- Email: [email protected]
Effects of the laser parameters on the mechanical properties and microstructure of weld joint in dissimilar pulsed laser welding of AISI 304 and AISI 420
Journal Pre-proofs Effects of the laser parameters on the mechanical properties and microstructure of weld joint in dissimilar pulsed laser welding of AISI 304 and AISI 420 Yuancheng Geng, Mohammad Akbari, Arash Karimipour, Alireza Karimi, Ahmad Soleimani, Masoud Afrand PII: DOI: Reference:
S1350-4495(19)30671-1 https://doi.org/10.1016/j.infrared.2019.103081 INFPHY 103081
To appear in:
Infrared Physics & Technology
Received Date: Revised Date: Accepted Date:
20 August 2019 12 October 2019 15 October 2019
Please cite this article as: Y. Geng, M. Akbari, A. Karimipour, A. Karimi, A. Soleimani, M. Afrand, Effects of the laser parameters on the mechanical properties and microstructure of weld joint in dissimilar pulsed laser welding of AISI 304 and AISI 420, Infrared Physics & Technology (2019), doi: https://doi.org/10.1016/j.infrared. 2019.103081
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Elsevier B.V. All rights reserved.
Effects of the laser parameters on the mechanical properties and microstructure of weld joint in dissimilar pulsed laser welding of AISI 304 and AISI 420 ` Yuancheng Geng1, Mohammad Akbari2,*, Arash Karimipour2, Alireza Karimi2, Ahmad
Soleimani2, Masoud Afrand3,4,* 1- Fujian Polytechnic of Information Technology, Fuzhou 350003, China 2-Department of Mechanical Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran 3-Laboratory of Magnetism and Magnetic Materials, Advanced Institute of Materials Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam 4- Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam * Corresponding author Corresponding author at: Ton Duc Thang University, Ho Chi Minh City, Vietnam Emails: [email protected] (Yuancheng Geng) [email protected] (Mohammad Akbari) [email protected] (Masoud Afrand)
Abstract Dissimilar laser welding of AISI 304 and AISI 420 stainless steel sheets was performed with pulsed Nd: YAG laser so as to evaluate the temperature distribution, microstructure and mechanical properties of the welded zone. The effects of pulsed laser process parameters including (pulse duration, frequency, current, focal length and welding speed) on the fusion zone temperature variation were investigated. Furthermore, the weld joint microstructure and mechanical properties were evaluated according to the temperature distribution of the fusion zone. Different thermal conductivity of the austenitic and ferritic stainless steel resulted in higher 1
temperature of the welded zone for austenitic sample because of more heat concentration. The focal length and peak power had the significant influence on the AISI 304 temperature rise. The fusion zone microstructure composed of coarse ferrite grains austenitic grains. The heat affected zone (HAZ) was including coarse ferrite grains near the ferritic side and austenitic grains with stringers of ferrite δ adjacent to the austenitic side of the weld. The fracture of the welded joint initiated at the HAZ region of AISI 420 stainless steel. Increasing the welding speed decreased the peak temperature of the fusion zone which clearly reduced the strength of the joint. Increasing the welding speed remarkably reduced the weld tensile strength of the about 30% and the temperature of the melt pool adjacent area about 140 C. Also, the porosity formed at the melt pool region when the temperature of the ferritic sample decreased from 250 C to 130 C during welding process. The fusion zone microstructure analysis shows that the predominant microstructure was composed of coarse ferritic grains that include more than 70% of the melt pool total volume. Keywords: Dissimilar laser welding, Stainless steels, Temperature distribution, Microstructure, Mechanical properties. 1. Introduction In recent years, the austenitic stainless steel (ASS) has been widely used in industrial and commercial areas because of its very high mechanical properties, high strain rate, and high toughness. Other properties of ASS with 18% chromium and 8% nickel, are high corrosion resistance, strength, and ductility. As a result, ASS is used in the automotive and nuclear industries and steam and gas turbine fabrication. As well as the production of some parts of the steam vessels, aircraft industries, liquids and gases transportation vehicles, pressure vessels, nuclear and chemical reactors, food preparation and maintenance tanks, and trailer transportation. Chromium and nickel are the main alloying elements of the austenitic steels. Since chromium tends to stabilize body-centred cubic (BCC) phase of the steel and results in desirable corrosion resistance. Moreover, nickel can be used to enhance the mechanical properties of the metal. It can stabilize the austenitic structure of the steel in low temperatures. The use of alloying elements at the same time can enhance corrosion resistance and mechanical properties of the metals [1, 2]. The high corrosion resistance and high mechanical strength of the ferrite stainless steel have always attracted industries. Due to high resistance of the ferrite 2
stainless steel compare to oxidizing materials, nitric acid, sulfuric acid, and alkaline fluids, with high chromium content, it is used in steam turbines, combustion systems, agricultural pesticide tanks, etc. The use of dissimilar materials in the manufacturing and construction industries is one of the most important practical issues [3-9]. This is because sometimes mechanical properties and sometimes corrosion resistance are the most important factors. It should be noted that joining two dissimilar steels is inevitable in some cases. This is because the material in the molten pool displays new properties at the end of the welding process; in addition, the geometry of the molten zone changes because of different melting points, chemical properties, and crystalline structures of these two types of steel [10-12]. Joining these two steel sheets made them applicable in such industries as automotive, nuclear, and shipbuilding, which depend on manufacturing composite sheets [13]. One of the famous joining methods of these two types of steels is laser beam welding (LBW). The advantages of LBW are controllability, reliable and high-quality welding, and the least welding distortions. This method allows controlling the laser beam radiation on the material surface by adjusting the amount of melting and freezing acting on the specific metal part to achieve a suitable weld. This is a characteristic that distinguishes this welding technique from the others. In this process, the size of the molten pool can be controlled; in addition, an assistant gas can be used to reduce the oxidation of the molten pool surface [1416]. Laser beam welding can be performed using pulsed wave (PW) and continuous wave (CW) lasers. The important parameters of CW laser welding are laser power, welding speed, and beam diameter. The important parameters of PW laser welding are the pulse width, pulse energy, and pulse frequency. Applying the laser welding method for two dissimilar materials increases the part production rate and reduces production costs [17-19]. In fact, the final product made from dissimilar materials has many advantages (e.g. technical, economic, and time) over a final product made from a single material [20-23]. Accordingly, Yan et al. [24] investigated the formation mechanism of welding microstructures and mechanical properties of welding of AISI 304 stainless steel using the tungsten inert gas (TIG). They found that the application of LBW technique creates the maximum tensile strength and minimum dendrite microstructure. A numerical study was performed using a finite element 3
method to estimate the thermal behaviour of the laser beam welding of a NiTi alloy by MehrPouya et al. [25]. They concluded that the development of a reliable finite element model can be a useful method to predict the thermal behaviour of the welding process. The employed numerical model can reduce the level of heat and molten area by estimating the optimal welding parameters, and ultimately producing high-quality welds. The numerical analysis of temperature distribution and molten pool geometry during the laser welding of AZR-1 NB alloy was done through the computational fluid dynamics (CFD) using the buoyancy, Marangoni stress, and properties of related materials [26]. Results showed that the 3D model of the heat transfer and fluid flow are useful for simulating the welding temperature and molten pool geometry. The numerical study of laser welding of ASS (AISI 304) at different contact angles was performed using the PW welding [27]. The results showed that 85.5° was the most optimal contact angle, resulting in an elliptical molten pool shape and the highest mean micro hardness (280HV). Due to physical, thermal, and chemical differences between dissimilar materials, such as thermal conductivity, melting point, and thermal expansion of two metals, welding of two dissimilar materials is a complicated process [28]. Lahdoa et al. [29] investigated how to control the penetration depth of high-power welding to join thick steel plates (S355) and aluminum plates (EN AW-6082). The results showed that an increased number of cracks in welding pool and weld material leakage are directly related to the penetration depth. An experimental and numerical study was conducted on the welding of ASS and FSS sheets and their microstructures and mechanical properties [30]. According to the results, the majority of microstructure in the molten zone was ferrite; in addition, the hardness and tensile strength of this zone were 350 HV and 391 MPa, respectively. Khan et al. [31] investigated the effects of laser parameters, such as the beam contact angle, focal length, welding speed, and shear strength of the weld, during laser welding of two dissimilar metals (AISI 304L and AISI 430). It can be concluded that the shear strength increased with increasing the laser power and reducing the welding speed. Although the previous studies have been investigated the effect of laser parameters on laser welding of the various materials, there is no comprehensive study so as to investigate the effect of different thermal properties of the austenitic and ferritic materials on the resultant temperature field according to the laser processing parameterssuch as speed, pulsed laser energy, pulse width, and frequency. The present study aims to assess the effect of pulsed laser welding process parameters on the temperature distribution and the resultant predominated microstructure of the 4
fusion zone and mechanical properties of the weld joint by considering different material properties of the joint. 2. Experimental setup The materials samples used in the experiments are AISI 304 and AISI 420 in the form of sheet with 1.5mm thickness. The materials properties are shown in Table1.
