Welding International 2003 17 (8) 659–660 Selected from Svarochnoe Proizvodstvo 2003 50 (3) 31–33; Reference SP/03/3/31; Translation 3185
Special features of fabrication technology of sheet welded structures from titanium alloys
M N S H U S H P A N O V, A B K O L O M E N S K I I and A N ROSHCHUPKIN Voronezh Aircraft Construction Holding Company
In Refs 1 and 2 it was reported that it would be possible to apply incomplete annealing of welded sheet titanium structures instead of complete annealing. At the same time, it is necessary to determine more accurately the temperature ranges of incomplete annealing, ensuring the maximum service endurance of structures. In addition to this, the rational methods of fabrication of sheet components in a number of cases include the processes of the formation of elements of structures with welded joints (straightening, shaping). These problems are investigated in this work. Specimens after welding and annealing for 1 h were subjected to brightening pickling to a depth of 80–90 µm in order to remove the gas-saturated layer and prevent its effect on the test results. Some other specimens produced only a transverse welded joint with the working zone 15 mm wide, and other specimens contained crossing welded joints with the working zone 40 mm wide. The longitudinal joint was produced last on the completed specimens. Consequently, the specimens of the first batch retained only the stresses of the second kind; the stresses of the first kind were almost completely removed when cutting specimens from blanks. The specimens of the second batch retained residual stresses of both the first and second kind. Tests in repeated static tensile loading were carried out in equipment UMM-10 at R = 0.1 and f = 0.6–0.8 Hz. The nature of the dependence of endurance N on annealing temperature T for the investigated alloys is almost identical for both the ductile low-strength technical titanium VT1-0 and also for VT6ch high-strength alloy
N10 3, cycle
N103, cycle
N103, cycle
a
(Fig. 1). At a temperature of 350–450 °C, there is a maximum of endurance, and with the annealing temperature increasing to 850 °C, the maximum gradually decreases. For specimens with the transverse welded joints, these tendencies are stronger. Thus, the maximum cyclic endurance is obtained in annealing, ensuring only partial removal of the residual stresses. Annealing at higher temperatures, resulting in the complete removal of the stresses, leads to a significant decrease in endurance. This may be explained by softening of the heat affected zone during annealing. During welding, the heat affected zone is subjected to plastic deformation increasing the yield limit of metal. The examination results of the microstructure of welded joints, showed slight coalescence of the ductile α'-phase in the V1-0 titanium and OT 4 alloy after annealing at 650 °C. The detected low-temperature maximum of incomplete annealing is evidently explained by the effect of precrystallisation hardening.3 This effect was associated with fixing of the mobile dislocations in the initial cold-deformed metal and dislocation walls, formed during polygonionisation, and by the atoms of dissolved impurities and alloying elements. It is well-known that producers supply sheet semifinished products in the partially cold hardened condition after smoothing (deformation 2–6 %). In annealing, the metal softens and this may influence the service characteristics of the component. In the shortterm fracture tests of sheets of titanium alloys after annealing at 350–450 °C, examination showed a stable
b
c
1 The dependence of repeated-static endurance of welded specimens with transverse joints in titanium alloys with thickness δ on annealing temperature: a VT1-0 alloy, δ = 12 mm, σmax = 320 MPa; b OT4 alloy, 0.8 mm, 660 MPa; c alloy VT6ch, 0.8 mm, 820 MPa.
660
Shushpanov et al
N103, cycle
N103, cycle
a
N10 3, cycle
c
b
2 The dependence of repeated-static endurance of welded joints in titanium alloys subjected to preliminary bending with strain ε, on annealing temperature: a VT1-0, ε = 13.4–15 %, σmax = 270 MPa; b OT4 alloy, 5 %, 500 MPa; c VT6ch alloy, 0.8– 2 %, 640 MPa.
maximum of the strength properties associated with the effect of pre-recrystallisation hardening. In cyclic tests, it was established that to ensure the maximum repeated static strength of sheets of titanium alloys, supplied in the hardened condition after smoothing, it is sufficient to carry out incomplete annealing at the 350–450 °C range. The most important factors in examination of the effect of residual stresses on the efficiency of welded structures are, firstly, the scale factor and secondly, the stress-strain state in the tests. Consequently, additional tests were carried out on full-size specimens of welded pipes with a diameter of 80 mm produced from VT 1-0 commercial titanium and OT 4 alloy with a thickness of 0.8 mm by pulsating internal pressure with f = 0.25 Hz, pmax = 5 MPa, and R = 0. The biaxial stress-strain state was produced. The highest endurance was recorded in the case of welded pipes after incomplete annealing in air at a temperature of 430–450 °C, and also on the sheet specimens. Complete annealing (550 °C in the case of titanium VT 1-0 and 660 °C for OT 4 alloy) decreased the endurance of the pipes by, approximately, a factor of 1.5. Annealing of welded joints in a number of cases was carried out not only to remove the residual stresses but also to restore the ductility properties of the metal essential for carrying out shaping operations. The experimental results show that the maximum technological plasticity in both the welded joints and the parent metal is obtained at temperatures higher than the recommended temperature range of complete annealing and close to the temperatures of the start of recrystallisation. Investigations were carried out into the effect of the degree of deformation and temperature of consecutive annealing on the repeatic-static endurance of welded joints. The possibility of cold deformation of welded joints in titanium alloys in practice, was determined. In bending, commercial titanium can be deformed to 18 % without any significant loss of endurance, and the OT 4 and VT6ch alloys can be deformed to 7 and 2 %, respectively. Tensile deformation of these alloys by 23, 3 and 1.5 % respectively, is accompanied by a large increase in the endurance of VT1-0 alloy, by retaining the level of endurance in the case of the OT4 alloy, and, by approximately, a 30 % decrease in the case of VT6ch alloy. Figure 2 shows the examination results of the effect
of annealing temperature on the repeated-static endurance of welded specimens subjected to preliminary bending with a controlled degree of deformation. The specimens were produced with a longitudinal welded joint and a central hole–stress concentrator. The hole with a diameter of 2.5 mm and width of the working zone of 50 mm (the stress concentration coefficient according to Neuber 2.6) was produced directly prior to testing. The experimental results show that low-temperature annealing increases more sufficiently the repeated-static endurance of deformed welded joints in titanium alloys. One of the reasons for this is associated with the effect of precrystallisation hardening. This stable dependence is recorded in all investigated technological variants: in both deformation methods (bending and tensile loading), the absence and presence of the geometrical concentrators, for the alloys of different strength groups, welded joints and the parent metal. Softening at higher temperatures has, in all cases, a stronger negative effect on the cyclic strength. Conclusions 1 The results indicate that it is not rational to subject cold-deformed welded structures to expensive complete annealing which is usually carried out in vacuum furnaces or in containers with inert gases. 2 When the conditions of the technological process, preventing hydrogen saturation of metal, are adhered to, and also if it is not necessary to carry out thermal straightening of the structure, it is efficient to use economical low-temperature annealing in air, resulting in the highest service endurance in the conditions of repeated-static loading.
References 1 2
3
Kolomenskii A B and Muraev'ev I I: ‘Current problems of welding non-ferrous metals’. Naukova dumka Kiev 1985 183– 184. Kolomenskii A B and Muraev'ev I I: ‘Effect of annealing method on the fatigue characteristics of welded joints in sheets of VT6ch alloy’. In: ‘National Conference on welding nonferrous metals’. TPI Tol'yatii 1986 47. Novikov I I: ‘Theory of heat treatment of metals’. Metallurgiya Moscow 1986 480.
