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Energy efficiency evaluation of hot-wire laser welding based on process characteristic and power consumption
Journal of Cleaner Production xxx (2014) 1e8
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Energy efficiency evaluation of hot-wire laser welding based on process characteristic and power consumption Haiying Wei*, Yi Zhang, Lipeng Tan, Zhihua Zhong State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, Hunan 410082, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 January 2014 Received in revised form 6 September 2014 Accepted 4 October 2014 Available online xxx
Energy conservation has become one of the priorities in manufacturing industries, while the energy efficiency at process level may provide deep understanding of the energy consumption during the manufacturing process. Hot-wire laser welding of double galvanized high-strength steel DP800 has a wide range of potential applications in the automobile manufacturing field. Based on the introduction of equivalent laser power, a mathematical model was developed to obtain the energy efficiency of hot-wire laser welding. The results of theoretical calculation and experimental verification indicate that the optimum pre-heating temperature range of the welding wire in hot-wire laser welding is from 600 to 800 C. The preheating temperature of the welding wire increases almost parabolically with the heating current. Furthermore, the developed technique can be used to realize a maximum energy savings of 16% over cold-wire laser welding. The conclusions allowed to formulate the energy efficiency evaluation as green manufacturing of novel process is concerned. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Energy efficiency evaluation Hot-wire laser welding Equivalent laser power Mathematical model
1. Introduction Energy conservation is an effective means of reducing the imbalance between energy supply and demand, while being necessary for the protection of the natural environment and ~ os et al., 2011; Bunse et al., 2011). Statisecological resources (Ban tical data from the International Energy Agency (IEA) shows that the energy consumption of the manufacturing industry accounts approximately one-third of global final energy use (IEA, 2011). IEA, 2011, global electricity consumption produced 1582 Mtons of CO2, 42.6% of which resulted from electricity consumption in the manufacturing industry (IEA, 2013). Hence, energy conservation and efficiency improvement have become a focal point and must be addressed by manufacturing industries. Especially, the energy consumption of laser welding is quite high. ‘A 1-kW laser requires 35 kW of electric power, and its energy availability is unusually quite low’ stated Zhao et al. (2010). And the measurement and management of energy efficiency represent the environmental and economic benefits of laser machining (Zhang et al., 2006). Just as the Director of the Division of Design & Manufacturing of the U.S. National Science Foundation (NSF), Dr. MartineVega stated in his keynote that by 2020, the manufacturing
* Corresponding author. E-mail address: [email protected] (H. Wei).
industry will encounter six major challenges, one of which will be environmental compatibility (Martin-Vega, 1999). Meanwhile, green manufacturing may offer an effective and sustainable solution. Therefore, the need for improvements in energy efficiency of manufacturing process will double over the next 40 years (Allwood et al., 2011). Moreover, studies revealed that the assessment of energy consumption have become an urgent matter for manufacturing industry (Wang et al., 2013), while the energy efficiency at process level have been considered rather interesting and may provide the deep understanding of the energy transformation during the manufacturing process (Apostolos et al., 2013). And it is essential for the manufacturing industry to improve energy efficiency of both its current technological processes and novel manufacturing processes (Apostolos et al., 2013; Herring, 2006). Advanced manufacturing is being developed in the direction of using high-power lasers, which uses “optical energy sources” and offers the significant advantages of high energy density focusing, ease of operation, high flexibility, efficiency, and high quality. In manufacturing processes, lasers have a wide range of applications. Some types of laser welding can replace traditional connection techniques and have gradually been applied in the fields of transportation, aerospace, and nuclear power. In aircraft manufacturing, for example, laser welding techniques have been applied to form the entire connection of the inner bulkheads and reinforce the ribs of the aircraft fuselages and wings of the Airbus A380 (Witik et al., € tzer, 2005). Laser welding greatly reduces the aircraft 2012; Ro
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H. Wei et al. / Journal of Cleaner Production xxx (2014) 1e8