Table1. Chemical composition and physical properties of the materials [32]. Chemical composition
Ni%
C%
p%
Si%
Cr%
Fe%
Mn%
AISI 304
8.9
0.08
0.03
0.34
18.4
Balanced
1.06
AISI 420
0.13
0.15
0.04
0.46
13
Balanced
0.17
Physical properties
Density
Specific heat capacity
Thermal
(g/cc)
(J/g-°C)
Conductivity(W/m-K)
AISI 304
7.8
0.5
16.2
AISI 420
8
0.46
24.9
For heating experiments, the sample dimensions have 20mm width and 50mm length. For measuring the temperature some grooves have been engraved on the samples in order to facilitate the thermocouple installation. For laser welding process, the Nd:YAG pulsed laser Model IQL10 with a maximum average power of 450W and wavelength of 1.06mm was used. The range of laser parameters was 1–250 Hz for pulse frequency, 0.2–25 ms for pulse duration, and 0–40 J for pulse energy and its beam was delivered through a fiber optic cable. A 3 axis CNC table with special fixture for mounting sample was utilized to arrange the samples and movement of them to do welding operation. Argon gas under pressure of 3 bars was applied to shield and protect the fusion zone. Measuring the temperature was performed by using two k-type thermocouples with 1mm diameter. To monitor the temperature, the thermocouple tip was installed 2mm away from 5
the centre of molten pool. The thermocouple temperature data were grabbed via data logger Advantech USB4718 thermocouple module and labview software to monitor and record the temperature data. A schematic and actual view of the experimental setup was shown in Fig. 1. For tensile tests the specimens were prepared as per ASTM E8 substandard for dimensions of the samples and tensile testing was performed at room temperature on a tensile test system made by GOTECH.
Fig.1. experimental setup, a) Schematic, b) actual view.
The microstructure of the fusion zone of welded samples was investigated using the optical microscope in accordance with ASTM E883-17. The samples were mounted in Bakelite and polished using standard metallographic techniques. Etching was performed with Vilella reagent and Glyceregia solution regarding ASTM E 407 for AISI 420 and AISI 304 respectively. 3. Experimental procedure The experiments were conducted to assess the effect of pulsed laser process parameters on the temperature field in both austenitic and ferritic stainless steel. Meanwhile, another consideration is to investigate the effect of different thermal conductivity and microstructure of materials on the temperature field and fusion zone microstructure. Therefore, a series of the experiment was performed as shown in Table 2. Based on some preliminary tests the levels of each parameter selected. The pulsed laser process parameters are including the welding speed, focal length, laser peak power, pulse duration and pulse frequency. The temperature around the melt pool, the resultant fusion zone microstructure and strength of joint are considered the responses. 6
Table 2. Laser welding process parameters case
Welding speed
Current
Pulse frequency
Pulse
Focal
(mm/sec)
(A)
(Hz)
duration
length
(ms)
(mm)
1
4.3
130
15
10
0
2
4.3
130
15
10
-1
3
4.3
130
10
12
-1
4
3.1
130
10
12
-1
5
3.1
130
10
12
0
6
3.1
130
15
8
0
7
6.2
130
15
8
0
8
6.2
120
15
8
0
9
6.2
120
20
9
0
10
3.1
120
20
9
0
11
3.1
120
15
12
0
12
3.1
120
10
8
0
13
6.2
120
15
9
0
4. Result and discussion The effect of different laser parameters on the temperature field around the melt pool and microstructure has been investigated. 4.1. Effects of welding speed Generally, by increasing the welding speed, the measured temperature around the molten pool clearly diminishes. Evidently, the temperature of austenitic stainless steel was higher than ferritic sample particularly at the point the temperature reached to the peak point as shown in Fig. 2. By changing the welding speed, it could be observed that the slope of the temperature graph for both materials is similar although the exact values are different. According to the Table1, the thermal conductivity coefficients are 16.2 W/m.K and 24.9 W/m.K for AISI 304 and AISI 420 respectively that has effect on the temperature values when the laser welding speed varies. The main discrepancy is seen during the cooling cycle because of the fact that the laser heating cycle is relatively lower than the time needed for the heat to dissipat into the adjacent areas of the melt 7
pool monitored by thermocouples. Furthermore, it is observed that increasing the welding speed from 3.1 to 4.3 mm/s results in reduction of temperature about 150°C for both austenitic and ferritic steels. 350
200 Ferrite Austenite
150
250
Temperature ( C)
Temperature ( C)
300
Ferrite Austenite
o
o
200 150 100
100
50
50 0
0
5
10
15
Time (s)
20
25
30
(a)
0
0
5
10
15
Time (s)
20
25
(b)
Fig. 2. The temperature history of welded samples at frequency 10 Hz, pulse duration 12ms, Focal length (-1mm) and welding speed a) 3.1mm/s, b) 4.3mm/s.
4.2. Effect of focal length Variation of focal length changes the power density which results in changing the temperature field and the laser beam absorption on the melt pool. By changing the focal point about 1mm from subsurface of the sample to the surface, the peak temperature measured by thermocouples notably increased for the ferritic sample. The temperature increased from 250 to 340°C by increasing the focal length about 1mm from subsurface of the materials (see Fig. 3). On the contrary, the temperature variation was insignificant for austenitic sample. Hence, it could be said that in case the focal point position is on the surface, the temperature and thereby the beam absorption is higher for ferritic material.
8
350 300
Ferrite Austenite
300
250
Temperature ( C)
Temperature ( C)
350
Ferrite Austenite
o
250
o
200 150 100 50 0
200 150 100 50
0
5
10
15
Time (s)
20
25
0
30
0
5
10
(a)
15
20
Time (s)
25
30
35
(b)
Fig. 3. The temperature history of welded samples at frequency 10 Hz, pulse duration 12ms and welding speed 3.1mm/s for focal length of a) -1 mm, b) 0.
4.3. Effect of pulse frequency Generally, the laser pulse frequency has a direct impact on the laser heating and cooling cycles. On the other hand, the laser average power is function of frequency. Seemingly, increasing the laser frequency result in augmentation of laser average power when other parameters kept constant. With regard to this, increasing the laser frequency leads to increasing the laser average power and the temperature as well. As it is observed in Fig. 4, by increasing the laser frequency, the peak temperature of the austenitic stainless steel around the melt pool remarkably increased from 190 to 250°C whereas the ferritic sample has much lower temperature rise about 20° C. By increasing the laser frequency, the laser irradiation time to the sample will be increased which in turn created higher temperature in austenitic sample that could be related to lower thermal conductivity and concentration of more heat in the fusion zone. Moreover, because of the higher temperature, the beam absorption will increase and the fusion zone temperature reaches the higher levels.
9
300
250
Ferrite Austenite
Ferrite Austenite
250
Temperature ( C)
Temperature ( C)
200 o
o
150
100
50
0
200
150
100
50
0
5
10
15
20
Time (s)
25
30
35
40
0
0
5
10
15
(a)
20
Time (s)
25
30
35
40
(b)
Fig. 4. The temperature history of welded samples at pulse duration 9ms and welding speed of 6.2mm/s, focal length 0 and frequency of a) 15 Hz, b) 20 Hz.
4.4. Effect of pulse duration Variation of the pulse duration has a direct effect on the laser heating and cooling cycles. Besides, the bigger pulse duration results in more laser average power and higher temperature whereas at identical laser average power, increasing the laser peak power is clearly more influential than the pulse duration. Though the pulse duration augmentation will increase the laser mean power and the time of heating cycle, increasing the laser average power by peak power creates bigger power density and penetration in the fusion zone which results in higher temperature and bigger molten pool dimensions. Fig. 5 shows that the effect of increasing the pulse duration on the temperature rise for austenitic sample was about 40°C while this value for the ferritic sample was about 20°C. It could be concluded that the pulse duration effects on the austenitic sample through extending the heating time and lower thermal conductivity and heat dissipation rate created higher temperature for austenitic sample. These effects were negligible for ferritic sample and no temperature change was clearly observed.
10
350
350 Ferrite Austenite
300
250
Temperature ( C)
Temperature ( C)
300
Ferrite Austenite
250
o
o
200 150 100
150 100 50
50 0
200
0
5
10
15
Time (s)
20
25
0
0
5
10
(a)
15
Time (s)
20
25
30
(b)
Fig. 5. The temperature history of welded samples at frequency 15 Hz, welding speed of 3.1mm/s, focal length 0 and pulse duration of a) 8ms, b) 12ms.
