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Effect of shielding gases on microstructure and mechanical properties of super austenitic stainless steel by hybrid welding
Materials and Design 33 (2012) 203–212
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Effect of shielding gases on microstructure and mechanical properties of super austenitic stainless steel by hybrid welding P. Sathiya a,⇑, Mahendra Kumar Mishra a, B. Shanmugarajan b a b
Department of Production Engineering, National Institute of Technology, Tiruchirappalli 620 015, Tamil Nadu, India International Advanced Research Centre for Powder Metallurgy & New Materials, Hyderabad 500 005, Andhra Pradesh, India
a r t i c l e
i n f o
Article history: Received 8 March 2011 Accepted 28 June 2011 Available online 12 August 2011
a b s t r a c t This paper investigates the bead geometry, microstructure and mechanical properties of AISI 904 L super austenitic stainless steel joint by CO2 laser–GMAW hybrid welding process. Shielding gas is one of the important parameters for the process stability and efficient synergetic effects between laser and gas metal arc welding (GMAW). A detailed study of CO2 laser–GMAW hybrid welding with different shielding gas mixtures in different ratio (50%He + 50%Ar, 50%He + 45%Ar + 5%O2, and 45%He + 45%Ar + 10%N2) was carried out on AISI 904 L super austenitic stainless steel sheet of 5 mm thickness. The weld penetration of hybrid welding was determined by the plasma shape varying with the shielding gas parameters, especially the plasma height interacting with incident plasma. The X-ray diffraction was used to analyze the phase composition, while the microstructural characterization was performed by phase microscopy of the joints. The impact and tensile tests were performed and fracture surface morphology had been analyzed through scanning electron microscope (SEM). Finally, hardness test is performed along the longitudinal direction of the weld zone. The results showed that the joint by laser–GMAW hybrid had higher tensile and impact strength than the base metal. The fractrography observation showed the cup-cone shaped fracture while the hybrid welding joint mixed mode of fracture. The laser–GMAW hybrid welding is suitable for welding of AISI 904 L super austenitic stainless steel owing to their high welding speed and excellent mechanical properties. Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
1. Introduction The laser–arc hybrid welding which integrates laser beam and arc welding into one process eliminates the disadvantage of individual processes. At the same time it has its own particular advantages, viz., the deeper welding penetration, high welding speed, less deformation, the ability to bridge relatively large gaps and the capability of handling highly reflective materials [1–4]. For these advantages, laser–arc hybrid welding technology has received significant attention in recent years. But there were only a few studies that reported the industrial applications of the laser–arc hybrid welding. Most of the researches [3–8] carried out so far have dwelt on the improvement of the weldability of a particular material. In order to get the most efficient synergetic effects between the laser and the arc, many problems like lack of penetration and gap bridge ability about the matching of numerous hybrid welding parameters must be solved. Laser CO2–GMAW is suitable for shipbuilding and transport industry and panel fabrication in aerospace industry. The two welding sources, coupled to perform a hybrid welding process, re-
⇑ Corresponding author. Tel.: +91 431 2503510; fax: +91 431 2500133. E-mail address: [email protected] (P. Sathiya).
quire a fine tuning of both sets of technological parameters: laser welding parameter and GMAW parameters in order to obtain a stable, repeatable and productive process. Based on hybrid welding, many studies have been carried out regarding power-related parameters [9,10] viz., coupled arc voltage, laser beam power and on source positioning related ones [11] such as defocus position and distance between the sources, in order to trace out the basics regarding the applicability of the process. More specific studies have been carried out considering plasma interaction [12] and molten pool fluid dynamics [13] with the aim of tuning the complex equilibrium which stands behind this kind of processes. The shielding gas and its protecting methods were also important for the behaviors of CO2 laser–MIG hybrid welding such as the laser–arc interaction and the energy transfer of the two heat sources, which directly impacts the laser–arc synergetic effects. Fellman et al. had discussed how the shielding gas composition (helium–argon–CO2) affected the weld quality, weld profile and process stability of the CO2 laser–metal active gas (MAG) hybrid welding [14]. Super austenitic stainless steel is Fe-based system highly alloyed with Cr, Ni, Mo, and N to produce excellent pitting and crevice corrosion-resistant properties at high temperatures and in seawater. When exposed to elevated temperatures for long periods of time, large amounts of precipitates, including carbides, nitrides,
0261-3069/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.06.065
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and intermetallic phases, are formed in this steel. The most commonly observed secondary phases include M23C6 carbide, and intermetallic r, v and Laves phases. Other less commonly observed secondary phases include M6C, p, R, and Cr2N. High amount of these phases reduces the corrosion resistance and mechanical properties of the stainless steel. The type of precipitate strongly depends on the local composition, heating time and temperature. At temperatures ranging from 700 to 1100 °C, carbides are usually the first to form. At longer duration, these carbides are usually replaced by intermetallic compounds, such as r, v and Laves. Low carbon solubility has been found in r, while a high solubility of interstitial elements in the austenite (c) matrix tends to favor the formation of v and Laves phases. Increased Mo percentage favors the precipitation of intermetallic at higher temperatures. High amount of N (0.5 wt.%) has been found to prevent the v phase from forming at all; instead, the formation of the Laves phase is reported. However, the Laves-phase formation is delayed with additional nitrogen [15–18]. It is well known that nitrogen has a greater solid solubility than carbon, and it is a strong austenite stabilizing element [19]. Thus, the addition of nitrogen not only increases the mechanical properties of these materials but also proves useful in delaying or even preventing the formation of harmful precipitates. The microstructural and fatigue properties of different super austenitic stainless steel like AISI 904 L, 254 SMO and AMANOX1.3964 are studied when they are welded by plasma arc and electron beam-welding. The microstructures were fully dendritic and austenitic although the interdendritic materials are richer in ferrite stabilizing elements (Cr, Mo). The HAZ was very narrow in the electron beam welded joints due to the high cooling rate. The fatigue strength of such welded joints was found to be superior to that of the joints of ferritic–pearlitic steel [15,18,20]. The absence of precipitates in the region near the plate surface, close to the fusion line, was justified by the cooling rate which was fast enough to prevent the precipitate formation. However, closer to the plate centers, the welding process has led to a temperature exposure at which precipitation occurred [21]. The temperature and stress fields which are computed in the laser welded region of mild steel sheets using finite element method were studied. It is found that, the temperature decay rate in the molten zone is lower than in the solid. The grain coarsening occurs in the heat affected zone and grain refinement and recrystallization takes place in the fusion zone [22]. CO2 laser welding was performed on super austenitic stainless steel sheets with optimized process parameters. High beam power (3.5 kW) and low travel speed (1200 and 2400 mm/min) with nitrogen as the shielding gas are the parameters that are taken into consideration. It was found that, gas porosity as well as discontinuities undercuts occurred alongside the bead, even in the case of complete penetration [23]. However, the researches in the field of CO2 laser–GMAW hybrid welding were very few. The effect of the shielding gases and its protecting methods which affect the process stability, the weld penetration, mechanical and metallurgical properties, so far have not been discussed in detail in any previous work. Therefore, in this paper, a detailed experiment of CO2 laser–GMAW hybrid welding was carried out on the 904 L super austenitic stainless (SASS) sheets to investigate the effect of the shielding gases and their effects on the mechanical and metallurgical properties of super austenitic stainless steel weld.
set of welding parameters. This model is able to predict the 3D geometry of the weld reinforcement observed at the top of welded samples and the deformation of the melt pool induced by the arc pressure and droplets. Finally, the numerical results are compared with corresponding experimental observations for a wide range of welding parameters [24]. Laser–tungsten inert gas (TIG) hybrid welding were carried out on ultra-fine grained (UFG) steel to investigate the effects of welding parameters on weld shape, microstructure, grain growth in heat-affected zone (HAZ) and mechanical performance. It is concluded that, the Laser–TIG hybrid welding is an effective process for ultrafine grained steel. Compared to laser welding, the hybrid welding can obtain higher welding speed, better weld shape, and lower microhardness resulting in higher toughness [25]. In this study, 3 mm thick 304 stainless steel plates were welded by TIG welding, laser welding and laser–TIG hybrid welding. Characterization of macrostructure, microstructure and phase composition had been investigated. Finally, the mechanical properties and fractured surface morphology had been analyzed. Mechanical properties of 304 stainless steel joints by TIG welding, laser welding and laser–TIG hybrid welding had been studied comparatively [26]. A new 5.2 kW continuous wave (CW) fiber laser combined with a MAG welding of 9.3 mm thick high strength low alloy steel-65 plates are successfully applied to fully penetrate using single-pass hybrid laser–arc welding (HLAW) process to evaluate the welds with respect to microstructure, hardness and distortion. It is concluded that, laser-leading HLAW process can produce higher quality welds due to less under fill and porosity defects than arcleading HLAW. The fusion zone microstructure in the HLAW process consisted of acicular ferrite and bainite phase constituents that restricted the increase in hardness of the welds relative to the base metal [27]. In order to investigate the effect of welding process, thermal elasto-plastic behavior of the hybrid welded joints was compared with a welded joints obtained by conventional submerged arc welding (SAW) process. The results show that the longitudinal residual stress in the hybrid welded joints is less (13–15%) than that of the submerged arc welded joints. Weld metal formed in both welding processes shows very fine dendritic structure. Lower heat input faster cooling rate and smaller volume of the weld metal than SAW process were the reason for lower residual stresses in hybrid welds. Because of sufficiently large cooling rate, the weld metal of both hybrid and SA welded joints exhibits very fine dendritic structure [28]. To compare and analysis the autogenous laser welding with hybrid laser TIG welding of weld joint efficiency and magnitude of residual strain generation. It is reported that, the extra energy input will result in widening the weld bead width, but the depth of penetration will not increase. This means that the joining efficiency decreases and the width of the longitudinal strain profile broadens. For large penetration depths, higher laser powers and the travel speeds give lower energy input and lower residual strains compared to using lower laser power and travel speed [29]. A brief account of the above mentioned researchers carried out in the field of laser–arc welding leads to the present research.