weight, fuel consumption, and exhaust and produces significant economic benefits in the form of lower operating costs. In the auto manufacturing field, laser welding technologies have been widely used to create connections for new materials for a higher strength and a lower weight of body-in-white. By 2000, 50% of the resistance spot welding production lines of the Big Three automakers in the United States had been replaced by laser welding production lines (Li et al., 2006). The FAW-Volkswagen Automotive Company in China has employed laser welding in the manufacturing of almost all of its brand-name auto models (Han, 2008). Advocates of the use of laser welding of body-in-white acknowledge that it offers many advantages. But the strict machining precision and assembly accuracy requirements of autobody parts have limited the applications of laser welding to some extent. For example, in butt-joint welding, the butt gap cannot exceed 10%e15% of the plate thickness (Dilthey et al., 1995). Thus, laser welding with a filler wire can considerably reduce the requirements on the butt gap for parts. The filler-wire laser welding technique decreases the fitting precision requirements on parts, generates a smaller heat-affected zone than conventional welding techniques, and produces a weld seam whose performance is similar to that using a conventional laser welding. In body-in-white welding, especially, butt-style filler-wire laser welding has the potential to replace the lapped-style resistance spot welding since it can increase the strength of the weld seam and lower the autobody weight. On the other hand, another hitch that might limit the applications of laser welding is low efficiencies in the energy conservation. To increase the laser welding energy efficiency, the characteristics of laser welding and the methods of proper improvements have been recently discussed in the studies (Apostolos et al., 2012; Dahmen et al., 2010; Daub et al., 2010). Apostolos et al. (2012) discussed that laser machining is a thermal process with relatively low energy efficiency because materials machining and cleaning primarily rely on melting or vaporization mechanisms. By measurement of separated boundaries laser systems, this research revealed that higher laser power and higher pulsing frequency improved the energy efficiency. By compared technical aspects of various laser welding, Dahmen et al. (2010) proposed heat input and different laser system had influence on the energy efficiency, which could be increased by using laser light sources with high energy efficiencies, as using fiber lasers or disk lasers. Daub et al. (2010) studied the relationship between the beam spot geometry and the laser energy efficiency. Despite these methods discussed, at process level, the laser welding are still not well documented in terms of energy efficiency, especially lack of analysis of process energy efficiency in hot-wire laser welding when replacing coldwire laser welding. Also, energy efficiency, as an important driver for manufacturing industry, has been debated and rethought recently. Herring (2006) specified the terms ‘energy efficiency’ as the ratio of energy services out to energy input, and by the review of literature, this research pointed out that efficiency gains meant higher levels of energy service. Patterson (1996) classified the operational indicators of energy efficiency into four groups: thermodynamic indicator, physical-thermodynamic indicator, economic-thermodynamic indicator and economic indicator. And Apostolos et al. (2013) divided energy efficiency into four elemental level: process level, machine level, line level and factory level. By comparative analysis of different system scale levels of discrete part manufactory, Duflou et al. (2012) proposed that while at unit process level, redesign of machine tools and optimization of process parameter settings allow improving the energy efficiency. Ingarao et al. (2012) focused on the machine tool architecture at process level and investigated the energy consumption by comparative analysis of the different
load capacities. When the methods and models to evaluate the energy efficiency are concerned, some quantitative analysis have been reported in the studies (Bhushan, 2013; Musa et al., 2009). Bhushan (2013) performed experiments to study the effect of the process parameters on energy consumption and tool life. This research present a statistical methodology based model for selecting optimum machining conditions, and the result showed that the energy consumption was reduced by 13.55% and the tool life was boosted by 22.12%. Musa et al. (2009) quantitatively compared two sintering techniques, the hot pressing and spark plasma sintering, in terms of energy efficiency with experimental study and focused on operating conditions and end-product characteristics. More recently, some quantitative analysis of the energy efficiency of laser welding, a typical example of non-conventional processes, have been made in the studies (Um and Stroud, 2013; Apostolos et al., 2012). Um and Stroud (2013) provided an estimation model for remote laser welding based on the analysis of robot motion and laser source, and the results showed that