PSI - Issue 16

Olena Berdnikova et al. / Procedia Structural Integrity 16 (2019) 89–96 Olena Berdnikova et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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4

5.2. Laser welding

In laser welding, studies have shown that at 880 J/mm (#5) a bainite-ferrite structure is formed in the weld metal and HAZ, mainly the B U (correspondingly 55% and 35%), Fig. 1e. When the heat input decreases to 316 J/mm (#7), the phase composition of the weld metal and HAZ changes from bainite-ferrite to bainite-martensite, Fig. 1f – h. Also crushing of the grain and sub-grain structure in 1.3...1.4 times occurs with a certain increase in microhardness. Fine grained grain structure of lower bainite (B L ) is mostly (45...50%) formed under conditions of uniform redistribution of the volume dislocation density in the weld metal ρ = (6...8)×10 10 cm – 2 , and in the metal of HAZ ρ = (8...9)×10 10 cm – 2 , Fig. 1f – h.

Table 2. Welds and HAZ parameters and their mechanical and structural properties. No. Weld HAZ

I HAZ

Width (mm)

Micro- hardness (MPa) Grain sizes (μm)

Dislocation density (cm − 2 )

Width (mm)

Width (mm)

Micro- hardness (MPa)

Grain size (μm) 40…110 30…80 40…100 30…70 50…90 30…80 30…60 30…60 25…50 20…40

Dislocation density (cm − 2 ) 8·1010…10 11

3090…4010

#1 #2 #3 #4 #5 #6 #7 #8 #9

15.4 2740...3360

50…200×200…700 60…180×200…400 50…180×200…400 40…150×180…300 50…90×150…410 30…100×120…400 20…40×100…400 30…120×170…350 30…80×150…300 20…80×150…250

(4…6)·10 10

2.9 2.8 2.0 1.5 2.0 1.9 0.8 1.9 1.6 1.4

0.9 0.8 0.4 0.4 0.4 0.3

15.0 10.0

2640…3090 3850…3220 3630…4050 2850…3510 3450…4010 3870…4330 3800…4010 4050…4420 3360…3940

−

3660

−

(4…6)·10 10

3600…3870 3800…4010 3830…4170 4010…4420 4010…4250 3540…3900 3830…4010 3360…4010

(6…8)·10 10

8.0 5.0 4.0 3.3 4.0 4.3

−

−

(2…4)·10 10

(4…6)·10 10

−

−

(4…6)·10 10 (4…6)·10 10

0.25

(8…10)·10 10 (6…8)·10 10

0.3

−

0.35 0.23

−

#10 4.2

(6…8)·10 10

(8…10)·10 10

5.3. Hybrid laser-arc welding

Studies of the structure and phase composition of welded joints in hybrid laser-arc welding have shown that when going from 363 J/mm (#8) to 314 J/mm (#10), the phase composition of metal of the weld and the overheated HAZ remains the same (bainite-martensite). However, the volume fraction of B L decreases noticeably (up to 10...20%). Herewith, in the case of 314 J/mm, the integral value of the dislocation density increases to ρ = 1.5×10 11 cm – 2 and the mostly structure of the B U is formed, Fig. 1i. And the most uniform distribution of dislocation density ρ = (4…6)×10 10 cm – 2 is characteristic for the B L structures at 363 J/mm, Fig. 1j – l. It should be noted that in this case an increase in the density of dislocations is observed while a decrease in heat input from 363 J/mm to 314 J/mm. Perhaps this is due to a change in the ratio of the contribution of the arc and laser components of the hybrid process in the resulting value of heat input, namely, with a relative increase in the contribution of the arc component from 39% to 54%. Thus, our results showed that in the case of transition from arc to laser and hybrid welding and corresponding change in modes, the ratio of phase components changes. This concerns the B L , B U and martensite formed in the welding zones, their parameters, volume fraction, as well as dislocation density and their distribution. So, under conditions of arc and laser welding under conditions with high heat input, the B U structures are mostly formed with a general increase in the size of the grain and sub-grain structures with an uneven distribution of the dislocation density. The decrease of heat input contributes to the predominant formation of B L structures with significant grain and subgrain refinement and uniform distribution of dislocation density in the volume and on grain boundaries. The greatest dispersion of the grain structure with the most uniform distribution of the dislocation density is characteristic of welded joints produced by laser and hybrid welding. As a result of mechanical testing of welded 5.4. Comparison of welding results