3. Materials and methods 3.1. Selection of base material and filler wire
2. Review of literature A numerical model is developed to calculate the thermal cycles in S355 steel welded by the hybrid laser–MAG process for a given
The butt welding trials were carried out on a 5 mm thick sheet of AISI 904 L super austenitic stainless steel and a 1.2 mm diameter of super austenitic stainless steel solid filler wire was used. The chemical composition of the base material and filler wire is
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P. Sathiya et al. / Materials and Design 33 (2012) 203–212 Table 1 Base material and filler material chemical composition (weight in %). Material
Si
Mn
P
S
Cr
Ni
Mo
C
Cu
Base material Filler material
0.374 0.35
1.522 1.7
0.018 –
0.004 –
19.893 20.0
25.557 25.5
4.124 4.5
0.018 0.01
1.65 1.5
Fig. 1. Experimental set up.
presented in Table 1. Butt welding experiments were conducted on 110 55 5 mm sheets along the longitudinal edge. Prior to welding, abutting surfaces were cleaned with fresh stainless steel wire brush followed by acetone swabbing.
selected under the present study and constant process parameters are shown in Table 2. Butt welding trials were conducted for three different shielding gas mixtures and the typical welded samples are presented in Fig. 2a–c.
3.2. Hybrid welding (laser and GMAW-Synergic)
3.5. Hybrid weld experiments
A Diffusion cooled slab 3.5 kW CO2 laser welding system along with KEMPPI PROMIG 530 (Synergic) welding machine with direct current electrode positive (DCEP) was used and presented in Fig. 1. The Gaussian mode laser (K > 0.9) with a wave length of 10.9 lm and a focal spot size of 180 lm was used throughout the study.
Weld profiles were obtained by sectioning and polishing with suitable abrasive and diamond paste. Weld samples were etched with 10% oxalic acid, an electrolyte, to reveal and increase the contrast of the fusion zone with respect to the base metal. The tensile samples were prepared according to the ASTM E8 [30] sub size and tests were carried out using 40 ton universal testing machine at room temperature. The impact test (ASTM-E 23-07) [31] was conducted at room temperature using a pendulum type impact testing machine. In order to detect various phases present in the weldment, X-ray diffraction (XRD) studies were carried out on different welds for different shielding gases. X-ray diffraction was carried out on planar surfaces of these weld fusion zones. Rigaku Ultimate 3 (Japan) X-ray diffractometer with Cu Ka radiation of wavelength 1.544 Å was used. Microhardness survey was carried out using a Matsuzawa-MMT-X7 Vickers hardness tester at 500 g load for 20 s. The microhardness tests were performed on a transverse section of the weld bead centre parallel to the surface of the sheet in order to identify the possible effects of microstructural heterogeneities both in the weld beads and in the base metal. Samples for characterization were prepared using standard metallographic techniques. The phases were analyzed with optical microscopy and JEOL JSM-5610 LV scanning electron microscope (SEM). In addition the microstructure characterization was studied.
3.3. Shielding gas mixtures 50%He + 50%Ar, 50%He + 45%Ar + 5%O2, 45%He + 45%Ar + 10%N2 at a constant flow rate of 2.5 was used for during experiment. The gases were supplied through a tube of 4 mm diameter in trailing arrangement with the nozzle angle at 45° and standoff distance of 4.5 cm. Torch inclination angle of 53°, standoff distance 15 mm and stick out length 10 mm were used in GMAW process. 3.4. Butt welding parameters A large number of trials have been conducted by varying one of the process parameters and keeping others constant. The working range of hybrid welding parameters like beam power (BP), travel speed (TS), focal position (FP), current (I), wire feed rate (WFR) and voltage (V) has been explored by inspecting bead appearance and the full penetration. The working process parameters were
Table 2 Process parameters. Sample no.
1 2 3
Shielding gas
50%He + 50%Ar 50%He + 45%Ar + 5%O2 45%He + 45%Ar + 10%N2
GMAW parameters
Laser parameters
I (A)
V (V)
Torch angle (°)
WFR (m/min)
BP (Kw)
TS (m/min)
FP (mm)
206 206 206
40 40 40
53 53 53
6 6 6
3.5 3.5 3.5
3 3 3
2 2 2
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P. Sathiya et al. / Materials and Design 33 (2012) 203–212
Fig. 2. (a–c) Hybrid welded (different shielding gas mixtures) photographic views.
namely upper zone (GMAW) and lower zone (laser). The upper zone of the weld profile can easily be described by a semi hemispherical shape. The root part of the weld shows a finger like penetration clearly illustrating the laser keyhole effect.
4. Experimental results 4.1. Macrostructure of the welded joints The butt welding trials were carried out and the photo graphic views of the welded sample for three different shielding gas mixtures are presented in Fig. 2a–c. Fig. 2a–c illustrates a different size bead width with some spattering (Fig. 2c). Typical macrostructures of the three different shielding gas bead profiles are presented in Fig. 3a–c. The bead profiles were measured using Pixel-fox software (German) were loaded with optical microscope and their values are presented in Table 3. In fact the weld characteristic such as the bead width and depth of penetration are the key factors for estimating the weld quality. Indeed, insufficient penetration reduces the weld strength resulting in a poor weld quality. And it is observed that the total weld depth in the laser–GMAW welded joint produced in a single pass is the summation of weld depth of arc (GMAW) at the top and laser at the bottom. It is observed from Fig. 3a–c, that a full penetration is achieved. When examining the hybrid welds, two distinct morphologies were seen as in Fig. 3a–c. Hybrid welds had a two fusion zone
4.2. Microstructures of the welded joints The base material microstructure is presented in Fig. 4. It exposed the well defined grain boundaries. Typical microstructure near the weld junction of the hybrid joints are presented in Fig. 5. From Fig. 5, parent material and fusion zone could be discriminated easily and the heat affected zone remains negligible. The quantitative chemical compositions of the weld metal for different shielding gases were confirmed by electro dispersive spectroscopy (EDS) investigations. The weld metal chemical compositions are presented in Table 4. The chromium and nickel equivalents (Creq and Nieq) with respect to the compositions of the laser-welded zone were determined using the following empirical relation (Eqs. (1) and (2)) [32]:
Creq ¼ %Cr þ %Mo þ 0:7%Nb
ð1Þ
Fig. 3. (a–c) Macrograph of the hybrid welded joints.
Table 3 Bead profile measured values. Experiment no.
Height of GMAW part (mm)
Height of laser part (mm)
Width of GMAW part (mm)
Width of laser part (mm)
Weld area of GMAW part (mm2)
Weld area of laser part (mm)
1 2 3
2.6079 1.1860 2.1964
2.6986 3.6668 2.8015
5.1855 4.4534 4.4110
0.7019 1.2344 0.6535
6.1490 3.1862 4.7385
1.9252 2.9319 1.7114
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P. Sathiya et al. / Materials and Design 33 (2012) 203–212
Fig. 4. Base material microstructure.
and
Nieq ¼ %Ni þ 35%C þ 20%N þ 0:25%Cu
ð2Þ
The symbols denote the chemical compositions in weight percentage. The ratio Creq/Nieq for each shielded weld metals is given in Table 4. A modified Schaffler diagram, which incorporates higher cooling rates involved in welding process, is made use of. Pseudobinary diagram for the prediction of equilibrium microstructure in stainless steels is presented in Fig. 6. This diagram indicates a Creq/Nieq equivalents and the phases present in the microstructure [33,34]. It has been reported [35] that, when the ratio of Creq/Nieq were lower than 1.35, solidification results in austenitic formation and when it was greater than 1.35, ferrite was formed. From Table 4, it is clearly understood that the ratio of Creq/Nieq was lesser than 1.35 for all the three different shielded weld metals (50%He + 50%Ar-; 50%He + 45%Ar + 5%O2-; and 45%He + 45%Ar + 10%N2-). From the weld bead chemical compositions, as shown in Table 4, it is clear that the alloys are in equilibrium and under high cooling rates they get solidified in austenite mode. The weld bead was investigated by means of X-ray diffraction for phase identification as presented in Fig. 7. The peaks in all these patterns correspond only to austenite. Thus any other possible phases like delta ferrite, carbides of
Fig. 6. Pseudo-binary diagram for the prediction of equilibrium microstructure in stainless steels [33].
intermetallic could not be revealed by X-ray diffraction patterns indicating completely austenitic mode of solidification. In all the diffraction patterns, the diffraction peak of c (1 1 1) in weld bead has the highest intensity and more amount of phase. But the diffraction peak of c (2 0 0) and c (2 2 0) in the entire weld bead has the lowest intensity and fewer amounts of phase. The Quantitative determination of the volume fraction (%) of c (1 1 1), c (2 0 0) and c (2 2 0) was estimated by using the following empirical relation (Eq. (3)) [36]:
Xð%Þ ¼
Ið1 1 1Þ 100 Ið1 1 1Þ þ Ið2 0 0Þ þ Ið2 2 0Þ
ð3Þ
where I refer to the X-ray intensity of the corresponding peaks and X is the percentage of volume fraction. From above formula, it is estimated that the more amount of volume fraction for (1 1 1) phase is presented (55.7% for 50%He + 50%Ar, 72.1% for 50%He + 45%Ar + 5%O2 and 74.2% for 45%He + 45%Ar + 10%N2). And less
Fig. 5. Weld junction micrographs.
