

4.3 Hardness test results

The hardness distributions of dissimilar joints by TIG-FSW is shown in Fig.4.3. A drop in hardness from base metal hardness is evident in the welding zone for Al6061-T6. Precipitation hardening alloys such as the Al6061-T6 show a loss of hardness in the HAZ, with some recovery in the nugget because a lower hardness due to the absence of strengthening precipitates at heat affected zone(HAZ) and the dissolved precipitates do re-precipitate or recrystallize subsequently at higher temperatures in the nugget zone. The hardness of Al6061-T6 at the thermo-mechanically affected zone(TMAZ) and stir zone(SZ) is more than HAZ because of the mechanical effect of plastic flow during weld formation.

On Ti-6Al-4V, the hardness value at weld zone is more than that of base metal due to work hardening effect by TIG preheating and frictional heating. The base metal hardness of Al6061-T6 and Ti-6Al-4V are 97HV and 375HV respectively. The hardness values HAZ, TMAZ and SZ of Al6061-T6 are 62, 73 and 78HV respectively.

The hardness of the weld nugget shows variable values because of the presence of the fine or coarse dispersed stainless steel particles in the weld nugget.

When comparing hardness results(Fig. 4.4) of TIG-FSW hybrid with FSW at each top position, hardness values are found more in TIG-FSW.

This is because, plastic flow is increased by TIG preheating making finer recrystallized grains at the TMAZ and SZ.

(a) FSW (b) TIG-FSW hybrid welding Fig. 4.3 Hardness distribution of dissimilar materials welded joints both

welding process

Fig. 4.4 Comparison of hardness distribution of FSW and TIG-FSW hybrid welding joints

4.4 Microstructure analysis

4.4.1 Microstructure of FS welded joints

Microstructure of FSW dissimilar joint is shown in Fig. 4.4. The base metal (point 'a') and HAZ (point 'b') exhibits almost similar microstructure. Grain size little bigger in HAZ than base metal. The TMAZ (point 'c') is characterized by a highly deformed structure. The base metal elongated grains were deformed in an upward flowing pattern around the nugget zone. The Al6061-T6 alloy in the weld nugget consists of fine, equiaxed, recrystallized grains. The fine recrystallized grains in the stirred zone are attributed to the generation of high deformation and temperature during FSW.

The weld nugget exhibits a mixture of Al6061-T6 alloy and Ti-6Al-4V particles pulled away by forge of tool probe from the Ti-6Al-4V surface. Therefore, the weld nugget has a composite structure of Ti-6Al-4V particles reinforced Al6061-T6 alloy.

Ti-6Al-4V particles inclusions were more in the Al6061-T6 weld nugget and have an irregular shape and inhomogeneous distribution within the weld nugget.

Fig. 4.5 Microstructure of FS welded joints

4.4.2 Microstructure of TIG-FSW hybrid welded joints

Microstructure of TIG-FSW hybrid joints are shown in Fig. 4.5. From the microstructure image of Al6061-T6 base metal, it is clear that the base metal have the same microstructure with homogeneous grain distribution as in FSW. Compared to FSW process, only finer particles of Ti-6Al-4V inclusions were found in TIG-FSW microstructure. At nugget zone, coarse grain size is appeared in TIG-FSW hybrid when compared to FSW because of more plastic flow due to TIG preheating effect. The microstructure of acicular α and α′(dark) is exhibited at point f. Also precipitate in β grains(light) at fusion zone(point f)

Fig. 4.6 Microstructure of TIG-FSW hybrid welded joints

4.5 Analysis of weld specimen using SEM and EDS observation

4.5.1 SEM of fractured specimens(FSW and TIG-FSW hybrid)

The SEM microstructures of the fractured specimens have been illustrated in Fig. 4.6. The images of top, middle and bottom side of the fractured surface of Al6061 and Ti-6Al-4V were observed for weld joints by FSW. The fracture surface shows a dimple pattern on all points. But middle and bottom points are subjected to brittle fracture. At Ti-6Al-4V side, Top point is subjected to ductile fracture and middle point is subjected to ductile and brittle fracture, where as at bottom point, complete brittle fracture is occurred.

From the SEM image observation of as-received dissimilar joint by TIG-FSW hybrid (Fig. 4.7.), dimple pattern is observed at the fractured surface in Al6061-T6 side associated with ductile fracture. Titanium inclusions were found in the middle and bottom part of Al6061-T6 side.

At Ti-6Al-4V side, top side shows dimple patterns associated with ductile fracture wherein middle side mixed mode of cleavage area and ductile fracture was observed. At bottom side, is subjected to ductile and brittle fracture.

(a) Top (b) Middle (c) Bottom

(a) FSW - Al 6061-T6 side

(a) Top (b) Middle (c) Bottom

(b) FSW - Ti-6Al-4V side

Fig. 4.7. SEM of dissimilar materials FS welded joints

(a) Top (b) Middle (c) Bottom

(a) TIG-FSW hybrid welding - Al 6061-T6 side

(a) Top (b) Middle (c) Bottom

(b) TIG-FSW hybrid welding - Ti-6Al-4V side

Fig. 4.8. SEM of dissimilar materials TIG-FSW hybrid welded joints

4.5.2 SEM and EDS observation of dissimilar welded joints by TIG-FSW hybrid

The SEM and EDS photographs of the welded joints have been illustrated in Figs 4.8. 4.9 and 4.10 respectively. The possibility of appearing inter-metallic compounds in the interface between titanium and aluminum is studied from the SEM images. Judging from SEM and EDS analysis, no inter-metallic compounds was observed on the check points.

But, previous research works report the possibility of appearing TiAl3

mainly at the upper part of dissimilar joint interface of aluminum and titanium. Here, only Wt% of Al and Ti at aluminum side where titanium inclusion are found at weld interface has been analysed.

(a) (b) (c)

(d) (e) (f)

Fig. 4.9. SEM of TIG-FSW hybrid welded joints

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Fig. 4.10. EDS of dissimilar materials TIG-FSW hybrid welded joints