4.5. Effect of peak power Commonly, by increasing the current, the laser peak power will increase which means that the laser pulse energy has enhanced. As shown in Fig. 6, when the laser current and thereby peak power has increased, the temperature around the fusion zone clearly increased in austenitic sample. Increasing the current from 120 to 130 A, led to growing the austenitic sample temperature about two folds from 100 to 200°C. It could be said that increasing the peak power has had more significant influence on the temperature of the austenitic sample and the volume of the molten pool in the fusion zone. Additionally, a higher temperature of austenitic sample could be related to lower thermal conductivity rate and concentration of more heat when the laser pulse energy remarkably increased. The effect of increasing the peak power on the temperature variation of the ferritic sample was negligible due to having about 45°C increase in temperature. Differences in thermal conductivity and microstructure of the ferritic sample could be the reason for creating lower temperature rise in this sample.
11
250
200
Ferrite Austenite
Ferrite Austenite
200
Temperature ( C)
Temperature ( C)
150 o
o
100
50
0
150
100
50
0
5
10
Time (s)
15
20
(a)
0
0
5
10
15
Time (s)
20
25
30
(b)
Fig. 6. The temperature history of welded samples at frequency 15 Hz, welding speed. of 6.2mm/s and pulse duration of 8ms for the current of a) 120A, b) 130A.
4.6. Characterization of weld microstructure The microstructure of the welded samples was analyzed in order to evaluate the effect of the pulsed laser heating and cooling cycles on the microstructural changes of the melt pool and HAZ regions of both steels. The microstructure of AISI 420 stainless steel at annealed condition composed of ferrite grains with small carbide particles (see Fig. 7a). The microstructure of the AISI 304 includes the austenitic structure with stringers of delta ferrite (see Fig. 7b). Generally, the microstructure of the HAZ zone plays a key role in the determination of the narrow area for initiation of cracking or metal grain size variations. The HAZ zone adjacent to the ferritic stainless steel shows a gradual grain growth in this region including coarse ferrite grains and grain boundary ferrite as shown in Fig. 8. Clearly, gradual grain growth is observed with increasing the temperature gradient. Hence, excessive grain growth could lead to decrease in mechanical strength of the joint. The fusion zone microstructure welded sample composed of coarse ferrite grains and grain boundary ferrite (see Fig. 9). Clearly, the ferrite grain size could have a relation with temperature distribution and temperature gradient of this region to the extent that grain growth rate is influenced by the peak temperature of the fusion zone and distance from the fusion zone. According to Fig. 9, the predominant microstructure of the fusion zone was including coarse ferrite grains whereas lower volumes of the austenitic microstructure are clearly 12
seen. It shows that ferritic samples showed more participation and solubility in formation of the dissimilar weld fusion zone. It could be said that a bulk of laser energy absorbed by ferritic sample led to microstructural change while it created more heat and higher temperature gradient on austenitic sample. The HAZ between austenitic and fusion zone interface has the austenitic grains with stringers of ferrite δ which in turn can have effect on AISI 304 mechanical and corrosion properties [33].
Fig. 7. The microstructure of base metals a) ferritic microstructure of AISI 420, b) austenitic microstructure of AISI 304.
Fig. 8. The microstructure of HAZ region adjacent to ferritic base at different magnifications, a) 500 x, b) 1000 x.
13
Fig. 9. The microstructure of fusion zone at different magnifications, a) low 200x, b) Coarse ferrite grains of fusion zone at 1000x.
Fig. 10. The microstructure of fusion zone at different welding speeds of a) 3.1mm/s, b) 4.3mm/s.
Besides, there is porosity in the fusion for some of the samples that experienced lower temperature due to receiving low levels of pulse energy or high welding speed. As it is observed in Fig. 10, there is porosity in this sample welded with high speed. In this case, there was not enough time for escaping porosities from the molten pool. Comparison of Fig. 10a and Fig. 10b implies the fact that raising welding speed will increase the possibility of porosity formation as reported in [34]. Formation of porosity can have some drawbacks on the mechanical strength of the joint. In general, decreasing the temperature of the melt pool created an unstable condition 14
for keyhole formation and thereby excessive pressure of welding has entrapped gases in the molten pool during the solidification process [35]. Therefore, the possibility of the porosity formation has been increased when the minimum values of temperature being created either by low energy input or high welding speed. Evidently, increasing the welding speed from 3.1 to 4.1 mm/s has significantly reduced the temperature and caused the porosities in the molten pool region. 4.7. Tensile properties of the joints Generally, the tensile test determines the effect of laser process parameters on the mechanical properties of the weld joint. Due to having two different ferritic and austenitic materials in dissimilar laser welding, evaluation the effect of process parameters led to the formation of more coarse grain ferrite microstructure at fusion zone and HAZ adjacent to the ferritic material is important. Therefore, welding efficiency and selection of appropriate process parameters could be evaluated through tensile test of weld joint. Commonly, the formation of coarse grain ferrite in the HAZ of ferritic stainless steel may lead to the fracture of the joint in this region [30]. Hence, the grain size in the HAZ and fusion zone could be a measurement to assess the weld strength. The tensile tests of some samples at different welding speed and power density were performed. The tensile stress-strain diagrams were shown in Fig.11 and the values of the yield stress and strain of the samples were presented in Table 3. Fig. 12 showed that the fracture occurred clearly in ferritic sample and HAZ region of the ferritic stainless steel. As it is observed in this figure, increasing the welding speed will increase the possibility of the fracture of the sample from the fusion zone (Fig. 12c). It could be concluded that increasing the welding speed not only decreased dimension of the fusion zone but also grew the possible formation of porosities and grain growth of HAZ and fusion zone that all could have impact on decreasing the joint strength. Thereby cracks started from the HAZ region and extended along the joint. Furthermore, the joint yield strength and ultimate tensile strength have been the lowest among all samples as shown in table 3 (sample c) and Fig. 11. According to Table 3 and Fig. 11, the highest level of tensile strength and also the elongation were seen for the sample (a), having the lowest speed and higher pulse energy and higher 15
temperature among the samples which in turn remarkably reduced the formation of porosity as well. 400
Sample A Sample B Sample C
350
Stress (MPa)
300 250 200 150 100 50 0
5
10
15
Strain (%)
20
25
o
0
Fig. 11. Tensile stress-strain diagrams at the frequency of 10 Hz, pulse duration 12ms and welding speed of 3.1mm/s and focal length of a) 0 mm, b) -1, c) welding speed of 6.2mm and focal length of 0 mm.
Fig. 12. Fracture appearance of specimens after tensile tests at the frequency of 10 Hz, pulse duration 12ms and welding speed of 3.1mm and focal length of a) 0 mm, b) -1, c) welding speed of 6.2mm/s and focal length of 0 mm.
16
Table.3 Tensile properties for samples a, b, and c. sample UTS (Mpa) Yield Strength ε max a
320
150
17
b
240
100
8
c
220
85
16
Fig.13 shows the effect of welding speed on the tensile strength of the welded samples. With increasing the welding speed from 3.1 to 6.2mm/s, the tensile strength diminished from 320 to 220 Mpa which is about 30 percent. Clearly the necking and even cracking initiated from the ferritic sample due to having ferrite microstructure. Apart from this, in case of 6.2mm/s, the temperature of the melt pool was decreased about 110°C which in turn was less than half of the sample with 3.1mm/s (250°C). Furthermore, the porosity formation in molten pool may decrease the tensile strength of the weld.
Tensile strength (MPa)
400 350 300 250 200 150 100 50 0
3.1
4.1 welding speed (mm/s)
6.2
Fig. 13. Tensile strength of the welded samples at different welding speeds, pulse duration 12ms and focal length of 0 mm and frequency of 10 Hz.
5. Conclusion This study represents a clear relation between variation of pulsed laser process parameters and the temperature distribution of the weld zone in dissimilar laser welding of austenitic and ferritic stainless steels (AISI 304 and AISI 420 orderly). Not only the temperature field variation can 17
monitor different interactions among the process parameters but also other weld joint properties such as fusion zone microstructure and mechanical properties could be evaluated by means of measuring the temperature field in order to predict the weld joint condition with lower consumption of time and cost. The findings of this study are as follow,
The peak temperature measured by thermocouples showed that differences in thermal conductivity of austenitic and ferritic stainless-steel result in higher temperature for austenitic sample because of more heat concentration. Clearly, variation of the welding speed changes the peak temperature of both samples at the same rate to the extent the slope of the heating and cooling cycles for both AISI 304 and AISI 420 are similar.
Changing the focal length led to the different trend of temperature variation for AISI 304 and AISI 420. In case the focal point is 1mm under the surface of the sheets, the peak temperature was about 70°C higher for austenitic sample. For the condition the focal point is 0mm, there is no clear discrepancy for the peak temperature of both austenitic and ferritic samples.