Table 4 Weld metal chemical composition (weight in %).
*
Sl. no.
Shielding gas*
Cr
Mo
Nb
Ni
C
N
Cu
Creq
Nieq
Creq/Nieq
1 2 3
A B C
19.67 18.27 17.99
2.62 2.79 3.09
0.35 0.37 0.30
20.09 19.96 20.08
0.09 0.11 0.08
0.21 0.39 0.43
0.79 0.83 0.85
22.53 21.31 21.29
27.63 31.81 31.69
0.81 0.67 0.67
A-50%He + 50%Ar; B-50%He + 45%Ar + 5%O2; C-45%He + 45%Ar + 10%N2.
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P. Sathiya et al. / Materials and Design 33 (2012) 203–212
Fig. 7. X-ray diffraction spectrum of the cross section of the weld bead with identification of the diffraction reflections.
amount of volume fraction is observed for (2 0 0) phase and (2 2 0) phase i.e., (33.4% for 50%He + 50%Ar, 19.8% for 50%He + 45%Ar + 5%O2 and 16.8% for 45%He + 45%Ar + 10%N2), (10.9% for 50%He + 50%Ar, 8.1 % for 50%He + 45%Ar + 5%O2 and 9% for 45%He + 45%Ar + 10%N2) respectively. The weld metal microstructures for upper zone (GMAW) and lower zones (laser) are presented in Figs. 8a–c and 9a–c. From Figs. 8a–c and 9a–c, it is clearly seen that the grains are finer in lower zone (laser) and the grains are coarser in upper zone (GMAW). The microstructures in the weld metal (both upper and lower zone) are the result of solidification behavior and subsequent solid phase transformation, which are controlled by composition and weld cooling rates. In the weld metal, a fully long columnar dendritic structure with well developed primary arms and clearly distinguishable short secondary arms are present.
From Fig. 8a–c and Fig. 9a–c, it can be observed that the laser (lower) weld zone microstructure had finer grains size with equiaxed grains are observed. A higher percentage of long columnar grains (Fig. 8a–c) are observed as compared to the lower zone (Fig. 9a–c). The austenitic stabilizing elements like Ni, N and C are partitioned into the primary dendrites (darker phase). This leads to ferrite stabilizing elements like Cr, Mo and Si partitioning into the liquid phase. Along with these elements, partition of impurities is also formed in the molten state and becomes part of interdendritic phases (lighter one). From image analysis, the percentage of amount of phases (dendritic and interdendritic) was presented in the weld metal in the different shielding gas mixtures. The phases were analyzed by Metgi Inverter microscope loaded with DHS image data base (German made). After capturing the micrograph, it can be launched for the metallographic analysis from the convenient DHS image data base plug-in interface. The original microstructure image colors are first converted to gray-scale values. The area detection expansion module uses gray-scale detection (phase analysis) to automatically calculate and process the area ratio by assigning a label and color layer to each phase identification. Then, numeric values are entered and slides are adjusted to define the upper and lower gray-scale limits of each material phase. The percentage of phases present in the weld metal is presented in Table 5. 4.3. Microhardness of the welds The microhardness (VHN) test was performed on the etched transverse cross-section of the laser–GMAW hybrid weld zone using a load of 1 kg, which was applied for duration of 20 s. Five measurements in each hybrid weld zone (base material across the weld toward its opposite side) were taken at regular intervals. The hardness values were measured 1 mm below the upper surface (GMAW zone) and the 1 mm above the lower surface (laser zone). The comparative hardness profile is presented in Fig. 10. From
Fig. 8. (a–c) Weld metal microstructure for upper zone (GMAW).
Fig. 9. (a–c) Weld metal microstructure for lower zone (laser).
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P. Sathiya et al. / Materials and Design 33 (2012) 203–212 Table 5 Percentage amounts of phases in the weld bead. Experiment no.
Shielding gas mixtures
1 2 3
50%He + 50%Ar 50%He + 45%Ar + 5%O2 45%He + 45%Ar + 10%N2
Primary dendritic phase (%)
Secondary interdendritic phase (%)
GMAW zone
Laser zone
GMAW zone
Laser zone
32 37 35
34 30 38
68 65 63
64 62 70
260
A B C
250 240
260
Microhardness (HV)
Microhardness (HV)
A B C
280
230 220 210 200
240
220
200
190 180
180 -0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
-0.8
Distance from weld centreline (mm)
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Distance from weld centreline (mm)
Fig. 10. (a and b) Hardness profile (A-50%He + 50%Ar, B-50%He + 45%Ar + 5%O2 and C-45%He + 45%Ar + 10%N2).
Fig. 10a and b, it is seen that the hardness values are higher in laser zone compared to GMAW zone. From Fig. 10a and b it can be seen an increase in the hardness in the weld zone may be induced by precipitation hardening effects, residual stresses or microstructure refinement due to the rapid solidification of the weld pool. From Fig. 10a it is observed that in GMAW zone; the 50%He + 45%Ar + 5%O2 hardness values are higher than the other two shielded weld metal hardness’s. This is due to more amount of oxygen content (5%) present in the GMAW weld zone and also a slow cooling rate. From Fig. 10b in laser zone, it is clearly seen that the 45%He + 45%Ar + 10%N2 hardness values are high than the others.
Fig. 11. From Fig. 11, it is clearly seen that the fractures occur away from the joint in the weld joint. Among the three shielded weld metals, tensile strength is higher than the base material and in particular in the 50%He + 45%Ar + 5%O2 shielded weld, tensile strength is much higher than the others. The fractured surface of tensile specimen is analyzed using SEM. The fracture surface of the tensile tested fractrograph is shown in Fig. 12. The 50%He + 45%Ar + 5%O2 shielded laser– GMAW hybrid joints as revealed by the fractrograph consist of smaller dimples, but some coarse dimples are seen distributed sparsely among fine dimples (Fig. 12b).
4.4. Tensile properties
4.5. Impact properties
The main aim of the tensile test is to evaluate the strength and plasticity of welding joints and examine the influence of welding defects on the joint performance. The test is done on computer interface electronic universal testing machine. The transverse tensile strength, yield strength and percentage of elongation of all joints have been evaluated. The tensile test results of laser–GMAW hybrid welding joints are presented in Table 6. From Table 6, it is clear that the weld metal tensile strength is much higher than the base metal tensile strength which is in line with the microhardness results. The tensile tested samples are presented in
In order to evaluate the Charpy impact toughness values of welded joint, series of Charpy V-notch test were carried out from specimen welded with different shielding gases at room temperature. Notches were prepared in the base metal and weld zone. The value obtained from the testing is shown in Table 7. The impact toughness of unwelded base metal is found to be 60 Joules, which is comparatively lower than the other three shielded weld metal impact strength. From Table 7 it is clear that among the three different shielded laser–GMAW hybrid joints, 50%He + 45%Ar + 5%O2 shielded weld joints exhibit higher impact toughness values,
Table 6 Tensile test results. Sl. no.
Shielding gas
Yield strength (MPa)
Ultimate tensile strength (MPa)
% of elongation
Fracture location
1 2 3 Base metal
50%He + 50%Ar 50%He + 45%Ar + 5%O2 45%He + 45%Ar + 10%N2
470 491 479 340
579 594 585 570
40.0 48.3 46.8 38.0
Base metal Base metal Base metal
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P. Sathiya et al. / Materials and Design 33 (2012) 203–212 Table 7 Impact test results.
Fig. 11. Tensile tested samples.
and the enhancement in toughness value is approximately 36% compared to base metal. The fractured surface of impact specimens is analyzed using SEM. The fractured surface of the impact tested fractrograph is shown in Fig. 13a–c. 5. Discussions From the above results, it is conceivable that the bottom of the laser keyhole is at least partially solidified and not remelted by the GMAW process. This is due to the incomplete mixing of the molten weld pool from the GMAW in the back to the front and down the laser key hole [37]. From Fig. 3a–c and Table 3, it is also clear that the weld shape is shallow and wide under 50% He + 50% Ar shielding gas mixtures. When a small amount of oxygen (5%) and nitrogen (10%) is mixed into the 50%He + 45%Ar and 45%He + 45% Ar shielding gas mixtures, the weld shape changes from the shallow
Sl. no.