laser power and process time were important factors in energy efficiency estimation. Apostolos et al. (2012) investigated laser machining energy efficiency under two different boundaries with using an index Eef1 to measure the energy efficiency of the entire laser machine system and using an index Eef for only the workpiece and laser beam interaction respectively. This research efforts produced that the process parameters and processing strategies influenced the energy efficiency. From the literature review in the domain of energy efficiency of manufacturing processes, the laser welding process, which were mainly defined as the thermal efficiency and whose efficiency vastly depended on the machining conditions, required to accurately evaluate the result, especially, the mathematical models based on the characteristics of the process. As pointed out by Apostolos et al. (2012), the laser based processes energy efficiency might be difficult to be analytically modeled. In this paper, a quantitative evaluation of the energy consumption and energy efficiency during hot-wire laser welding of double galvanized high-strength steel, which has a wide range of applications in the auto manufacturing field, was obtained in this work based on the essence of energy transformation and technological characteristics of the laser welding process. The results were theoretically evaluated and experimentally verified. The paper offered a method to formulate the energy efficiency evaluation as green manufacturing of novel process is concerned. The paper proceeds as follows: Section 2 describes the process characteristics and parameters for hot-wire laser welding, and provides such experimental details as methods and materials. Section 3 develops a model to calculate and analyze the wire feed rate, preheating temperature, and energy efficiency in hot-wire laser welding based on the measurement results of the wire preheating temperature and the introduction of the equivalent laser power. Section 4 offers the conclusions of the study. 2. Experiments 2.1. Process features and parameters The process parameters of laser welding include the laser power, the welding speed, the defocusing distance, and the characteristics of gas protection such as the gas type, the blowing mode (coaxial or side-blown) and the flow rate. In laser welding with a hot filler wire, process control in joining materials is much more complicated by the preheating and the addition of the welding wire. Various process parameters such as the direction, the angle, the feed placement and the wire extension cause such four hot filler wire transfer modes in the hot-wire laser welding as drop transfer, fusing transfer, continuous transfer and top thread transfer.
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For the wire feed direction in hot-wire laser welding, the wire can be fed in such two directions as a dragging feed and a trailing feed relative to the direction of the welding speed. The smooth transitions in the wire melting process can be easily obtained, resulting in an expected stable welding process under dragging feed mode. However, even small vibration in the trailing feed process could cause the adhesion between the wire and the weld bead and destabilizing the welding process. Also, the hot filler wire in trailing feed mode can only get the energy from the melting pool for being further heated and melted. Size limitations imposed by the wire feed nozzle and the laser head result in a wire feed angle between the wire and the laser beam. When the wire feed angle is fairly large, wire extension must be increased to prevent interference between the wires feed nozzle and the workpiece, easily leading to vibration at the wire tip. However, when the wire feed angle is fairly small, the feed placement of the hot filler wire is difficult to adjust. The reason was that a fairly small deviation in the position of the welding wire produces a sharp increase in the deviation of the beam-wire coupling along the beam propagation direction. The vibration at the wire tip and the deviation in the beam-wire coupling can both destabilize and even interrupt the welding process. On the other hand, the wire feed angle also affects the area of the heat affected zone (HAZ). The heat input to the welding wire under the laser irradiation varies with the wire feed position. The deviation of the welding wire from the laser light in the direction of cross section should be minimized to obtain a symmetric weld seam and higher melting efficiency of the welding wire. The higher the laser power and the wider the weld bead formed, the larger is the allowable deviation of the wire feed placement in the direction of cross section. If there is zero deviation in the direction of cross section for the wire feed placement, the wire feed placement can also be described in terms of the beam-wire distance, which corresponds to the deviation of the center of the welding wire tip from the center of the laser beam in the welding direction. In the dragging feed mode, the placement of the welding wire usually coincides