The laser pulse duration and frequency had induced almost similar changes for heating and cooling cycles.
Increasing the laser peak power has had a significant influence on the temperature rise of the austenitic samples. At identical condition, increasing the peak power created 100°C temperature rise which is two times higher than only 50°C of ferritic sample. It shows that more laser beam energy or power density notably changed the austenitic fusion zone temperature distribution.
The microstructure of the fusion zone composed of coarse ferrite grains acicular ferrite and grain boundary ferrite in all the welded samples. The HAZ zone adjacent to the ferritic stainless steel shows a gradual grain growth in this region including coarse ferrite grains and grain boundary ferrite. The HAZ between austenitic and fusion zone interface has had the austenitic grains with stringers of ferrite δ.
The higher fusion zone temperature due to decreasing welding speed or increasing laser energy evidently decreased the formation of porosities in the fusion zone.
Increasing the welding speed from 3.1 to 6.2mm/s, remarkably reduced the tensile strength of the weld from 320 to 220 Mpa which is about 30 percent. Simultaneously, the 18
temperature of the melt pool was decreased from 250°C to 110°C when the welding speed increased from 3.1mm/s to 6.2mm/s.
The porosity formed at the melt pool region when the temperature of the ferritic samples decreased to the 130°C at a welding speed of 4.1mm/s.
6. Aknowledgents This research is partially supported by Youth Education Research Program of Fujian (JAT170932) 7. References [1] A.S.M. International, ASM Metals Handbook Volume 3 Alloy Phase Diagrams. [2] Shanthos Kumara, G., Saravananb, S., Vetriselvanc, R., Raghukandan, K., 2018. Numerical and experimental studies on the effect of varied pulse energy in Nd: YAG laser welding of Monel 400 sheets. Infrared Physics & Technology, 93 (2018) pp. 184–191. [3] Maio, L., Liberini, M., Campanella, D., Astarita, A., Esposito, S., Boccardi, S., Meola, C., 2017. Infrared thermography for monitoring heat generation in a linear friction welding process of Ti6Al4V alloy. Infrared Physics & Technology, 81 (2017) pp. 325–338. [4] Asséko, A. C. A., Cosson, B., Schmidt, F., Maoult, Y. L., Gilblas, R., Lafranche, E., 2015. Laser transmission welding of composites – Part B: Experimental validation of numerical model. Infrared Physics & Technology, 73 (2015) pp. 304–311. [5] Yang, Y., Gaob, Z., Cao, L. Identifying optimal process parameters in deep penetration laser welding by adopting Hierarchical-Kriging model. Infrared Physics & Technology, 92 (2018) pp. 443–453. [6] Shanthos Kumar, G., Saravanan, S., Vetriselvan, R., Raghukandan, K. Numerical and experimental studies on the effect of varied pulse energy in Nd:YAG laser welding of Monel 400 sheets. Infrared Physics and Technology, 93 (2018) pp. 184–191.
19
[7] Shanthos Kumar, G., Raghukandan, K., Saravanan, S., Sivagurumanikandan, N. Optimization of parameters to attain higher tensile strength in pulsed Nd: YAG laser welded Hastelloy C-276–Monel 400 sheets. Infrared Physics and Technology, 100 (2019) 1–10. [8] Wu, W., Hu, S., Shen, J. Microstructure, mechanical properties and corrosion behavior of laser welded dissimilar joints between ferritic stainless steel and carbon steel. Materials & Design, 65 (2015), 855-861. [9] Feng, J., Guo, W., Irvine, N., Li, L. Understanding and elimination of process defects in narrow gap multi-pass fiber laser welding of ferritic steel sheets of 30 mm thickness. The International Journal of Advanced Manufacturing Technology, 88 (2017), 1821–1830. [10] Olowinsky A.M., Kramer T. and Durand F. SPIE Proc. Photon Processing in Microelectronics and Photonics 4637 (2002), 571-580. [11] Sun Z. and Ion J.C. Journal of Materials Science 30 (1995), 4205-4214. [12] Sepold G., Schubert E. and Zerner E. Laser beam joining of dissimilar materials. IIW IV 739-99, Lissabon, 1999. [13] Mecklein M., Johannes M., lechner M. and Kuppert A. A review on tailored blanksProduction, applications and evaluation. Journal of Materials Processing Technology 214 (2014), 151-164. [14] Norish J. Advanced welding Processes, 1992. [15] Dawes C.T. Laser welding: a practical guide, 1992, PP. 107–121. [16] Kilgore A.B., Koehler M.L., Metzler J.W. and Sturges S.R. Welding, Brazing, and Soldering Handbook vol. 6, ASM International, 1993 (Chapter 47). [17] Liu W., Ma J., Atabaki M.M., Pillai R., Kumar B. and Vasudevan U. Hybrid laserarcwelding of 17-4 PH martensitic stainless steel. Lasers in Manufacturing and Materials Processing 2 (2015), 74-90.
20
[18] Kumar S. and Shahi A.S. Effect of heat input on the microstructure and mechanical properties of gas tungsten arc welded AISI 304 stainless steel joints. Materials and Design 32 (2011), 3617-3623. [19] ASM handbook volume 6: welding, brazing and soldering. 9th 1993. USA . [20] Karagiannis S. and Chryssolouris G. Nd:YAG laser welding–an overview, Third GR– Iinternational conference on new laser technologies and applications. Proceedings of SPIE 2003. [21] Casalino G., Leo P., Mortello M., Perulli P. and Varone A. Effects of laser offset and hybrid welding on microstructure and IMC in Fe-Al dissimilar welding. Metals 7 (2017), 1-17. [22] Casalino G., Guglielmi P., Lorusso V.D., Mortello M., Peyre P. and Sorgente D. Laser offset welding of AZ31B magnesium alloy to 316 stainless steel. Journal of Materials Processing Technology 242 (2017), 49-59. [23] Casalino G. Advances in welding metal alloys, dissimilar metals and additively manufactured parts. Metals (2017), 7-32. [24] Yan J., Gao M. and Zeng X. Study on microstructure and mechanical properties of 304 stainless steel joints by TIG, laser and laser-TIG hybrid welding. Optics and Lasers in Engineering 48 (2010), 512-517. [25] Mehrpouya M., Gisario A., Huang H., Rahimzadeh A. and Elahinia M. Numerical study for prediction of optimum operational parameters in laser welding of NiTi alloy. Optics and Laser Technology 118 (2019), 159-169. [26] Satyanarayana G., Narayana K.L. and Nageswara Ra B. Numerical investigation of temperature distribution and melt pool geometry in laser beam welding of a Zr–1% Nb alloy nuclear fuel rod end cap. Bulletin of Materials Science (2019), 42:170. [27] Kumar N., Mukherjee M. and Bandyopadhyay A. Study on laser welding of austenitic stainless steel by varying incident angle of pulsed laser beam. Optics and Laser Technology 94 (2017), 296-309. [28] Zhang L. and Fontana G. Autogenous laser welding of stainless steel to free-cutting steel for the manufacture of hydraulic valves. Journal of Materials Processing Technology 74 (1-3) (1998), 174-182.
21
[29] Lahdo R., Seffer O., Kaierle S. and Overmeyer L. Investigations on in-process control of penetration depth for high-power laser welding of thick steel-aluminum joints. Journal of Laser Applications 31(2019), 022418. [30] Casalino G., Angelastro A., Perulli P., Casavola C. and Moramarco V. Study on the fiber laser/TIG weldability of AISI 304 and AISI 410 dissimilar weld. Journal of Manufacturing Processes 35 (2018), 216-225. [31] Khan M.M.A., Romoli L., Fiaschi M., Dini G. and Sarri F. Laser beam welding of dissimilar stainless steels in a fillet joint configuration. Journal of Materials Processing Technology 212 (2012), 856-867. [32] Casalino G., Angelastro A., Patrizia P., Casavola C. and Moramarco V. Study on the fiber laser/TIG weldability of AISI 304 and AISI 410 dissimilar weld. Journal of Manufacturing Processes 35 (2018), 216–225. [33] Hong H., Rho BS. and Nam SW. A study on the crack initiation and growth from dferrite/ c phase interface under continuous fatigue and creep–fatigue conditions in type 304L stainless steels. International Journal of Fatigue 24 (2002), 1063–1070. [34] Yan J., Gao M. and Zeng X. Study on microstructure and mechanical properties of 304stainless steel joints by TIG, laser and laser-TIG hybrid welding. Optics and Lasers in Engineering 48 (2010), 512–517. [35] Kawahito Y, Mizutani M, Katayama S. Elucidation of high power fibre laser welding phenomena of stainless steel and effect of factors on weld geometry. J Phys D Appl Phys 40 (2007),5854–5859.