Shielding gas
Impact strength (Joules)
1 2 3
50%He + 50%Ar 50%He + 45%Ar + 5%O2 45%He + 45%Ar + 10%N2
66 82 62
type to the deep narrow one as shown in Fig. 3b and c. This is due to plasma plume formation suppressed in the oxygen and nitrogen shield weld. The depth penetration is higher in laser (lower) zone. From Table 5, it is clearly observed that, in GMAW upper zone 50%He + 45%Ar + 5%O2 shielded weld metal primary dendritic is more and in the lower zone (laser) weld metal a high amount of primary dendritic is present in 45%He + 45%Ar + 10%N2. And no carbide precipitation and intermetallic phases from the microstructure are found. This is due to addition of nitrogen and oxygen to decrease the tendency of formation of precipitates and intermetallic phases. Nitrogen is a more effective solid solution strengthening element than carbon and it also enhances grain refining. Nitrogen is a strong austenite stabilizer, thereby reducing the amount of nickel required for austenite stabilization and lowering the tendency to form ferrite. It has been [38] reported that, in gas tungsten arc welding of a super austenitic stainless steel with shielding gases and arc lengths yielding nitrogen contents in excess of 0.2%, more porosity is observed. The microhardness values are higher in laser zone compared to GMAW zone. This may be the result of the effect of rapid solidification. Rapid solidification increases under cooling and nucleation probability leads to very fine microstructure. At the same time it extends solute solubility which results in a supersaturated
Fig. 12. (a–c) SEM fractrograph microstructure.
P. Sathiya et al. / Materials and Design 33 (2012) 203–212
211
Fig. 13. (a–c) SEM fractrograph microstructure.
solid solution. During the rapid solidification of the weld zone, the material generally loses its original strength by strain hardening effect. In particularly, 45%He + 45%Ar + 10%N2 hardness values are higher than the others. This could be possible due to the formation of hard nitride phases. On the other hand, the higher amount of primary austenite phase is present in GMAW (top) zone is 50%He + 45%Ar + 5%O2 and laser (bottom) zone is 45%He + 45%Ar + 10%N2. Due to formation of higher amount of primary austenitic (Table 5), the hardness values are higher in 45%He + 45%Ar + 10%N2 for laser zone and 50%He + 45%Ar + 5%O2 for GMAW zone. Most of the tensile specimens failed in the base region and it suggests that the weld region is stronger than the base metal region. This fact is also evident from the hardness measurements profile (weld region shows higher hardness than the base metal region) for all different shielded weld metals. There exists appreciable difference between the three different shielded laser–GMAW hybrid joints in the appearance of dimples. The 45%He + 45%Ar + 10%N2 shielded hybrid joints invariably consist of fine and uniform dimples (in Fig. 13c). The 50%He + 50% Ar shielded joints consists of coarser dimple size than the others. Due to coarser dimple size, the tensile strength is less when compared to other shielded weld tensile strength. In particular, 50%He + 45%Ar + 5%O2 shielded fracture surface appears in ductile with partially cleavage fracture (Fig. 12b) and also from Fig. 12a–c, it is clearly seen that the fracture surface appeared in pure shear fracture. The impact properties are of greater importance if the materials are to be employed for temperature applications. This is due to the least amount of nitrogen in the weld metal and also due to the presence high amount of interdendritic phases in the weld metal.
The addition of nitrogen to shielded weld joint lowers the toughness due to presence of hard nitride phases. The precipitation of intermetallic phases has a major role to play in affecting the impact toughness of the weld metal. From XRD patterns (Fig. 7), all the peaks are in austenitic phase and no other intermetallic phases are present. Due to the absence of intermetallic phases in the weld metal, the toughness of the weld metal is higher than the base metal. An appreciable amount of dimples and elongated fine dimples are seen in 50%He + 45%Ar + 5%O2 SEM structure. The dimple size exhibits a directly proportional relationship with strength and ductility, that is, if the dimple size is finer, the strength and ductility of the respective joint is higher and vice versa. The high impact toughness value of the weld is due to the presence of finer grain structure compared to the base material. However, the impact specimen fracture surfaces of hybrid welded joints show mixed mode fractures, that is, ductile and cleavage fractures and not a pure ductile fracture which is due to the presence of small interdendritic phases. It is clearly seen from Figs. 8 and 9a–c, that the percentage of secondary interdendritic phases present in the weld metal is much higher than the primary dendritic phases for all shielded weld metals. Due to high amount of interdendritic, austenite is attributed to the high cooling rates, which makes the relevant coarsening of primary austenite dendrites difficult. Due to higher ionization potential, fast cooling rate and also the high amount of nitrogen content in the weld metal, the percentage of secondary interdendritic phases present in nitrogen addition shielded weld metal is much higher than the other shielded weld metals.
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6. Conclusions Based on the Laser GMAW hybrid welding with different shielding gas mixtures (AISI 904 L SASS) the following points are deduced: 1. The full penetration is achieved without any defects. The weld shape is shallow and wide under 50% He + 50% Ar shielding gas mixtures. When a small amount of oxygen (5%) and nitrogen (10%) is mixed into the 50%He + 45%Ar and 45%He + 45% Ar shielding gas mixtures, the weld shape changes from the shallow type to the deep narrow shape. 2. Due to rapid cooling with high heat energy in laser zone, the grains are getting finer refinement. The laser (lower) weld zone microstructure had finer grains with equi axed grains are present. A higher percentage of long columnar grains are present in upper zone (GMAW). 3. In GMAW upper zone, 50%He + 45%Ar + 5%O2 shielded weld metal has more primary dendritic and in the lower zone (laser) weld metal a high amount of primary dendritic is present in 45%He + 45%Ar + 10%N2 shielding gas. 4. The 50%He + 45%Ar + 5%O2 hardness values are higher than the other two shielded weld metal hardness in GMAW (upper) weld zone. Due to plasma plume suppression in addition of nitrogen shielding gas, the hardness values are higher in 45%He + 45%Ar + 10%N2 shielded laser zone. 5. The tensile strength is less in 50% He + 50% Ar shielded weld when compared to other two shielded weld tensile strength. The fracture surface appeared in pure shear fracture in 45%He + 45%Ar + 10%N2 and in 50%He + 45%Ar + 5%O2 shielded fracture surface appears ductile with partially cleavage fracture occurring. 6. 50%He + 45%Ar + 5%O2 shielded weld joints exhibit higher impact toughness values, and the enhancement in toughness value is approximately 36% compared to base metal. The impact specimen fracture surfaces of the different shielding gas laser– GMAW hybrid welded joints show mixed mode fractures, that is, ductile and cleavage fractures.
Acknowledgments We would like to express our deepest appreciation to the Department of Science & Technology (DST), New Delhi, India for their guidance and financial assistance to this research under Fast Track Proposal SR/FTP/ETA-11/2007. We further wish to convey our gratitude to Dr. G. Padmanabham for offering us the distinguished opportunity to use the laser welding trials at the International Advanced Research Centre for powder metallurgy & New materials, Hyderabad – 500 005, Andhra Pradesh, India. References [1] Steen WM. Arc augmented laser processing of materials. J Appl Phys 1980;51:5636–41. [2] Bagger C, Olsen FO. Review of laser hybrid welding. J Laser Appl 2005;17:2–14. [3] Tusek J, Suban M. Hybrid welding with arc and laser beam. Sci Technol Weld Join 1999;4:308–11. [4] Shinn BW, Farson DF, Denney PE. Laser stabilization of arc cathode spots in titanium welding. Sci Technol Weld Join 2005;10:475–81. [5] Bagger C, Sondrup LC, Olsen FO, Laser/TIG hybrid welding of pot for induction heater. In: Proceedings of ICALEO 2004, hybrid laser welding, San Francisco, California; 2004. p. 60–9. [6] Fujinaga S, Ohashi R, Katayama S. Improvements of welding characteristics of aluminum alloys with YAG laser and TIG arc hybrid system. In: Miyamoto I editor. Proc. SPIE 2002;4831:301–6.