with the center of the light spot such that the beam-wire distance is zero, and the hot filler wire heats up and melts under the irradiation of the laser. At the same time, in thin-plate hot-wire laser welding, the welding wire is usually placed on the workpiece surface, i.e., the placement deviation in the direction of the weld depth is zero. The wire heating extension is the distance between the wire nozzle and the workpiece along the feeding direction. The wire heating extension affects the rigidity and the directivity of the welding wire. Under normal conditions, the wire heating extension should not be too long because the wire heating extension can affect the preheating time of the welding wire, thereby affecting the preheating temperature of the welding wire. For the same heating current, the larger the wire heating extension is, the longer the preheating time and the higher the preheating temperature are. Therefore, the dragging feed mode was used in the following experiments with a wire feed angle of 45 , a zero beam-wire distance, and wire heating extension of 17 mm. 2.2. Experimental details Experiments were performed on high-strength automobile steel sheets DP800 with double zinc coating and dual phase. This test
3
material has a tensile strength Rm 800 MPa, a yield strength Rp0.2 450 MPa, and test-piece dimensions of 60 mm 35 mm 1.2 mm. The edges of the test-piece were sanded before testing to ensure that the butt gap was even, and acetone was used to clean the butt position. The welding filler wire was high-strength steel as well and had a 1.0-mm diameter. The chemical constituents of the DP800 and the high-strength-steel welding wire are listed in Table 1. Fig. 1 is a schematic of the hot-wire laser welding test apparatus. Welding tests were carried out with a continuous wave fiber laser (IPG YLR 4000) with a wavelength of 1070 nm, TEM00 mode, and an optical fiber core diameter of 0.3 mm. The welding head was a Precitec YW50, which was fitted with a coaxial air blow protection device; the alignment and focusing system was composed of an alignment lens with a 150 mm focal length and a focusing lens with a 200 mm focal length, which produced a light spot with a 0.4-mm diameter at the laser focal spot. The hot wire equipment was a Fronius TransPuls Synergic 2700 hot-wire machine: the heating current of the welding wire ranged from 0 to 270 A, and the wire feed rate was could be regulated from 0.5 to 22 m/min. The welding head and the wire feed head were grasped by a welding robot (ABB IRB 2400). Controllers could be used to set the process parameters, including the laser power, the welding speed, the defocus, and the heating current. The optimal parameters were obtained by orthogonal testing. The hot-wire anode of the wire heating source was connected to the welding wire through the wire feed head, and the other side of cathode was connected to the workpiece. The heating circuit connection of the welding wire was maintained during welding, and coaxial argon (Ar) gas protection was used during welding at a flow rate of 15 L/min. The orthogonal test results for hot-wire laser welding produced the following optimum welding process parameters for a butt gap of 0.6 mm: laser power, 1400 W; heating current, 70e80 A; welding speed, 20 mm/s; and defocusing distance, þ8 mm. Under these conditions, good weld seams were formed, and the welding wire fused and mixed uniformly with the parent material in the weld seams. The weld seam zone consisted primarily of lath martensite with a small amount of bainite and austenite; the crystalline grains were quite fine and had a uniform elements distribution. The tensile test showed that the welded joint had a higher tensile strength than that of the basement material. The laser power and the welding speed determined the energy input for the welding process and thus affected the heat input and the width of the weld seam. The maximum width of the weld seam was obtained at a welding speed of 15 mm/s; however, a welding speed of 30 mm/s was excessive and caused the heat input insufficient, resulting in a narrow weld seam and lack of fusion. The defocusing distance affects the size of the laser light spot and the laser energy density distribution. When defocusing distance was zero, the welding wire did not melt, and it was difficult to maintain the stability of the welding process. The small diameter of the laser light spot placed a strict precision requirement on the placement of the feed wire. When there was a slight vibration at the wire tip or a small placement deviation between the wire and the laser beam, the welding wire was not heated well for absence of the irradiation of the full laser, and then lack of fusion occurred. Increasing the defocusing distance could improve the stability of the welding process. However, increasing the defocusing distance enlarged the
Table 1 Chemical constituents of DP800 and welding wire. Element
C
Si
Mn
P
S
Al
Ni
Mo
Ti
Fe
DP800 (%) Welding wire (%)
0.13 0.028
0.17 0.49
1.96 1.48
0.012 0.0094
<0.0050 0.0090
0.050 e
e 2.36
e 0.24
e 0.12
Bal. Bal.