22
Dissimilar laser welding of AISI 304 and AISI 420 stainless steel was performed. The focal length and pick power had the significant influence on temperature rise. Increasing the welding speed decreased the pick temperature of the fusion zone. Differences in thermal conductivity caused to higher temperature for austenitic sample. The pulse duration and frequency had similar changes for heating and cooling cycles.
23
S1350-4495(19)30671-1 https://doi.org/10.1016/j.infrared.2019.103081 INFPHY 103081
To appear in:
Infrared Physics & Technology
Received Date: Revised Date: Accepted Date:
20 August 2019 12 October 2019 15 October 2019
Please cite this article as: Y. Geng, M. Akbari, A. Karimipour, A. Karimi, A. Soleimani, M. Afrand, Effects of the laser parameters on the mechanical properties and microstructure of weld joint in dissimilar pulsed laser welding of AISI 304 and AISI 420, Infrared Physics & Technology (2019), doi: https://doi.org/10.1016/j.infrared. 2019.103081
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Elsevier B.V. All rights reserved.
Effects of the laser parameters on the mechanical properties and microstructure of weld joint in dissimilar pulsed laser welding of AISI 304 and AISI 420 ` Yuancheng Geng1, Mohammad Akbari2,*, Arash Karimipour2, Alireza Karimi2, Ahmad
Soleimani2, Masoud Afrand3,4,* 1- Fujian Polytechnic of Information Technology, Fuzhou 350003, China 2-Department of Mechanical Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran 3-Laboratory of Magnetism and Magnetic Materials, Advanced Institute of Materials Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam 4- Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam * Corresponding author Corresponding author at: Ton Duc Thang University, Ho Chi Minh City, Vietnam Emails: [email protected] (Yuancheng Geng) [email protected] (Mohammad Akbari) [email protected] (Masoud Afrand)
Abstract Dissimilar laser welding of AISI 304 and AISI 420 stainless steel sheets was performed with pulsed Nd: YAG laser so as to evaluate the temperature distribution, microstructure and mechanical properties of the welded zone. The effects of pulsed laser process parameters including (pulse duration, frequency, current, focal length and welding speed) on the fusion zone temperature variation were investigated. Furthermore, the weld joint microstructure and mechanical properties were evaluated according to the temperature distribution of the fusion zone. Different thermal conductivity of the austenitic and ferritic stainless steel resulted in higher 1
temperature of the welded zone for austenitic sample because of more heat concentration. The focal length and peak power had the significant influence on the AISI 304 temperature rise. The fusion zone microstructure composed of coarse ferrite grains austenitic grains. The heat affected zone (HAZ) was including coarse ferrite grains near the ferritic side and austenitic grains with stringers of ferrite δ adjacent to the austenitic side of the weld. The fracture of the welded joint initiated at the HAZ region of AISI 420 stainless steel. Increasing the welding speed decreased the peak temperature of the fusion zone which clearly reduced the strength of the joint. Increasing the welding speed remarkably reduced the weld tensile strength of the about 30% and the temperature of the melt pool adjacent area about 140 C. Also, the porosity formed at the melt pool region when the temperature of the ferritic sample decreased from 250 C to 130 C during welding process. The fusion zone microstructure analysis shows that the predominant microstructure was composed of coarse ferritic grains that include more than 70% of the melt pool total volume. Keywords: Dissimilar laser welding, Stainless steels, Temperature distribution, Microstructure, Mechanical properties. 1. Introduction In recent years, the austenitic stainless steel (ASS) has been widely used in industrial and commercial areas because of its very high mechanical properties, high strain rate, and high toughness. Other properties of ASS with 18% chromium and 8% nickel, are high corrosion resistance, strength, and ductility. As a result, ASS is used in the automotive and nuclear industries and steam and gas turbine fabrication. As well as the production of some parts of the steam vessels, aircraft industries, liquids and gases transportation vehicles, pressure vessels, nuclear and chemical reactors, food preparation and maintenance tanks, and trailer transportation. Chromium and nickel are the main alloying elements of the austenitic steels. Since chromium tends to stabilize body-centred cubic (BCC) phase of the steel and results in desirable corrosion resistance. Moreover, nickel can be used to enhance the mechanical properties of the metal. It can stabilize the austenitic structure of the steel in low temperatures. The use of alloying elements at the same time can enhance corrosion resistance and mechanical properties of the metals [1, 2]. The high corrosion resistance and high mechanical strength of the ferrite stainless steel have always attracted industries. Due to high resistance of the ferrite 2
stainless steel compare to oxidizing materials, nitric acid, sulfuric acid, and alkaline fluids, with high chromium content, it is used in steam turbines, combustion systems, agricultural pesticide tanks, etc. The use of dissimilar materials in the manufacturing and construction industries is one of the most important practical issues [3-9]. This is because sometimes mechanical properties and sometimes corrosion resistance are the most important factors. It should be noted that joining two dissimilar steels is inevitable in some cases. This is because the material in the molten pool displays new properties at the end of the welding process; in addition, the geometry of the molten zone changes because of different melting points, chemical properties, and crystalline structures of these two types of steel [10-12]. Joining these two steel sheets made them applicable in such industries as automotive, nuclear, and shipbuilding, which depend on manufacturing composite sheets [13]. One of the famous joining methods of these two types of steels is laser beam welding (LBW). The advantages of LBW are controllability, reliable and high-quality welding, and the least welding distortions. This method allows controlling the laser beam radiation on the material surface by adjusting the amount of melting and freezing acting on the specific metal part to achieve a suitable weld. This is a characteristic that distinguishes this welding technique from the others. In this process, the size of the molten pool can be controlled; in addition, an assistant gas can be used to reduce the oxidation of the molten pool surface [1416]. Laser beam welding can be performed using pulsed wave (PW) and continuous wave (CW) lasers. The important parameters of CW laser welding are laser power, welding speed, and beam diameter. The important parameters of PW laser welding are the pulse width, pulse energy, and pulse frequency. Applying the laser welding method for two dissimilar materials increases the part production rate and reduces production costs [17-19]. In fact, the final product made from dissimilar materials has many advantages (e.g. technical, economic, and time) over a final product made from a single material [20-23]. Accordingly, Yan et al. [24] investigated the formation mechanism of welding microstructures and mechanical properties of welding of AISI 304 stainless steel using the tungsten inert gas (TIG). They found that the application of LBW technique creates the maximum tensile strength and minimum dendrite microstructure. A numerical study was performed using a finite element 3
method to estimate the thermal behaviour of the laser beam welding of a NiTi alloy by MehrPouya et al. [25]. They concluded that the development of a reliable finite element model can be a useful method to predict the thermal behaviour of the welding process. The employed numerical model can reduce the level of heat and molten area by estimating the optimal welding parameters, and ultimately producing high-quality welds. The numerical analysis of temperature distribution and molten pool geometry during the laser welding of AZR-1 NB alloy was done through the computational fluid dynamics (CFD) using the buoyancy, Marangoni stress, and properties of related materials [26]. Results showed that the 3D model of the heat transfer and fluid flow are useful for simulating the welding temperature and molten pool geometry. The numerical study of laser welding of ASS (AISI 304) at different contact angles was performed using the PW welding [27]. The results showed that 85.5° was the most optimal contact angle, resulting in an elliptical molten pool shape and the highest mean micro hardness (280HV). Due to physical, thermal, and chemical differences between dissimilar materials, such as thermal conductivity, melting point, and thermal expansion of two metals, welding of two dissimilar materials is a complicated process [28]. Lahdoa et al. [29] investigated how to control the penetration depth of high-power welding to join thick steel plates (S355) and aluminum plates (EN AW-6082). The results showed that an increased number of cracks in welding pool and weld material leakage are directly related to the penetration depth. An experimental and numerical study was conducted on the welding of ASS and FSS sheets and their microstructures and mechanical properties [30]. According to the results, the majority of microstructure in the molten zone was ferrite; in addition, the hardness and tensile strength of this zone were 350 HV and 391 MPa, respectively. Khan et al. [31] investigated the effects of laser parameters, such as the beam contact angle, focal length, welding speed, and shear strength of the weld, during laser welding of two dissimilar metals (AISI 304L and AISI 430). It can be concluded that the shear strength increased with increasing the laser power and reducing the welding speed. Although the previous studies have been investigated the effect of laser parameters on laser welding of the various materials, there is no comprehensive study so as to investigate the effect of different thermal properties of the austenitic and ferritic materials on the resultant temperature field according to the laser processing parameterssuch as speed, pulsed laser energy, pulse width, and frequency. The present study aims to assess the effect of pulsed laser welding process parameters on the temperature distribution and the resultant predominated microstructure of the 4
fusion zone and mechanical properties of the weld joint by considering different material properties of the joint. 2. Experimental setup The materials samples used in the experiments are AISI 304 and AISI 420 in the form of sheet with 1.5mm thickness. The materials properties are shown in Table1.
