510_cas.pdf

RESIDUAL STRESS ON TITANIUM ALLOY
WELDED JOINTS
Caterina Casavola, Carmine Pappalettere
Politecnico di Bari – Dip. di Ingegneria Meccanica e Gestionale
Viale Japigia, 182 – 70126 Bari (ITALY)
[email protected], [email protected]
ABSTRACT
Recent improvements in welding technique (laser and electron beam welding) allow the utilization of high quality titanium
welded components in naval industry. However some problems related to the welding, as deteriorated material properties,
residual stresses and distortions, need further investigations. Residual stresses magnitude and distribution play an important
role in determining the effective working strength of a welded component. In this work titanium grade 2 and 5 butt-plates
welded by laser and electron beam techniques have been studied: residual stresses have been measured by hole drilling
method using electrical strain-gage rosettes bonded at different position, in order to evaluate magnitude and distribution of
residual stresses along the cord.
Introduction
Titanium and its alloys present an elevated strength/weight ratio, good mechanical properties at high temperatures too and
high resistance to corrosion and attack of many acids. These characteristics makes this material attractive for numerous
applications.
Thanks to recent improvements in welding techniques, which allow the realization of high quality welded joints, the use of
titanium alloys is spreading in numerous field ranging from aerospace to chemical industry. Naval industry, for which the
corrosion in the aggressive sea environment constitutes a discriminating element in the choice of materials, is weighing
possible applications of titanium. Structural application of titanium alloy are still under study, even if some coatings of ship
bottoms are already realized as protection against corrosion, instead of the traditional protective paints, which can give
pollution.
Even if the costs for supplying titanium are still more expensive than aluminium or steel (but they are rapidly decreasing) and
there are technological difficulties in welding operations (risk of contamination of melted bath from atmosphere), benefits in
terms of improvements in mechanical and corrosion strength and in weigh reduction are important aspects which give reasons
for studying this material and increasing its use in mechanical constructions.
At present, there are no authorized rules or recommendations for titanium structures and some researches are ongoing,
particularly directed on the comprehension of all phenomena related to welded components behaviour.
Residual stresses practically exist in all rigid parts as consequence of the “history” of the manufacturing process of the
structure. The heat supplied by a welding cycle, in addition to modification of the material’s microstructure, produces
distortions and residual stresses in welded joints [1-2]. Although residual stresses are self-equilibrated systems, their effects
on mechanical and fatigue [3] strength of materials are often relevant and requires to contemplate them in joint design.
Residual stress field, superposed to a field of service stress, could produce local overload till to reach the yield point of
material. Then it is important to know residual stress distribution, magnitude and orientation in order to consider their effects on
performance of structures. Nothing is more wrong that assuming idealistic conditions of material in welded joint design: it is
well known that compressive residual stresses decrease the buckling strength and that tensile residual stresses alter the load
ratio during fatigue cycle and change component life [4]. Many problems on residual stress are still open.
Titanium and its alloys, due to the low thermal conductivity, are suitable for laser arc and electron beam welding which above
all have a limited heat affected zone (HAZ). But this do not prevent the development of residual stresses.
The role of residual stresses on titanium welded components is not clearly explained. So, whereas new industrial application,
particularly naval application, push for titanium utilization, the problem of residual stresses, which seems to be greater on
material with high mechanical properties [5] like this material, represses its use. Large experimental data and/or reliable
prediction model are needed to let titanium utilization more frequent.
The incremental hole drilling method (HDM) has been used in this work to measure residual stresses [6] on welded plates in
titanium grade 2 and 5 [7] using electrical strain gage rosettes. The plates are 3 and 1.5 mm thick, the welding technique are
laser arc and electron beam.
Materials
Two different types of titanium alloy have been considered: titanium grade 2 and titanium grade 5.
Titanium grade 2 is commercially pure titanium (CP). At room temperature it is in a hexagonal close-packed form (hcp) called
α phase. At about 882°C allotropic transformation occurs and titanium changes in β phase (up to 1670°C) characterized by a
body centred cubic lattice (bcc). The addition of alloying elements may modify the transition temperatures and determines
which microstructure (α, α−β, β) is predominantly at working temperature. Titanium grade 5 is α−β phase.