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Interaction of both plasmas in CO2 laser–MAG hybrid welding of carbon steel. Proc SPIE 2003;4831:341–6. [13] Fellman A, Salminen A, Kujanpaa V. A study of the molten filler material movements during CO2–laser–MAG hybrid welding. In: Proceedings of the 3rd international WLT conference on lasers in manufacturing; 2005. p. 171–6. [14] Fellman A, Jernstrom P, Kujanpaa V, CO2–GMA hybrid welding of carbon steel – the effect of shielding gas composition. In: Proceedings of ICALEO 2003, laser materials processing conference, Jacksonville, FL; 2003. p. 56–65. [15] Heino S, Knutson-Wedel M, Karlsson B. Precipitation behavior in heat affected zone of welded super austenitic stainless steel. Mater Sci Technol 1999;15: 101–8. [16] Heino S, Knutson-Wedel M, Karlsson B. Precipitation in a high nitrogen super austenitic stainless steel. Mater Sci Technol Forum 1999;318–320:143–8. [17] Svoboda M, Kroupa A, Soousek J, Vrestal J, Miodownik P. Phase changes in super austenitic steels after long-term annealing. Zeitschrift Fur Metallkunde 2004;95:1025–30. [18] Lee TH, Kim SJ, Jung YC. Crystallographic details of precipitates in Fe–22Cr– 21Ni–6Mo–(N) super austenitic stainless steels aged at 900 °C. Metal Mater Trans A 2000;31:1713–23. [19] Simmons JW. Overview: high nitrogen alloying of stainless steels. Mater Sci Eng A 1996;207:159–69. [20] Bonollo F, Tiziani A, Tovo T, Volpone LM. Super austenitic stainless steel: the microstructure and fatigue strength a welded joint. Weld Int 2004;18:24–30. [21] Nastac L, Stefanescu LM. An analytical model for solute redistribution during solidification of planar, columnar, or equiaxed morphology. Metal Trans A 1993;24:2107–18. [22] Yilbas BS, Arif AFM, Abdul Aleem BJ. Laser welding of low carbon steel and thermal stress analysis. Opt Laser Technol 2010;42:760–8. [23] Zambon A, Ferro P, Bonollo F. Microstructural, compositional and residual stress evaluation of CO2 laser welded super austenitic AISI 904 L stainless steel. Mater Sci Eng A 2006;424:117–27. [24] Le Guen Emilie, Muriel, Fabbro Rémy, Costea Frédéric, Le Masson Philippe. 3D heat transfer model of hybrid laser Nd:Yag-MAG welding of S355 steel and experimental validation. Int J Heat Mass Transfer 2011;54:1313–22. [25] Gao Ming, Zeng Xiaoyan, Hu Qianwu, Yan Jun. Laser–TIG hybrid welding of ultra-fine grained steel. J Mater Process Technol 2009;209:785–91. [26] Yan Jun, Gao Ming, Zeng Xiaoyan. Study on microstructure and mechanical properties of 304 stainless steel joints by TIG, laser and laser–TIG hybrid welding. Opt Lasers Eng 2010;48:512–7. [27] Cao X, Wanjara P, Huang J, Munro C, Nolting A. Hybrid fiber laser – arc welding of thick section high strength low alloy steel. Mater Des 2011;32:3399–413. [28] Bang Hee Seon, Bang Han Sur, Kim You Chul, Oh Ik Hyun. A study on mechanical and microstructure characteristics of the STS304L butt joints using hybrid CO2 laser-gas metal arc welding. Mater Des 2011;32:2328–33. [29] Suder W, Ganguly S, Williams S, Paradowska AM, Colegrove P. Comparison of joining efficiency and residual stresses in laser and laser hybrid welding. Sci Technol Weld Join 2011;16(3):244–8. [30] ASTM E8 M-04. Standard test method for tension testing of metallic materials. ASTM International; 2006. [31] ASTM E 23-07. Standard test methods for notched bar impact testing of metallic materials. ASTM International; 2007. p. 184. [32] Molian PA. Solidification behavior of laser welded stainless steel. Mater Sci Lett 1985;4:281–3. [33] Bonollo F, Tiziani A, Tovo T, Volpone LM. Super austenitic stainless steel: the microstructure and fatigue strength a welded joints. Weld Int 2004;18:24–30. [34] David SA, Vitek JM. Laser in metallurgy. In: Conference proceedings of the metallurgical society of AIME; 1981. p. 147–50. 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Technical Report
Effect of shielding gases on microstructure and mechanical properties of super austenitic stainless steel by hybrid welding P. Sathiya a,⇑, Mahendra Kumar Mishra a, B. Shanmugarajan b a b
Department of Production Engineering, National Institute of Technology, Tiruchirappalli 620 015, Tamil Nadu, India International Advanced Research Centre for Powder Metallurgy & New Materials, Hyderabad 500 005, Andhra Pradesh, India
a r t i c l e
i n f o
Article history: Received 8 March 2011 Accepted 28 June 2011 Available online 12 August 2011
a b s t r a c t This paper investigates the bead geometry, microstructure and mechanical properties of AISI 904 L super austenitic stainless steel joint by CO2 laser–GMAW hybrid welding process. Shielding gas is one of the important parameters for the process stability and efficient synergetic effects between laser and gas metal arc welding (GMAW). A detailed study of CO2 laser–GMAW hybrid welding with different shielding gas mixtures in different ratio (50%He + 50%Ar, 50%He + 45%Ar + 5%O2, and 45%He + 45%Ar + 10%N2) was carried out on AISI 904 L super austenitic stainless steel sheet of 5 mm thickness. The weld penetration of hybrid welding was determined by the plasma shape varying with the shielding gas parameters, especially the plasma height interacting with incident plasma. The X-ray diffraction was used to analyze the phase composition, while the microstructural characterization was performed by phase microscopy of the joints. The impact and tensile tests were performed and fracture surface morphology had been analyzed through scanning electron microscope (SEM). Finally, hardness test is performed along the longitudinal direction of the weld zone. The results showed that the joint by laser–GMAW hybrid had higher tensile and impact strength than the base metal. The fractrography observation showed the cup-cone shaped fracture while the hybrid welding joint mixed mode of fracture. The laser–GMAW hybrid welding is suitable for welding of AISI 904 L super austenitic stainless steel owing to their high welding speed and excellent mechanical properties. Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
1. Introduction The laser–arc hybrid welding which integrates laser beam and arc welding into one process eliminates the disadvantage of individual processes. At the same time it has its own particular advantages, viz., the deeper welding penetration, high welding speed, less deformation, the ability to bridge relatively large gaps and the capability of handling highly reflective materials [1–4]. For these advantages, laser–arc hybrid welding technology has received significant attention in recent years. But there were only a few studies that reported the industrial applications of the laser–arc hybrid welding. Most of the researches [3–8] carried out so far have dwelt on the improvement of the weldability of a particular material. In order to get the most efficient synergetic effects between the laser and the arc, many problems like lack of penetration and gap bridge ability about the matching of numerous hybrid welding parameters must be solved. Laser CO2–GMAW is suitable for shipbuilding and transport industry and panel fabrication in aerospace industry. The two welding sources, coupled to perform a hybrid welding process, re-
⇑ Corresponding author. Tel.: +91 431 2503510; fax: +91 431 2500133. E-mail address: [email protected] (P. Sathiya).
quire a fine tuning of both sets of technological parameters: laser welding parameter and GMAW parameters in order to obtain a stable, repeatable and productive process. Based on hybrid welding, many studies have been carried out regarding power-related parameters [9,10] viz., coupled arc voltage, laser beam power and on source positioning related ones [11] such as defocus position and distance between the sources, in order to trace out the basics regarding the applicability of the process. More specific studies have been carried out considering plasma interaction [12] and molten pool fluid dynamics [13] with the aim of tuning the complex equilibrium which stands behind this kind of processes. The shielding gas and its protecting methods were also important for the behaviors of CO2 laser–MIG hybrid welding such as the laser–arc interaction and the energy transfer of the two heat sources, which directly impacts the laser–arc synergetic effects. Fellman et al. had discussed how the shielding gas composition (helium–argon–CO2) affected the weld quality, weld profile and process stability of the CO2 laser–metal active gas (MAG) hybrid welding [14]. Super austenitic stainless steel is Fe-based system highly alloyed with Cr, Ni, Mo, and N to produce excellent pitting and crevice corrosion-resistant properties at high temperatures and in seawater. When exposed to elevated temperatures for long periods of time, large amounts of precipitates, including carbides, nitrides,
0261-3069/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.06.065
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and intermetallic phases, are formed in this steel. The most commonly observed secondary phases include M23C6 carbide, and intermetallic r, v and Laves phases. Other less commonly observed secondary phases include M6C, p, R, and Cr2N. High amount of these phases reduces the corrosion resistance and mechanical properties of the stainless steel. The type of precipitate strongly depends on the local composition, heating time and temperature. At temperatures ranging from 700 to 1100 °C, carbides are usually the first to form. At longer duration, these carbides are usually replaced by intermetallic compounds, such as r, v and Laves. Low carbon solubility has been found in r, while a high solubility of interstitial elements in the austenite (c) matrix tends to favor the formation of v and Laves phases. Increased Mo percentage favors the precipitation of intermetallic at higher temperatures. High amount of N (0.5 wt.%) has been found to prevent the v phase from forming at all; instead, the formation of the Laves phase is reported. However, the Laves-phase formation is delayed with additional nitrogen [15–18]. It is well known that nitrogen has a greater solid solubility than carbon, and it is a strong austenite stabilizing element [19]. Thus, the addition of nitrogen not only increases the mechanical properties of these materials but also proves useful in delaying or even preventing the formation of harmful precipitates. The microstructural and fatigue properties of different super austenitic stainless steel like AISI 904 L, 254 SMO and AMANOX1.3964 are studied when they are welded by plasma arc and electron beam-welding. The microstructures were fully dendritic and austenitic although the interdendritic materials are richer in ferrite stabilizing elements (Cr, Mo). The HAZ was very narrow in the electron beam welded joints due to the high cooling rate. The fatigue strength of such welded joints was found to be superior to that of the joints of ferritic–pearlitic steel [15,18,20]. The absence of precipitates in the region near the plate surface, close to the fusion line, was justified by the cooling rate which was fast enough to prevent the precipitate formation. However, closer to the plate centers, the welding process has led to a temperature exposure at which precipitation occurred [21]. The temperature and stress fields which are computed in the laser welded region of mild steel sheets using finite element method were studied. It is found that, the temperature decay rate in the molten zone is lower than in the solid. The grain coarsening occurs in the heat affected zone and grain refinement and recrystallization takes place in the fusion zone [22]. CO2 laser welding was performed on super austenitic stainless steel sheets with optimized process parameters. High beam power (3.5 kW) and low travel speed (1200 and 2400 mm/min) with nitrogen as the shielding gas are the parameters that are taken into consideration. It was found that, gas porosity as well as discontinuities undercuts occurred alongside the bead, even in the case of complete penetration [23]. However, the researches in the field of CO2 laser–GMAW hybrid welding were very few. The effect of the shielding gases and its protecting methods which affect the process stability, the weld penetration, mechanical and metallurgical properties, so far have not been discussed in detail in any previous work. Therefore, in this paper, a detailed experiment of CO2 laser–GMAW hybrid welding was carried out on the 904 L super austenitic stainless (SASS) sheets to investigate the effect of the shielding gases and their effects on the mechanical and metallurgical properties of super austenitic stainless steel weld.