Table1. Chemical composition and physical properties of the materials [32]. Chemical composition
Ni%
C%
p%
Si%
Cr%
Fe%
Mn%
AISI 304
8.9
0.08
0.03
0.34
18.4
Balanced
1.06
AISI 420
0.13
0.15
0.04
0.46
13
Balanced
0.17
Physical properties
Density
Specific heat capacity
Thermal
(g/cc)
(J/g-°C)
Conductivity(W/m-K)
AISI 304
7.8
0.5
16.2
AISI 420
8
0.46
24.9
For heating experiments, the sample dimensions have 20mm width and 50mm length. For measuring the temperature some grooves have been engraved on the samples in order to facilitate the thermocouple installation. For laser welding process, the Nd:YAG pulsed laser Model IQL10 with a maximum average power of 450W and wavelength of 1.06mm was used. The range of laser parameters was 1–250 Hz for pulse frequency, 0.2–25 ms for pulse duration, and 0–40 J for pulse energy and its beam was delivered through a fiber optic cable. A 3 axis CNC table with special fixture for mounting sample was utilized to arrange the samples and movement of them to do welding operation. Argon gas under pressure of 3 bars was applied to shield and protect the fusion zone. Measuring the temperature was performed by using two k-type thermocouples with 1mm diameter. To monitor the temperature, the thermocouple tip was installed 2mm away from 5
the centre of molten pool. The thermocouple temperature data were grabbed via data logger Advantech USB4718 thermocouple module and labview software to monitor and record the temperature data. A schematic and actual view of the experimental setup was shown in Fig. 1. For tensile tests the specimens were prepared as per ASTM E8 substandard for dimensions of the samples and tensile testing was performed at room temperature on a tensile test system made by GOTECH.
Fig.1. experimental setup, a) Schematic, b) actual view.
The microstructure of the fusion zone of welded samples was investigated using the optical microscope in accordance with ASTM E883-17. The samples were mounted in Bakelite and polished using standard metallographic techniques. Etching was performed with Vilella reagent and Glyceregia solution regarding ASTM E 407 for AISI 420 and AISI 304 respectively. 3. Experimental procedure The experiments were conducted to assess the effect of pulsed laser process parameters on the temperature field in both austenitic and ferritic stainless steel. Meanwhile, another consideration is to investigate the effect of different thermal conductivity and microstructure of materials on the temperature field and fusion zone microstructure. Therefore, a series of the experiment was performed as shown in Table 2. Based on some preliminary tests the levels of each parameter selected. The pulsed laser process parameters are including the welding speed, focal length, laser peak power, pulse duration and pulse frequency. The temperature around the melt pool, the resultant fusion zone microstructure and strength of joint are considered the responses. 6
Table 2. Laser welding process parameters case
Welding speed
Current
Pulse frequency
Pulse
Focal
(mm/sec)
(A)
(Hz)
duration
length
(ms)
(mm)
1
4.3
130
15
10
0
2
4.3
130
15
10
-1
3
4.3
130
10
12
-1
4
3.1
130
10
12
-1
5
3.1
130
10
12
0
6
3.1
130
15
8
0
7
6.2
130
15
8
0
8
6.2
120
15
8
0
9
6.2
120
20
9
0
10
3.1
120
20
9
0
11
3.1
120
15
12
0
12
3.1
120
10
8
0
13
6.2
120
15
9
0
4. Result and discussion The effect of different laser parameters on the temperature field around the melt pool and microstructure has been investigated. 4.1. Effects of welding speed Generally, by increasing the welding speed, the measured temperature around the molten pool clearly diminishes. Evidently, the temperature of austenitic stainless steel was higher than ferritic sample particularly at the point the temperature reached to the peak point as shown in Fig. 2. By changing the welding speed, it could be observed that the slope of the temperature graph for both materials is similar although the exact values are different. According to the Table1, the thermal conductivity coefficients are 16.2 W/m.K and 24.9 W/m.K for AISI 304 and AISI 420 respectively that has effect on the temperature values when the laser welding speed varies. The main discrepancy is seen during the cooling cycle because of the fact that the laser heating cycle is relatively lower than the time needed for the heat to dissipat into the adjacent areas of the melt 7
pool monitored by thermocouples. Furthermore, it is observed that increasing the welding speed from 3.1 to 4.3 mm/s results in reduction of temperature about 150°C for both austenitic and ferritic steels. 350
200 Ferrite Austenite
150
250
Temperature ( C)
Temperature ( C)
300
Ferrite Austenite
o
o
200 150 100
100
50
50 0
0
5
10
15
Time (s)
20
25
30
(a)
0
0
5
10
15
Time (s)
20
25
(b)
Fig. 2. The temperature history of welded samples at frequency 10 Hz, pulse duration 12ms, Focal length (-1mm) and welding speed a) 3.1mm/s, b) 4.3mm/s.
4.2. Effect of focal length Variation of focal length changes the power density which results in changing the temperature field and the laser beam absorption on the melt pool. By changing the focal point about 1mm from subsurface of the sample to the surface, the peak temperature measured by thermocouples notably increased for the ferritic sample. The temperature increased from 250 to 340°C by increasing the focal length about 1mm from subsurface of the materials (see Fig. 3). On the contrary, the temperature variation was insignificant for austenitic sample. Hence, it could be said that in case the focal point position is on the surface, the temperature and thereby the beam absorption is higher for ferritic material.
8
350 300
Ferrite Austenite
300
250
Temperature ( C)
Temperature ( C)
350
Ferrite Austenite
o
250
o
200 150 100 50 0
200 150 100 50
0
5
10
15
Time (s)
20
25
0
30
0
5
10
(a)
15
20
Time (s)
25
30
35
(b)
Fig. 3. The temperature history of welded samples at frequency 10 Hz, pulse duration 12ms and welding speed 3.1mm/s for focal length of a) -1 mm, b) 0.
4.3. Effect of pulse frequency Generally, the laser pulse frequency has a direct impact on the laser heating and cooling cycles. On the other hand, the laser average power is function of frequency. Seemingly, increasing the laser frequency result in augmentation of laser average power when other parameters kept constant. With regard to this, increasing the laser frequency leads to increasing the laser average power and the temperature as well. As it is observed in Fig. 4, by increasing the laser frequency, the peak temperature of the austenitic stainless steel around the melt pool remarkably increased from 190 to 250°C whereas the ferritic sample has much lower temperature rise about 20° C. By increasing the laser frequency, the laser irradiation time to the sample will be increased which in turn created higher temperature in austenitic sample that could be related to lower thermal conductivity and concentration of more heat in the fusion zone. Moreover, because of the higher temperature, the beam absorption will increase and the fusion zone temperature reaches the higher levels.
9
300
250
Ferrite Austenite
Ferrite Austenite
250
Temperature ( C)
Temperature ( C)
200 o
o
150
100
50
0
200
150
100
50
0
5
10
15
20
Time (s)
25
30
35
40
0
0
5
10
15
(a)
20
Time (s)
25
30
35
40
(b)
Fig. 4. The temperature history of welded samples at pulse duration 9ms and welding speed of 6.2mm/s, focal length 0 and frequency of a) 15 Hz, b) 20 Hz.
4.4. Effect of pulse duration Variation of the pulse duration has a direct effect on the laser heating and cooling cycles. Besides, the bigger pulse duration results in more laser average power and higher temperature whereas at identical laser average power, increasing the laser peak power is clearly more influential than the pulse duration. Though the pulse duration augmentation will increase the laser mean power and the time of heating cycle, increasing the laser average power by peak power creates bigger power density and penetration in the fusion zone which results in higher temperature and bigger molten pool dimensions. Fig. 5 shows that the effect of increasing the pulse duration on the temperature rise for austenitic sample was about 40°C while this value for the ferritic sample was about 20°C. It could be concluded that the pulse duration effects on the austenitic sample through extending the heating time and lower thermal conductivity and heat dissipation rate created higher temperature for austenitic sample. These effects were negligible for ferritic sample and no temperature change was clearly observed.
10
350
350 Ferrite Austenite
300
250
Temperature ( C)
Temperature ( C)
300
Ferrite Austenite
250
o
o
200 150 100
150 100 50
50 0
200
0
5
10
15
Time (s)
20
25
0
0
5
10
(a)
15
Time (s)
20
25
30
(b)
Fig. 5. The temperature history of welded samples at frequency 15 Hz, welding speed of 3.1mm/s, focal length 0 and pulse duration of a) 8ms, b) 12ms.