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H. Wei et al. / Journal of Cleaner Production xxx (2014) 1e8
the welding speed, in mm/s; d is the plate thickness, in mm; and d is the butt gap, in mm. Hence, substituting Equations (1) and (2) into Equation (3) yields the wire feed rate:
vf ¼
Fig. 1. Experimental schematic of hot-wire laser welding.
diameter of the laser light spot, resulting in the laser energy density being dropped. The defocusing distance with þ8 mm can obtain the beam diameter of the laser on the workpiece surface with 1.2 mm. At this moment, the diameter of the laser irradiation was slightly larger than the diameter of the welding wire, which maintained the stability of the welding process and produced good weld seams. Meanwhile, the heating current of the welding wire considerably affected the stability of the welding process as well. A FLIR A615 infrared (IR) thermal imaging system was used to measure the preheating temperature under different heating currents. The IR thermal imaging system was connected via a data line to a computer, and the temperature measurement process was controlled by a computer program. Before the temperature measurements were conducted, the parameters of the radiation rate, the air temperature, and the air humidity were set, followed by selecting a suitable image frame frequency and temperature measurement range. And the accuracy of the thermal imaging system is ±2% of the measured value. When the heating current was too low, the wire fusion was insufficient and then top thread occurred on the side of the weld seam. On the other side, too high heating current was apt to form molten drops at the tip of the welding wire, spattering and wire feed suspending occurred, and the welding process was discontinuous. For a suitable heating current, the welding wire formed transient molten drops at the tip of the welding wire. On this condition the welding process became stable, and then the optimum preheating temperature was obtained. 3. Results and discussion
kd $d $d$vw p$r 2
(4)
Equation (4) shows that under certain defined welding conditions, the wire feed rate has a positive linear correlation with the butt gap and the welding speed. At a fixed laser power, the value of the welding speed determines the energy input. The higher the welding speed, the lower the energy input. Hence, in hot-wire laser welding; the welding speed also has a fairly large influence on the stability of the wire feed. If the welding speed is too high, the wire feed rate must be increased to match the volume of plate gap. Then the feed wire will be easily led to vibration at the wire tip, destabilizing the welding process. Fig. 2 shows the wire feed rate as a function of the butt gap for a welding wire radius r of 0.5 mm, a plate thickness d of 1.2 mm, and a kd value of 1.1. 3.2. Measurement of wire preheating temperature A suitable preheating temperature of the welding wire is critical to obtain continuous transfer behavior of the welding wire and to ensure that the hot-wire laser welding process is stable. The experiment results showed that the preheating temperature of the welding wire was determined by several factors such as the heating current, the wire feed rate, etc. The results demonstrated that for a butt gap of 0.6 mm, the optimal heating current range was 70e80 A, and the heating temperature at these currents were in the optimal preheating temperature range for the welding wire. Fig. 3 shows the heat distribution on the welding wire, which had a heating extension of 17 mm. The temperature was measured at the sample point of the intersection between the welding wire and the workpiece. Actually, the measured position was the wire endpoint in the welding process, which was contacted with the melting pool. To minimize the measurement error, the temperature was recorded at 5 different times, and the mean value was used as the measured result. The wire feed rate was taken to be vf ¼ 1.2 m/min. Table 2 shows the measured preheating temperatures of the welding wire at different heating currents. When the heating current was in the 70e80 A range, the measured preheating temperature of the welding wire was 600e800 C, which was the optimal preheating temperature range of the welding wire for the welding process.