set of welding parameters. This model is able to predict the 3D geometry of the weld reinforcement observed at the top of welded samples and the deformation of the melt pool induced by the arc pressure and droplets. Finally, the numerical results are compared with corresponding experimental observations for a wide range of welding parameters [24]. Laser–tungsten inert gas (TIG) hybrid welding were carried out on ultra-fine grained (UFG) steel to investigate the effects of welding parameters on weld shape, microstructure, grain growth in heat-affected zone (HAZ) and mechanical performance. It is concluded that, the Laser–TIG hybrid welding is an effective process for ultrafine grained steel. Compared to laser welding, the hybrid welding can obtain higher welding speed, better weld shape, and lower microhardness resulting in higher toughness [25]. In this study, 3 mm thick 304 stainless steel plates were welded by TIG welding, laser welding and laser–TIG hybrid welding. Characterization of macrostructure, microstructure and phase composition had been investigated. Finally, the mechanical properties and fractured surface morphology had been analyzed. Mechanical properties of 304 stainless steel joints by TIG welding, laser welding and laser–TIG hybrid welding had been studied comparatively [26]. A new 5.2 kW continuous wave (CW) fiber laser combined with a MAG welding of 9.3 mm thick high strength low alloy steel-65 plates are successfully applied to fully penetrate using single-pass hybrid laser–arc welding (HLAW) process to evaluate the welds with respect to microstructure, hardness and distortion. It is concluded that, laser-leading HLAW process can produce higher quality welds due to less under fill and porosity defects than arcleading HLAW. The fusion zone microstructure in the HLAW process consisted of acicular ferrite and bainite phase constituents that restricted the increase in hardness of the welds relative to the base metal [27]. In order to investigate the effect of welding process, thermal elasto-plastic behavior of the hybrid welded joints was compared with a welded joints obtained by conventional submerged arc welding (SAW) process. The results show that the longitudinal residual stress in the hybrid welded joints is less (13–15%) than that of the submerged arc welded joints. Weld metal formed in both welding processes shows very fine dendritic structure. Lower heat input faster cooling rate and smaller volume of the weld metal than SAW process were the reason for lower residual stresses in hybrid welds. Because of sufficiently large cooling rate, the weld metal of both hybrid and SA welded joints exhibits very fine dendritic structure [28]. To compare and analysis the autogenous laser welding with hybrid laser TIG welding of weld joint efficiency and magnitude of residual strain generation. It is reported that, the extra energy input will result in widening the weld bead width, but the depth of penetration will not increase. This means that the joining efficiency decreases and the width of the longitudinal strain profile broadens. For large penetration depths, higher laser powers and the travel speeds give lower energy input and lower residual strains compared to using lower laser power and travel speed [29]. A brief account of the above mentioned researchers carried out in the field of laser–arc welding leads to the present research.
3. Materials and methods 3.1. Selection of base material and filler wire
2. Review of literature A numerical model is developed to calculate the thermal cycles in S355 steel welded by the hybrid laser–MAG process for a given
The butt welding trials were carried out on a 5 mm thick sheet of AISI 904 L super austenitic stainless steel and a 1.2 mm diameter of super austenitic stainless steel solid filler wire was used. The chemical composition of the base material and filler wire is
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P. Sathiya et al. / Materials and Design 33 (2012) 203–212 Table 1 Base material and filler material chemical composition (weight in %). Material
Si
Mn
P
S
Cr
Ni
Mo
C
Cu
Base material Filler material
0.374 0.35
1.522 1.7
0.018 –
0.004 –
19.893 20.0
25.557 25.5
4.124 4.5
0.018 0.01
1.65 1.5
Fig. 1. Experimental set up.
presented in Table 1. Butt welding experiments were conducted on 110 55 5 mm sheets along the longitudinal edge. Prior to welding, abutting surfaces were cleaned with fresh stainless steel wire brush followed by acetone swabbing.
selected under the present study and constant process parameters are shown in Table 2. Butt welding trials were conducted for three different shielding gas mixtures and the typical welded samples are presented in Fig. 2a–c.
3.2. Hybrid welding (laser and GMAW-Synergic)
3.5. Hybrid weld experiments
A Diffusion cooled slab 3.5 kW CO2 laser welding system along with KEMPPI PROMIG 530 (Synergic) welding machine with direct current electrode positive (DCEP) was used and presented in Fig. 1. The Gaussian mode laser (K > 0.9) with a wave length of 10.9 lm and a focal spot size of 180 lm was used throughout the study.
Weld profiles were obtained by sectioning and polishing with suitable abrasive and diamond paste. Weld samples were etched with 10% oxalic acid, an electrolyte, to reveal and increase the contrast of the fusion zone with respect to the base metal. The tensile samples were prepared according to the ASTM E8 [30] sub size and tests were carried out using 40 ton universal testing machine at room temperature. The impact test (ASTM-E 23-07) [31] was conducted at room temperature using a pendulum type impact testing machine. In order to detect various phases present in the weldment, X-ray diffraction (XRD) studies were carried out on different welds for different shielding gases. X-ray diffraction was carried out on planar surfaces of these weld fusion zones. Rigaku Ultimate 3 (Japan) X-ray diffractometer with Cu Ka radiation of wavelength 1.544 Å was used. Microhardness survey was carried out using a Matsuzawa-MMT-X7 Vickers hardness tester at 500 g load for 20 s. The microhardness tests were performed on a transverse section of the weld bead centre parallel to the surface of the sheet in order to identify the possible effects of microstructural heterogeneities both in the weld beads and in the base metal. Samples for characterization were prepared using standard metallographic techniques. The phases were analyzed with optical microscopy and JEOL JSM-5610 LV scanning electron microscope (SEM). In addition the microstructure characterization was studied.
3.3. Shielding gas mixtures 50%He + 50%Ar, 50%He + 45%Ar + 5%O2, 45%He + 45%Ar + 10%N2 at a constant flow rate of 2.5 was used for during experiment. The gases were supplied through a tube of 4 mm diameter in trailing arrangement with the nozzle angle at 45° and standoff distance of 4.5 cm. Torch inclination angle of 53°, standoff distance 15 mm and stick out length 10 mm were used in GMAW process. 3.4. Butt welding parameters A large number of trials have been conducted by varying one of the process parameters and keeping others constant. The working range of hybrid welding parameters like beam power (BP), travel speed (TS), focal position (FP), current (I), wire feed rate (WFR) and voltage (V) has been explored by inspecting bead appearance and the full penetration. The working process parameters were
Table 2 Process parameters. Sample no.
1 2 3
Shielding gas
50%He + 50%Ar 50%He + 45%Ar + 5%O2 45%He + 45%Ar + 10%N2
GMAW parameters
Laser parameters
I (A)
V (V)
Torch angle (°)
WFR (m/min)
BP (Kw)
TS (m/min)
FP (mm)
206 206 206
40 40 40
53 53 53
6 6 6
3.5 3.5 3.5
3 3 3
2 2 2
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Fig. 2. (a–c) Hybrid welded (different shielding gas mixtures) photographic views.
namely upper zone (GMAW) and lower zone (laser). The upper zone of the weld profile can easily be described by a semi hemispherical shape. The root part of the weld shows a finger like penetration clearly illustrating the laser keyhole effect.
4. Experimental results 4.1. Macrostructure of the welded joints The butt welding trials were carried out and the photo graphic views of the welded sample for three different shielding gas mixtures are presented in Fig. 2a–c. Fig. 2a–c illustrates a different size bead width with some spattering (Fig. 2c). Typical macrostructures of the three different shielding gas bead profiles are presented in Fig. 3a–c. The bead profiles were measured using Pixel-fox software (German) were loaded with optical microscope and their values are presented in Table 3. In fact the weld characteristic such as the bead width and depth of penetration are the key factors for estimating the weld quality. Indeed, insufficient penetration reduces the weld strength resulting in a poor weld quality. And it is observed that the total weld depth in the laser–GMAW welded joint produced in a single pass is the summation of weld depth of arc (GMAW) at the top and laser at the bottom. It is observed from Fig. 3a–c, that a full penetration is achieved. When examining the hybrid welds, two distinct morphologies were seen as in Fig. 3a–c. Hybrid welds had a two fusion zone
4.2. Microstructures of the welded joints The base material microstructure is presented in Fig. 4. It exposed the well defined grain boundaries. Typical microstructure near the weld junction of the hybrid joints are presented in Fig. 5. From Fig. 5, parent material and fusion zone could be discriminated easily and the heat affected zone remains negligible. The quantitative chemical compositions of the weld metal for different shielding gases were confirmed by electro dispersive spectroscopy (EDS) investigations. The weld metal chemical compositions are presented in Table 4. The chromium and nickel equivalents (Creq and Nieq) with respect to the compositions of the laser-welded zone were determined using the following empirical relation (Eqs. (1) and (2)) [32]:
Creq ¼ %Cr þ %Mo þ 0:7%Nb
ð1Þ
Fig. 3. (a–c) Macrograph of the hybrid welded joints.