4.5. Effect of peak power Commonly, by increasing the current, the laser peak power will increase which means that the laser pulse energy has enhanced. As shown in Fig. 6, when the laser current and thereby peak power has increased, the temperature around the fusion zone clearly increased in austenitic sample. Increasing the current from 120 to 130 A, led to growing the austenitic sample temperature about two folds from 100 to 200°C. It could be said that increasing the peak power has had more significant influence on the temperature of the austenitic sample and the volume of the molten pool in the fusion zone. Additionally, a higher temperature of austenitic sample could be related to lower thermal conductivity rate and concentration of more heat when the laser pulse energy remarkably increased. The effect of increasing the peak power on the temperature variation of the ferritic sample was negligible due to having about 45°C increase in temperature. Differences in thermal conductivity and microstructure of the ferritic sample could be the reason for creating lower temperature rise in this sample.
11
250
200
Ferrite Austenite
Ferrite Austenite
200
Temperature ( C)
Temperature ( C)
150 o
o
100
50
0
150
100
50
0
5
10
Time (s)
15
20
(a)
0
0
5
10
15
Time (s)
20
25
30
(b)
Fig. 6. The temperature history of welded samples at frequency 15 Hz, welding speed. of 6.2mm/s and pulse duration of 8ms for the current of a) 120A, b) 130A.
4.6. Characterization of weld microstructure The microstructure of the welded samples was analyzed in order to evaluate the effect of the pulsed laser heating and cooling cycles on the microstructural changes of the melt pool and HAZ regions of both steels. The microstructure of AISI 420 stainless steel at annealed condition composed of ferrite grains with small carbide particles (see Fig. 7a). The microstructure of the AISI 304 includes the austenitic structure with stringers of delta ferrite (see Fig. 7b). Generally, the microstructure of the HAZ zone plays a key role in the determination of the narrow area for initiation of cracking or metal grain size variations. The HAZ zone adjacent to the ferritic stainless steel shows a gradual grain growth in this region including coarse ferrite grains and grain boundary ferrite as shown in Fig. 8. Clearly, gradual grain growth is observed with increasing the temperature gradient. Hence, excessive grain growth could lead to decrease in mechanical strength of the joint. The fusion zone microstructure welded sample composed of coarse ferrite grains and grain boundary ferrite (see Fig. 9). Clearly, the ferrite grain size could have a relation with temperature distribution and temperature gradient of this region to the extent that grain growth rate is influenced by the peak temperature of the fusion zone and distance from the fusion zone. According to Fig. 9, the predominant microstructure of the fusion zone was including coarse ferrite grains whereas lower volumes of the austenitic microstructure are clearly 12
seen. It shows that ferritic samples showed more participation and solubility in formation of the dissimilar weld fusion zone. It could be said that a bulk of laser energy absorbed by ferritic sample led to microstructural change while it created more heat and higher temperature gradient on austenitic sample. The HAZ between austenitic and fusion zone interface has the austenitic grains with stringers of ferrite δ which in turn can have effect on AISI 304 mechanical and corrosion properties [33].
Fig. 7. The microstructure of base metals a) ferritic microstructure of AISI 420, b) austenitic microstructure of AISI 304.
Fig. 8. The microstructure of HAZ region adjacent to ferritic base at different magnifications, a) 500 x, b) 1000 x.
13
Fig. 9. The microstructure of fusion zone at different magnifications, a) low 200x, b) Coarse ferrite grains of fusion zone at 1000x.
Fig. 10. The microstructure of fusion zone at different welding speeds of a) 3.1mm/s, b) 4.3mm/s.
Besides, there is porosity in the fusion for some of the samples that experienced lower temperature due to receiving low levels of pulse energy or high welding speed. As it is observed in Fig. 10, there is porosity in this sample welded with high speed. In this case, there was not enough time for escaping porosities from the molten pool. Comparison of Fig. 10a and Fig. 10b implies the fact that raising welding speed will increase the possibility of porosity formation as reported in [34]. Formation of porosity can have some drawbacks on the mechanical strength of the joint. In general, decreasing the temperature of the melt pool created an unstable condition 14
for keyhole formation and thereby excessive pressure of welding has entrapped gases in the molten pool during the solidification process [35]. Therefore, the possibility of the porosity formation has been increased when the minimum values of temperature being created either by low energy input or high welding speed. Evidently, increasing the welding speed from 3.1 to 4.1 mm/s has significantly reduced the temperature and caused the porosities in the molten pool region. 4.7. Tensile properties of the joints Generally, the tensile test determines the effect of laser process parameters on the mechanical properties of the weld joint. Due to having two different ferritic and austenitic materials in dissimilar laser welding, evaluation the effect of process parameters led to the formation of more coarse grain ferrite microstructure at fusion zone and HAZ adjacent to the ferritic material is important. Therefore, welding efficiency and selection of appropriate process parameters could be evaluated through tensile test of weld joint. Commonly, the formation of coarse grain ferrite in the HAZ of ferritic stainless steel may lead to the fracture of the joint in this region [30]. Hence, the grain size in the HAZ and fusion zone could be a measurement to assess the weld strength. The tensile tests of some samples at different welding speed and power density were performed. The tensile stress-strain diagrams were shown in Fig.11 and the values of the yield stress and strain of the samples were presented in Table 3. Fig. 12 showed that the fracture occurred clearly in ferritic sample and HAZ region of the ferritic stainless steel. As it is observed in this figure, increasing the welding speed will increase the possibility of the fracture of the sample from the fusion zone (Fig. 12c). It could be concluded that increasing the welding speed not only decreased dimension of the fusion zone but also grew the possible formation of porosities and grain growth of HAZ and fusion zone that all could have impact on decreasing the joint strength. Thereby cracks started from the HAZ region and extended along the joint. Furthermore, the joint yield strength and ultimate tensile strength have been the lowest among all samples as shown in table 3 (sample c) and Fig. 11. According to Table 3 and Fig. 11, the highest level of tensile strength and also the elongation were seen for the sample (a), having the lowest speed and higher pulse energy and higher 15
temperature among the samples which in turn remarkably reduced the formation of porosity as well. 400
Sample A Sample B Sample C
350
Stress (MPa)
300 250 200 150 100 50 0
5
10
15
Strain (%)
20
25
o
0
Fig. 11. Tensile stress-strain diagrams at the frequency of 10 Hz, pulse duration 12ms and welding speed of 3.1mm/s and focal length of a) 0 mm, b) -1, c) welding speed of 6.2mm and focal length of 0 mm.
Fig. 12. Fracture appearance of specimens after tensile tests at the frequency of 10 Hz, pulse duration 12ms and welding speed of 3.1mm and focal length of a) 0 mm, b) -1, c) welding speed of 6.2mm/s and focal length of 0 mm.
16
Table.3 Tensile properties for samples a, b, and c. sample UTS (Mpa) Yield Strength ε max a
320
150
17
b
240
100
8
c
220
85
16
Fig.13 shows the effect of welding speed on the tensile strength of the welded samples. With increasing the welding speed from 3.1 to 6.2mm/s, the tensile strength diminished from 320 to 220 Mpa which is about 30 percent. Clearly the necking and even cracking initiated from the ferritic sample due to having ferrite microstructure. Apart from this, in case of 6.2mm/s, the temperature of the melt pool was decreased about 110°C which in turn was less than half of the sample with 3.1mm/s (250°C). Furthermore, the porosity formation in molten pool may decrease the tensile strength of the weld.
Tensile strength (MPa)
400 350 300 250 200 150 100 50 0
3.1
4.1 welding speed (mm/s)
6.2
Fig. 13. Tensile strength of the welded samples at different welding speeds, pulse duration 12ms and focal length of 0 mm and frequency of 10 Hz.
5. Conclusion This study represents a clear relation between variation of pulsed laser process parameters and the temperature distribution of the weld zone in dissimilar laser welding of austenitic and ferritic stainless steels (AISI 304 and AISI 420 orderly). Not only the temperature field variation can 17
monitor different interactions among the process parameters but also other weld joint properties such as fusion zone microstructure and mechanical properties could be evaluated by means of measuring the temperature field in order to predict the weld joint condition with lower consumption of time and cost. The findings of this study are as follow,
The peak temperature measured by thermocouples showed that differences in thermal conductivity of austenitic and ferritic stainless-steel result in higher temperature for austenitic sample because of more heat concentration. Clearly, variation of the welding speed changes the peak temperature of both samples at the same rate to the extent the slope of the heating and cooling cycles for both AISI 304 and AISI 420 are similar.
Changing the focal length led to the different trend of temperature variation for AISI 304 and AISI 420. In case the focal point is 1mm under the surface of the sheets, the peak temperature was about 70°C higher for austenitic sample. For the condition the focal point is 0mm, there is no clear discrepancy for the peak temperature of both austenitic and ferritic samples.
The laser pulse duration and frequency had induced almost similar changes for heating and cooling cycles.
Increasing the laser peak power has had a significant influence on the temperature rise of the austenitic samples. At identical condition, increasing the peak power created 100°C temperature rise which is two times higher than only 50°C of ferritic sample. It shows that more laser beam energy or power density notably changed the austenitic fusion zone temperature distribution.