The material properties are directly related to its microstructure: α phase titanium has very high corrosion strength and
excellent weldability with good ductility, but is relatively low in strength; α−β titanium alloy offers higher mechanical properties
because the bcc is stronger than hcp. Both of these alloys may be used for structural (particularly naval) applications and have
been chosen among the most easy available in adequate amount for naval industry.
Table 1 reports the chemical composition of these materials. Table 2 reports some chemical and physical properties of
titanium with respect to comparable steel, as indicated in data sheet of manufacturers. Density, coefficient of thermal linear
expansion (CTE) and thermal conductivity in titanium are quite low with respect to steel.
Ti grade 2
Ti grade 5
Ti
99.2
90
N
0.03
0.05
C
0.1
0.1
Fe
0.3
0.3
O
0.25
0.2
Al
6
Sn
-
Zr
-
Mo
-
V
4
Table 1 - Chemical composition
Ti grade 2
AISI 1010
Ti grade 5
AISI 4340
Tensile
Strength
Yield
[MPa]
275
305
1100
862
Tensile
Strength
Ultimate
[MPa]
345
365
1200
1282
Modulus of
Elasticity
[GPa]
Elongation
at Break
%
Hardness
Vickers
Density
[g/cc]
105
205
114
205
20
20
10
12
145
108
396
384
4.51
7.87
4.43
7.85
α
CTE linear
20°C
.
[µm/m- °C]
8.6
12
8.6
12
Thermal
Conductivity
.
[W/m K]
16.4
50
6.7
44.5
Table 2 - Chemical and physical properties
Welded plates
Chemical and physical properties of titanium alloys allow good weldability both by laser (LBW) and electron beam welding
(EBW). The main problem of EBW is how to deal with the evacuated chamber needed by the process seeing all the welding
operations must be performed in a shipyard. On the other hand, both LBW and EBW involves very high density power (do not
need filling metal) and can achieve very deep penetration. So specific devices have been designed in order to develop welding
systems suitable for realizing structures in naval environment [8].
Three types of welded plates and two specimens have been studied as indicated in Table 3.
Material
B
C
D
E
F
Ti gr 2
Ti gr 2
Ti gr 5
Ti gr 2
Ti gr 5
Thickness
t [mm]
1.5
1.5
3
1.5
3
Length x Width
L [mm] x W [mm]
300x40
300x40
400 x 380
400 x 380
400 x 380
Welding technique
Type of joint
LBW
LBW
LBW
EBW
EBW
butt
butt
butt
butt
butt
Table 3 – Welded plates tested
Both in EBW and LBW, the molten welded metal area has been shielded with a specific device and inert gas until 700°C in
order to avoid contamination from the air and absorption of impurities which may lead to early failures.
During welding operations, all plates have been constrained in 3 point in order to avoid the opening of the junction.
Preliminary visual inspection has been conducted on all welded plates in order to examine the quality of the cord: the color
appears silver and polished for all plates which indicates good gas protection without contaminations, the shape appears
regular with no evident spray or notch.
Then photomacrography and Vickers hardness (HV) measurements have been executed: both EBW and LBW butt joints
presents notch on the right side, but all of them appear smooth (fig. 1); heat affected zone (HAZ) is very restricted and it is
clearly visible only for 3 mm plates. HV values are slightly higher on LBW joint (fig.2-4).
HV sup
HV sup
HV inf
HV inf
But E (magnification 50X)
But F (magnification 25X)
But D (magnification 25X)
Fig. 1 – Photomacrography (by Leica DMRME microscope)
180
175
170
HV 0,5
165
160
155
150
145
140
-2
-1,5
-1
-0,5
0
0,5
1
1,5
2
Distance from midpoint cord [mm]
Fig. 2 - Vickers hardness measurements on But E (along horizontal line)
Distance from the top [mm]
380
HV sup
370
HV inf
360
HV1
350
340
330
320
310
-3
-2
-1
0
1
Distance from midpoint cord [mm]
2
3
0
340
-0,5
345
350
355
360
365
-1
-1,5
-2
-2,5
-3
HV1
Fig. 3 - Vickers hardness measurements on But F (along horizontal lines and vertical line)
370
375
380
Distance from the top [mm]
380
HV sup
370
HV inf
360
HV1
350
340
330
320
310
-3
-2
-1
0
1
2
0
340
-0,5
345
350
355
360
365
370
375
380
-1
-1,5
-2
-2,5
3
-3
HV1
Distance from midpoint cord [mm]
Fig. 4 – Vickers hardness measurements on But D (along horizontal lines and vertical line)
Test method
A variety of techniques may be used for measuring residual stresses. The HDM consists of machining a hole in sequential
steps and then measuring the resulting changes of strain with a 3-grid electrical strain gage rosette. During drilling, residual
stress are gradually relaxed and strain gages allow to calculate the magnitude and the orientation of in-plane principal
stresses.