Table 3 Bead profile measured values. Experiment no.
Height of GMAW part (mm)
Height of laser part (mm)
Width of GMAW part (mm)
Width of laser part (mm)
Weld area of GMAW part (mm2)
Weld area of laser part (mm)
1 2 3
2.6079 1.1860 2.1964
2.6986 3.6668 2.8015
5.1855 4.4534 4.4110
0.7019 1.2344 0.6535
6.1490 3.1862 4.7385
1.9252 2.9319 1.7114
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Fig. 4. Base material microstructure.
and
Nieq ¼ %Ni þ 35%C þ 20%N þ 0:25%Cu
ð2Þ
The symbols denote the chemical compositions in weight percentage. The ratio Creq/Nieq for each shielded weld metals is given in Table 4. A modified Schaffler diagram, which incorporates higher cooling rates involved in welding process, is made use of. Pseudobinary diagram for the prediction of equilibrium microstructure in stainless steels is presented in Fig. 6. This diagram indicates a Creq/Nieq equivalents and the phases present in the microstructure [33,34]. It has been reported [35] that, when the ratio of Creq/Nieq were lower than 1.35, solidification results in austenitic formation and when it was greater than 1.35, ferrite was formed. From Table 4, it is clearly understood that the ratio of Creq/Nieq was lesser than 1.35 for all the three different shielded weld metals (50%He + 50%Ar-; 50%He + 45%Ar + 5%O2-; and 45%He + 45%Ar + 10%N2-). From the weld bead chemical compositions, as shown in Table 4, it is clear that the alloys are in equilibrium and under high cooling rates they get solidified in austenite mode. The weld bead was investigated by means of X-ray diffraction for phase identification as presented in Fig. 7. The peaks in all these patterns correspond only to austenite. Thus any other possible phases like delta ferrite, carbides of
Fig. 6. Pseudo-binary diagram for the prediction of equilibrium microstructure in stainless steels [33].
intermetallic could not be revealed by X-ray diffraction patterns indicating completely austenitic mode of solidification. In all the diffraction patterns, the diffraction peak of c (1 1 1) in weld bead has the highest intensity and more amount of phase. But the diffraction peak of c (2 0 0) and c (2 2 0) in the entire weld bead has the lowest intensity and fewer amounts of phase. The Quantitative determination of the volume fraction (%) of c (1 1 1), c (2 0 0) and c (2 2 0) was estimated by using the following empirical relation (Eq. (3)) [36]:
Xð%Þ ¼
Ið1 1 1Þ 100 Ið1 1 1Þ þ Ið2 0 0Þ þ Ið2 2 0Þ
ð3Þ
where I refer to the X-ray intensity of the corresponding peaks and X is the percentage of volume fraction. From above formula, it is estimated that the more amount of volume fraction for (1 1 1) phase is presented (55.7% for 50%He + 50%Ar, 72.1% for 50%He + 45%Ar + 5%O2 and 74.2% for 45%He + 45%Ar + 10%N2). And less
Fig. 5. Weld junction micrographs.
Table 4 Weld metal chemical composition (weight in %).
*
Sl. no.
Shielding gas*
Cr
Mo
Nb
Ni
C
N
Cu
Creq
Nieq
Creq/Nieq
1 2 3
A B C
19.67 18.27 17.99
2.62 2.79 3.09
0.35 0.37 0.30
20.09 19.96 20.08
0.09 0.11 0.08
0.21 0.39 0.43
0.79 0.83 0.85
22.53 21.31 21.29
27.63 31.81 31.69
0.81 0.67 0.67
A-50%He + 50%Ar; B-50%He + 45%Ar + 5%O2; C-45%He + 45%Ar + 10%N2.
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Fig. 7. X-ray diffraction spectrum of the cross section of the weld bead with identification of the diffraction reflections.
amount of volume fraction is observed for (2 0 0) phase and (2 2 0) phase i.e., (33.4% for 50%He + 50%Ar, 19.8% for 50%He + 45%Ar + 5%O2 and 16.8% for 45%He + 45%Ar + 10%N2), (10.9% for 50%He + 50%Ar, 8.1 % for 50%He + 45%Ar + 5%O2 and 9% for 45%He + 45%Ar + 10%N2) respectively. The weld metal microstructures for upper zone (GMAW) and lower zones (laser) are presented in Figs. 8a–c and 9a–c. From Figs. 8a–c and 9a–c, it is clearly seen that the grains are finer in lower zone (laser) and the grains are coarser in upper zone (GMAW). The microstructures in the weld metal (both upper and lower zone) are the result of solidification behavior and subsequent solid phase transformation, which are controlled by composition and weld cooling rates. In the weld metal, a fully long columnar dendritic structure with well developed primary arms and clearly distinguishable short secondary arms are present.
From Fig. 8a–c and Fig. 9a–c, it can be observed that the laser (lower) weld zone microstructure had finer grains size with equiaxed grains are observed. A higher percentage of long columnar grains (Fig. 8a–c) are observed as compared to the lower zone (Fig. 9a–c). The austenitic stabilizing elements like Ni, N and C are partitioned into the primary dendrites (darker phase). This leads to ferrite stabilizing elements like Cr, Mo and Si partitioning into the liquid phase. Along with these elements, partition of impurities is also formed in the molten state and becomes part of interdendritic phases (lighter one). From image analysis, the percentage of amount of phases (dendritic and interdendritic) was presented in the weld metal in the different shielding gas mixtures. The phases were analyzed by Metgi Inverter microscope loaded with DHS image data base (German made). After capturing the micrograph, it can be launched for the metallographic analysis from the convenient DHS image data base plug-in interface. The original microstructure image colors are first converted to gray-scale values. The area detection expansion module uses gray-scale detection (phase analysis) to automatically calculate and process the area ratio by assigning a label and color layer to each phase identification. Then, numeric values are entered and slides are adjusted to define the upper and lower gray-scale limits of each material phase. The percentage of phases present in the weld metal is presented in Table 5. 4.3. Microhardness of the welds The microhardness (VHN) test was performed on the etched transverse cross-section of the laser–GMAW hybrid weld zone using a load of 1 kg, which was applied for duration of 20 s. Five measurements in each hybrid weld zone (base material across the weld toward its opposite side) were taken at regular intervals. The hardness values were measured 1 mm below the upper surface (GMAW zone) and the 1 mm above the lower surface (laser zone). The comparative hardness profile is presented in Fig. 10. From
Fig. 8. (a–c) Weld metal microstructure for upper zone (GMAW).
Fig. 9. (a–c) Weld metal microstructure for lower zone (laser).
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P. Sathiya et al. / Materials and Design 33 (2012) 203–212 Table 5 Percentage amounts of phases in the weld bead. Experiment no.
Shielding gas mixtures
1 2 3
50%He + 50%Ar 50%He + 45%Ar + 5%O2 45%He + 45%Ar + 10%N2
Primary dendritic phase (%)
Secondary interdendritic phase (%)
GMAW zone
Laser zone
GMAW zone
Laser zone
32 37 35
34 30 38
68 65 63
64 62 70
260
A B C
250 240
260
Microhardness (HV)
Microhardness (HV)
A B C
280
230 220 210 200
240
220
200
190 180
180 -0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
-0.8
Distance from weld centreline (mm)
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Distance from weld centreline (mm)
Fig. 10. (a and b) Hardness profile (A-50%He + 50%Ar, B-50%He + 45%Ar + 5%O2 and C-45%He + 45%Ar + 10%N2).
Fig. 10a and b, it is seen that the hardness values are higher in laser zone compared to GMAW zone. From Fig. 10a and b it can be seen an increase in the hardness in the weld zone may be induced by precipitation hardening effects, residual stresses or microstructure refinement due to the rapid solidification of the weld pool. From Fig. 10a it is observed that in GMAW zone; the 50%He + 45%Ar + 5%O2 hardness values are higher than the other two shielded weld metal hardness’s. This is due to more amount of oxygen content (5%) present in the GMAW weld zone and also a slow cooling rate. From Fig. 10b in laser zone, it is clearly seen that the 45%He + 45%Ar + 10%N2 hardness values are high than the others.
Fig. 11. From Fig. 11, it is clearly seen that the fractures occur away from the joint in the weld joint. Among the three shielded weld metals, tensile strength is higher than the base material and in particular in the 50%He + 45%Ar + 5%O2 shielded weld, tensile strength is much higher than the others. The fractured surface of tensile specimen is analyzed using SEM. The fracture surface of the tensile tested fractrograph is shown in Fig. 12. The 50%He + 45%Ar + 5%O2 shielded laser– GMAW hybrid joints as revealed by the fractrograph consist of smaller dimples, but some coarse dimples are seen distributed sparsely among fine dimples (Fig. 12b).