The microstructure of the fusion zone composed of coarse ferrite grains acicular ferrite and grain boundary ferrite in all the welded samples. The HAZ zone adjacent to the ferritic stainless steel shows a gradual grain growth in this region including coarse ferrite grains and grain boundary ferrite. The HAZ between austenitic and fusion zone interface has had the austenitic grains with stringers of ferrite δ.
The higher fusion zone temperature due to decreasing welding speed or increasing laser energy evidently decreased the formation of porosities in the fusion zone.
Increasing the welding speed from 3.1 to 6.2mm/s, remarkably reduced the tensile strength of the weld from 320 to 220 Mpa which is about 30 percent. Simultaneously, the 18
temperature of the melt pool was decreased from 250°C to 110°C when the welding speed increased from 3.1mm/s to 6.2mm/s.
The porosity formed at the melt pool region when the temperature of the ferritic samples decreased to the 130°C at a welding speed of 4.1mm/s.
6. Aknowledgents This research is partially supported by Youth Education Research Program of Fujian (JAT170932) 7. References [1] A.S.M. International, ASM Metals Handbook Volume 3 Alloy Phase Diagrams. [2] Shanthos Kumara, G., Saravananb, S., Vetriselvanc, R., Raghukandan, K., 2018. Numerical and experimental studies on the effect of varied pulse energy in Nd: YAG laser welding of Monel 400 sheets. Infrared Physics & Technology, 93 (2018) pp. 184–191. [3] Maio, L., Liberini, M., Campanella, D., Astarita, A., Esposito, S., Boccardi, S., Meola, C., 2017. Infrared thermography for monitoring heat generation in a linear friction welding process of Ti6Al4V alloy. Infrared Physics & Technology, 81 (2017) pp. 325–338. [4] Asséko, A. C. A., Cosson, B., Schmidt, F., Maoult, Y. L., Gilblas, R., Lafranche, E., 2015. Laser transmission welding of composites – Part B: Experimental validation of numerical model. Infrared Physics & Technology, 73 (2015) pp. 304–311. [5] Yang, Y., Gaob, Z., Cao, L. Identifying optimal process parameters in deep penetration laser welding by adopting Hierarchical-Kriging model. Infrared Physics & Technology, 92 (2018) pp. 443–453. [6] Shanthos Kumar, G., Saravanan, S., Vetriselvan, R., Raghukandan, K. Numerical and experimental studies on the effect of varied pulse energy in Nd:YAG laser welding of Monel 400 sheets. Infrared Physics and Technology, 93 (2018) pp. 184–191.
19
[7] Shanthos Kumar, G., Raghukandan, K., Saravanan, S., Sivagurumanikandan, N. Optimization of parameters to attain higher tensile strength in pulsed Nd: YAG laser welded Hastelloy C-276–Monel 400 sheets. Infrared Physics and Technology, 100 (2019) 1–10. [8] Wu, W., Hu, S., Shen, J. Microstructure, mechanical properties and corrosion behavior of laser welded dissimilar joints between ferritic stainless steel and carbon steel. Materials & Design, 65 (2015), 855-861. [9] Feng, J., Guo, W., Irvine, N., Li, L. Understanding and elimination of process defects in narrow gap multi-pass fiber laser welding of ferritic steel sheets of 30 mm thickness. The International Journal of Advanced Manufacturing Technology, 88 (2017), 1821–1830. [10] Olowinsky A.M., Kramer T. and Durand F. SPIE Proc. Photon Processing in Microelectronics and Photonics 4637 (2002), 571-580. [11] Sun Z. and Ion J.C. Journal of Materials Science 30 (1995), 4205-4214. [12] Sepold G., Schubert E. and Zerner E. Laser beam joining of dissimilar materials. IIW IV 739-99, Lissabon, 1999. [13] Mecklein M., Johannes M., lechner M. and Kuppert A. A review on tailored blanksProduction, applications and evaluation. Journal of Materials Processing Technology 214 (2014), 151-164. [14] Norish J. Advanced welding Processes, 1992. [15] Dawes C.T. Laser welding: a practical guide, 1992, PP. 107–121. [16] Kilgore A.B., Koehler M.L., Metzler J.W. and Sturges S.R. Welding, Brazing, and Soldering Handbook vol. 6, ASM International, 1993 (Chapter 47). [17] Liu W., Ma J., Atabaki M.M., Pillai R., Kumar B. and Vasudevan U. Hybrid laserarcwelding of 17-4 PH martensitic stainless steel. Lasers in Manufacturing and Materials Processing 2 (2015), 74-90.
20
[18] Kumar S. and Shahi A.S. Effect of heat input on the microstructure and mechanical properties of gas tungsten arc welded AISI 304 stainless steel joints. Materials and Design 32 (2011), 3617-3623. [19] ASM handbook volume 6: welding, brazing and soldering. 9th 1993. USA . [20] Karagiannis S. and Chryssolouris G. Nd:YAG laser welding–an overview, Third GR– Iinternational conference on new laser technologies and applications. Proceedings of SPIE 2003. [21] Casalino G., Leo P., Mortello M., Perulli P. and Varone A. Effects of laser offset and hybrid welding on microstructure and IMC in Fe-Al dissimilar welding. Metals 7 (2017), 1-17. [22] Casalino G., Guglielmi P., Lorusso V.D., Mortello M., Peyre P. and Sorgente D. Laser offset welding of AZ31B magnesium alloy to 316 stainless steel. Journal of Materials Processing Technology 242 (2017), 49-59. [23] Casalino G. Advances in welding metal alloys, dissimilar metals and additively manufactured parts. Metals (2017), 7-32. [24] Yan J., Gao M. and Zeng X. Study on microstructure and mechanical properties of 304 stainless steel joints by TIG, laser and laser-TIG hybrid welding. Optics and Lasers in Engineering 48 (2010), 512-517. [25] Mehrpouya M., Gisario A., Huang H., Rahimzadeh A. and Elahinia M. Numerical study for prediction of optimum operational parameters in laser welding of NiTi alloy. Optics and Laser Technology 118 (2019), 159-169. [26] Satyanarayana G., Narayana K.L. and Nageswara Ra B. Numerical investigation of temperature distribution and melt pool geometry in laser beam welding of a Zr–1% Nb alloy nuclear fuel rod end cap. Bulletin of Materials Science (2019), 42:170. [27] Kumar N., Mukherjee M. and Bandyopadhyay A. Study on laser welding of austenitic stainless steel by varying incident angle of pulsed laser beam. Optics and Laser Technology 94 (2017), 296-309. [28] Zhang L. and Fontana G. Autogenous laser welding of stainless steel to free-cutting steel for the manufacture of hydraulic valves. Journal of Materials Processing Technology 74 (1-3) (1998), 174-182.
21
[29] Lahdo R., Seffer O., Kaierle S. and Overmeyer L. Investigations on in-process control of penetration depth for high-power laser welding of thick steel-aluminum joints. Journal of Laser Applications 31(2019), 022418. [30] Casalino G., Angelastro A., Perulli P., Casavola C. and Moramarco V. Study on the fiber laser/TIG weldability of AISI 304 and AISI 410 dissimilar weld. Journal of Manufacturing Processes 35 (2018), 216-225. [31] Khan M.M.A., Romoli L., Fiaschi M., Dini G. and Sarri F. Laser beam welding of dissimilar stainless steels in a fillet joint configuration. Journal of Materials Processing Technology 212 (2012), 856-867. [32] Casalino G., Angelastro A., Patrizia P., Casavola C. and Moramarco V. Study on the fiber laser/TIG weldability of AISI 304 and AISI 410 dissimilar weld. Journal of Manufacturing Processes 35 (2018), 216–225. [33] Hong H., Rho BS. and Nam SW. A study on the crack initiation and growth from dferrite/ c phase interface under continuous fatigue and creep–fatigue conditions in type 304L stainless steels. International Journal of Fatigue 24 (2002), 1063–1070. [34] Yan J., Gao M. and Zeng X. Study on microstructure and mechanical properties of 304stainless steel joints by TIG, laser and laser-TIG hybrid welding. Optics and Lasers in Engineering 48 (2010), 512–517. [35] Kawahito Y, Mizutani M, Katayama S. Elucidation of high power fibre laser welding phenomena of stainless steel and effect of factors on weld geometry. J Phys D Appl Phys 40 (2007),5854–5859.
22
Dissimilar laser welding of AISI 304 and AISI 420 stainless steel was performed. The focal length and pick power had the significant influence on temperature rise. Increasing the welding speed decreased the pick temperature of the fusion zone. Differences in thermal conductivity caused to higher temperature for austenitic sample. The pulse duration and frequency had similar changes for heating and cooling cycles.
23