The measure device is simply composed of an automatic system (RESTAN, REsidual STresses ANalyzer) that carries out the
measure of residual stresses with the hole drilling method in conformity of ASTM E837. The equipment includes a high speed
turbine (300000 rpm) set in action from compressed air with carbide tungsten cutter and advance of 0.1 mm/min, an electric
step by step motor for the vertical advance of cutter so as to realize the measures in subsequent steps, a microscope
endowed of two orthogonal movements for the corrected positioning of cutter with the center of the hole of the 3 grid strain
gages, two centesimal comparator.
This equipment can execute precise drilling procedure [9-10]. This fact either limits introduction of additional stresses as a
result of the unavoidable plasticization around the hole and locates univocally the starting point of drilling. The eccentricity and
effective diameter of the hole can be optically measured.
The measure of eccentricity helps to suitably correct the calculated residual stresses values [11]. In order to consider the
variation of the residual stresses measured through the thickness, the power series method has been used [12-13].
The geometry of welded plates and strain gages locations are shown in figures 5-6-7 (rosette are bonded to the weld toe and
positioned so that the machine can drill at the maximum distance of 1.5 millimeters between the weld toe and the centre of the
hole).
Fig. 5 - Geometry of welded plates (D-E-F) and drilled holes location
Fig. 6 - Geometry of welded specimens (B-C) and
drilled hole location
Fig. 7 – Rosette bonded close to the weld toe
number of HDM
measurements
1
1
6
4
6
B
C
D
E
F
location
right side of plates
right side of plates
right side of plates
right side of plates
right side of plates
distance of the centre of the
hole from the cord
1.5 mm
1.5 mm
1.5 mm
1.5 mm
1.5 mm
Table 4 – Test plan
Results
Figures 8 through 10 show some of the relaxed strains measured at different steps during drilling operations and the
corresponding calculated residual stresses in the longitudinal and transversal directions with respect to the weld cord. Both
strains and stresses are plotted against the hole depth h. Plotted residual stresses have been calculated by power series
method [13].
Some strange values have been measured at the first drilling steps due to stresses probably produced during the surface
preparation for strain gage bonding. At other depth, residual stresses express a more regular trend.
Table 5 reports final residual stress values obtained both by ASTM and power series method. Using the ASTM procedure,
measured strains in many cases describe a “non uniform stress field” in the thickness of the specimens (particularly on Ti
grade 2 where HDM technique experienced some difficulties due to the material characteristics). However, table 5 shows that
ASTM and power series calculations reach results in good agreement. In few cases, as for specimen B, an elevated gap have
been found, but the magnitude of residual stresses is very low.
600.00
800
strain 1
strain 2
strain 3
Strain [me]
400
200
0
0
0.5
1
1.5
-200
2
s normal to the cord
s longitudinal to the cord
shear
400.00
2.5
Residual stress [Mpa]
600
200.00
0.00
0
0.5
1
1.5
-200.00
-400
-400.00
-600
-800
-600.00
Hole depth [mm]
Hole depth [mm]
Fig. 8 – Strain measured and calculated residual stresses on plate D (hole position: 2)
2
2.5
800
600.00
strain 1
strain 2
strain 3
400
400.00
Strain [me]
200
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
-200
Residual stress [MPa]
600
200.00
0.00
0
0.2
0.4
0.6
0.8
1
1.6
s normal to the cord
s longitudinal to the cord
shear
-400.00
-800
1.4
-200.00
-400
-600
1.2
-600.00
Hole depth [mm]
Hole depth [mm]
Fig. 9 – Strain measured and calculated residual stresses on plate E (hole position: 2)
600.00
800
strain 1
strain 2
strain 3
400
400.00
Strain [me]
200
0
0
0.5
1
1.5
2
2.5
-200
Residual stress [MPa]
600
200.00
0.00
0
0.5
1
1.5
2.5
-200.00
s normal to the cord
s longitudinal to the cord
shear
-400
-400.00
-600
-800
2
-600.00
Hole depth [mm]
Hole depth [mm]