4.4. Tensile properties
4.5. Impact properties
The main aim of the tensile test is to evaluate the strength and plasticity of welding joints and examine the influence of welding defects on the joint performance. The test is done on computer interface electronic universal testing machine. The transverse tensile strength, yield strength and percentage of elongation of all joints have been evaluated. The tensile test results of laser–GMAW hybrid welding joints are presented in Table 6. From Table 6, it is clear that the weld metal tensile strength is much higher than the base metal tensile strength which is in line with the microhardness results. The tensile tested samples are presented in
In order to evaluate the Charpy impact toughness values of welded joint, series of Charpy V-notch test were carried out from specimen welded with different shielding gases at room temperature. Notches were prepared in the base metal and weld zone. The value obtained from the testing is shown in Table 7. The impact toughness of unwelded base metal is found to be 60 Joules, which is comparatively lower than the other three shielded weld metal impact strength. From Table 7 it is clear that among the three different shielded laser–GMAW hybrid joints, 50%He + 45%Ar + 5%O2 shielded weld joints exhibit higher impact toughness values,
Table 6 Tensile test results. Sl. no.
Shielding gas
Yield strength (MPa)
Ultimate tensile strength (MPa)
% of elongation
Fracture location
1 2 3 Base metal
50%He + 50%Ar 50%He + 45%Ar + 5%O2 45%He + 45%Ar + 10%N2
470 491 479 340
579 594 585 570
40.0 48.3 46.8 38.0
Base metal Base metal Base metal
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P. Sathiya et al. / Materials and Design 33 (2012) 203–212 Table 7 Impact test results.
Fig. 11. Tensile tested samples.
and the enhancement in toughness value is approximately 36% compared to base metal. The fractured surface of impact specimens is analyzed using SEM. The fractured surface of the impact tested fractrograph is shown in Fig. 13a–c. 5. Discussions From the above results, it is conceivable that the bottom of the laser keyhole is at least partially solidified and not remelted by the GMAW process. This is due to the incomplete mixing of the molten weld pool from the GMAW in the back to the front and down the laser key hole [37]. From Fig. 3a–c and Table 3, it is also clear that the weld shape is shallow and wide under 50% He + 50% Ar shielding gas mixtures. When a small amount of oxygen (5%) and nitrogen (10%) is mixed into the 50%He + 45%Ar and 45%He + 45% Ar shielding gas mixtures, the weld shape changes from the shallow
Sl. no.
Shielding gas
Impact strength (Joules)
1 2 3
50%He + 50%Ar 50%He + 45%Ar + 5%O2 45%He + 45%Ar + 10%N2
66 82 62
type to the deep narrow one as shown in Fig. 3b and c. This is due to plasma plume formation suppressed in the oxygen and nitrogen shield weld. The depth penetration is higher in laser (lower) zone. From Table 5, it is clearly observed that, in GMAW upper zone 50%He + 45%Ar + 5%O2 shielded weld metal primary dendritic is more and in the lower zone (laser) weld metal a high amount of primary dendritic is present in 45%He + 45%Ar + 10%N2. And no carbide precipitation and intermetallic phases from the microstructure are found. This is due to addition of nitrogen and oxygen to decrease the tendency of formation of precipitates and intermetallic phases. Nitrogen is a more effective solid solution strengthening element than carbon and it also enhances grain refining. Nitrogen is a strong austenite stabilizer, thereby reducing the amount of nickel required for austenite stabilization and lowering the tendency to form ferrite. It has been [38] reported that, in gas tungsten arc welding of a super austenitic stainless steel with shielding gases and arc lengths yielding nitrogen contents in excess of 0.2%, more porosity is observed. The microhardness values are higher in laser zone compared to GMAW zone. This may be the result of the effect of rapid solidification. Rapid solidification increases under cooling and nucleation probability leads to very fine microstructure. At the same time it extends solute solubility which results in a supersaturated
Fig. 12. (a–c) SEM fractrograph microstructure.
P. Sathiya et al. / Materials and Design 33 (2012) 203–212
211
Fig. 13. (a–c) SEM fractrograph microstructure.
solid solution. During the rapid solidification of the weld zone, the material generally loses its original strength by strain hardening effect. In particularly, 45%He + 45%Ar + 10%N2 hardness values are higher than the others. This could be possible due to the formation of hard nitride phases. On the other hand, the higher amount of primary austenite phase is present in GMAW (top) zone is 50%He + 45%Ar + 5%O2 and laser (bottom) zone is 45%He + 45%Ar + 10%N2. Due to formation of higher amount of primary austenitic (Table 5), the hardness values are higher in 45%He + 45%Ar + 10%N2 for laser zone and 50%He + 45%Ar + 5%O2 for GMAW zone. Most of the tensile specimens failed in the base region and it suggests that the weld region is stronger than the base metal region. This fact is also evident from the hardness measurements profile (weld region shows higher hardness than the base metal region) for all different shielded weld metals. There exists appreciable difference between the three different shielded laser–GMAW hybrid joints in the appearance of dimples. The 45%He + 45%Ar + 10%N2 shielded hybrid joints invariably consist of fine and uniform dimples (in Fig. 13c). The 50%He + 50% Ar shielded joints consists of coarser dimple size than the others. Due to coarser dimple size, the tensile strength is less when compared to other shielded weld tensile strength. In particular, 50%He + 45%Ar + 5%O2 shielded fracture surface appears in ductile with partially cleavage fracture (Fig. 12b) and also from Fig. 12a–c, it is clearly seen that the fracture surface appeared in pure shear fracture. The impact properties are of greater importance if the materials are to be employed for temperature applications. This is due to the least amount of nitrogen in the weld metal and also due to the presence high amount of interdendritic phases in the weld metal.
The addition of nitrogen to shielded weld joint lowers the toughness due to presence of hard nitride phases. The precipitation of intermetallic phases has a major role to play in affecting the impact toughness of the weld metal. From XRD patterns (Fig. 7), all the peaks are in austenitic phase and no other intermetallic phases are present. Due to the absence of intermetallic phases in the weld metal, the toughness of the weld metal is higher than the base metal. An appreciable amount of dimples and elongated fine dimples are seen in 50%He + 45%Ar + 5%O2 SEM structure. The dimple size exhibits a directly proportional relationship with strength and ductility, that is, if the dimple size is finer, the strength and ductility of the respective joint is higher and vice versa. The high impact toughness value of the weld is due to the presence of finer grain structure compared to the base material. However, the impact specimen fracture surfaces of hybrid welded joints show mixed mode fractures, that is, ductile and cleavage fractures and not a pure ductile fracture which is due to the presence of small interdendritic phases. It is clearly seen from Figs. 8 and 9a–c, that the percentage of secondary interdendritic phases present in the weld metal is much higher than the primary dendritic phases for all shielded weld metals. Due to high amount of interdendritic, austenite is attributed to the high cooling rates, which makes the relevant coarsening of primary austenite dendrites difficult. Due to higher ionization potential, fast cooling rate and also the high amount of nitrogen content in the weld metal, the percentage of secondary interdendritic phases present in nitrogen addition shielded weld metal is much higher than the other shielded weld metals.
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6. Conclusions Based on the Laser GMAW hybrid welding with different shielding gas mixtures (AISI 904 L SASS) the following points are deduced: 1. The full penetration is achieved without any defects. The weld shape is shallow and wide under 50% He + 50% Ar shielding gas mixtures. When a small amount of oxygen (5%) and nitrogen (10%) is mixed into the 50%He + 45%Ar and 45%He + 45% Ar shielding gas mixtures, the weld shape changes from the shallow type to the deep narrow shape. 2. Due to rapid cooling with high heat energy in laser zone, the grains are getting finer refinement. The laser (lower) weld zone microstructure had finer grains with equi axed grains are present. A higher percentage of long columnar grains are present in upper zone (GMAW). 3. In GMAW upper zone, 50%He + 45%Ar + 5%O2 shielded weld metal has more primary dendritic and in the lower zone (laser) weld metal a high amount of primary dendritic is present in 45%He + 45%Ar + 10%N2 shielding gas. 4. The 50%He + 45%Ar + 5%O2 hardness values are higher than the other two shielded weld metal hardness in GMAW (upper) weld zone. Due to plasma plume suppression in addition of nitrogen shielding gas, the hardness values are higher in 45%He + 45%Ar + 10%N2 shielded laser zone. 5. The tensile strength is less in 50% He + 50% Ar shielded weld when compared to other two shielded weld tensile strength. The fracture surface appeared in pure shear fracture in 45%He + 45%Ar + 10%N2 and in 50%He + 45%Ar + 5%O2 shielded fracture surface appears ductile with partially cleavage fracture occurring. 6. 50%He + 45%Ar + 5%O2 shielded weld joints exhibit higher impact toughness values, and the enhancement in toughness value is approximately 36% compared to base metal. The impact specimen fracture surfaces of the different shielding gas laser– GMAW hybrid welded joints show mixed mode fractures, that is, ductile and cleavage fractures.
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