Fig. 10 – Strain measured and calculated residual stresses on plate F (hole position: 2)
position of HDM
measurements
B
C
D
D
D
D
D
D
E
E
E
E
F
F
F
F
F
F
1
2
3
4
5
6
1
2
5
6
1
2
3
4
5
6
ASTM
count
Power series
count
normal [MPa]
21,15
126,83
-70,13
69,06
120,06
101,50
119,93
113,12
59,44
96,20
39,56
24,64
26,83
51,29
67,82
113,93
75,46
108,17
normal [MPa]
10,10
136,59
-66,71
68,50
124,04
105,94
127,32
121,55
29,65
113,24
26,89
8,63
36,75
47,75
69,03
111,23
76,75
111,93
Error %
109,41
-7,15
5,13
0,82
-3,21
-4,19
-5,80
-6,94
100,47
-15,05
47,12
185,52
-26,99
7,41
-1,75
2,43
-1,68
-3,36
ASTM
count
longitudinal
[MPa]
42,48
16,91
20,69
204,34
204,16
206,29
239,93
233,96
344,02
209,76
230,67
100,03
68,72
148,33
139,41
213,65
126,50
201,99
Table 5 – Calculated residual stresses (hole depth 1.5 mm)
Power series
count
longitudinal [MPa]
42,09
18,36
0,59
216,84
209,39
221,83
252,97
257,38
359,32
256,22
218,31
108,60
131,28
163,45
150,46
221,23
134,05
219,27
Error %
0.93
-7.90
-5.76
-2.50
-7.01
-5.15
-9.10
-4.26
-18.13
5.66
-7.89
-47.65
-9.25
-7.34
-3.43
-5.63
-7.88
Figures 11 and 12 are referred to plates D and F respectively. Residual stresses values along the cord are reported in view of
underline possible distinctive effects between LBW and EBW.
500
s normal (side A)
s normal (side B)
Residual stress [MPa]
400
s longitudinal (side A)
s longitudinal (side B)
300
200
100
0
0
50
100
150
200
250
300
350
-100
Position along the cord [mm]
Fig. 11 - Residual stresses along the cord for But D (Ti grade 5 – LBW – 3mm)
500
Residual stress [MPa]
400
s normal (side A)
s longitudinal (side A)
s normal (side B)
s longitudinal (side B)
300
200
100
0
0
50
100
150
200
250
300
350
-100
Position along the cord [mm]
Fig. 12 – Residual stresses along the cord for But F (Ti grade 5 – EBW – 3mm)
Conclusion
Analysis of residual stresses measured values suggests the following considerations.
The magnitude and distribution of residual stresses found in titanium plates are affected by the material properties that rule
temperature changes, thermal expansions and consequent shrinkages. Residual stresses are due to thermal stresses and
non-elastic strains generated during welding. Thermal stress σ = α E T are lower for titanium than for steel plates (both Young
modulus E and α are lower and only fusion temperature T is higher for titanium). Besides, due to low thermal conductivity, the
heat move slowly inside the plates and this probably is responsible for a very narrow HAZ. Cooling is also slow. This could
explain the regular distribution of residual stresses along the cord (fig. 11-12) for titanium welded plates with respect to steel
ones [14-17].
As generally happen also on steel plates [14-17], longitudinal residual stresses measured are higher than transversal ones.
External constraints utilized during welding operations to inhibit the opening of the junction have little influence on the
transversal residual stress distribution.
EBW and LBW on Ti grade 5 do not show essential differences in residual stresses peak values. The magnitude of residual
stresses measured on Ti grade 2 is comparable with yield of material or even higher. Residual stresses on Ti grade 5 are
largely less than half of the material yield point.
The test done in this work are very expensive, but they are needed to perform a correct and realistic evaluation of stress field
due to welding operations. Recent progress in the field of welding technique and titanium alloy applications in naval
constructions suggest to deeply understand not only the genesis and the magnitude of residual stress due to welding, but also
to investigate on the interaction with other mechanical phenomena such as fatigue and to develop effective prediction model to
take residual stresses into account at the design stage.
Acknowledgments
All the LBW and EBW components have been realized respectively by Laser Laboratory of ENEA (Trisaia, Italy) and Electron
Beam Laboratory of ENEA (Casaccia, Italy). The authors are very grateful to Eng. Marco Brandizzi (CALEF) for his useful
suggestions.
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