3.1. Calculation of wire feed rate During laser butt welding with the hot filler wire, the required feed amount of the welding wire is determined by the volume of the gap to be filled (Sun and Salminen, 1997):
Vw ¼ kd $Vg
(1)
where:
Vw ¼ vf $p$r 2 $t
(2)
Vg ¼ vw $d$d$t
(3)
Vw is the required volume of the filler wire, in mm3; Vg is the volume of the plate gap, in mm3; kd is the form factor of weld that is determined by the reinforcement requirement and is usually selected from 1.1 to 1.15; vf is the wire feed rate, in mm/s; r is the radius of the welding wire, in mm; t is the time, in seconds; vw is
Fig. 2. Wire feed rate versus butt gap under different welding speed.
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5
Then, the resistance R is given as follows:
R¼
rre $Dx p$r 2
(9)
where rre is the resistivity of the welding wire, in U m. The heating time to preheat the welding wire is the time period which takes the welding wire to make contact with the workpiece after the welding wire exits the wire feed nozzle. Hence, this time can be written as follows:
t ¼ l vf
(10)
where l is the heating extension of the welding wire, in m, and vf is the wire feed speed, in m/s. Substituting Equations (6)e(10) into Equation (5) yields:
Fig. 3. Thermal image of welding wire.
rre l$I 2 $ p2 $r 4 $Cp $rden vf
3.3. Preheating temperature analysis
DT ¼
The preheating temperature of the welding wire depends primarily on the heating current as well as the wire feed rate, the heating extension, the geometric feature and thermo-physical characteristics of the welding wire. Current heating during preheating of the welding wire produces a temperature rise of the welding wire. Considering conduction as the main action of the heat transfer in preheating the wire, two thermodynamic assumptions were simplified as following to establish a model to calculate the preheating temperature.
Equation (11) shows that the temperature rise of the welding wire. DT is directly proportional to the heating extension of the welding wire and the square of heating current I and inversely proportional to the wire feed rate vf. Here,
rre ¼ r0 ð1 þ aTÞ
(11)
(12)
T þ 273 T þ 273 2 T þ 273 3 þ C$ þ D$ 1000 1000 1000 , 2 !, T þ 273 þE MFe 1000
Cp ¼ 1000$ A þ B$ 1) Ignore the heat loss from thermal radiation in welding process. 2) Ignore the heat loss from heat convection in welding process. Thus, the principle of energy conservation can be used to equate the work done by the electric current on the welding wire to the quantity of heat produced by the welding wire:
W¼Q
(5)
where
W ¼ I 2 $R$t
(6)
Q ¼ Cp $m$DT
(7)
W is the work done, in J, by the current on the welding wire; Q is the quantity of heat, in J, produced by the welding wire; I is the heating current, in A; R is the resistance of the welding wire, in U; t is the heating time, in s; Cp is the specific heat of the welding wire at atmosphere pressure, in J/kg C; m is the mass of a segment of the heated welding wire, in kg; and DT, which is the temperature rise of the welding wire, in C, can be described as DT ¼ T Ta, where T is the temperature on the wire endpoint, in C, and Ta is the air temperature, in C. Let us consider a small length Dx along the welding wire (as shown in Fig. 4) to investigate the factors that cause a temperature rise in a segment of the welding wire. The mass m of the Dx segment of welding wire is given as follows:
m ¼ rden $p$r 2 $Dx
(8)
where rden is the material density of the welding wire, in kg/m3, and r is the radius of the welding wire.
(13) where T is the temperature of the welding wire in C. It is known that s ¼ 17 mm, r ¼ 0.5 mm, and vf ¼ 1.2 m/min. A query of the National Institute of Standards and Technology (NIST) database (Chase, 1998) shows that at 0 C, the welding wire has a resistivity r0 ¼ 9.7 108 U m, a temperature coefficient of resistance a ¼ 6.51 103 /Co, a density rden ¼ 7.87 103 kg/m3, specific heat coefficients at atmosphere pressure A ¼ 23.97, B ¼ 8.368, C ¼ 2.77 104, D ¼ 8.6 105 , and E ¼ 5 106, and that the molar mass of iron (Fe) MFe ¼ 55.85 g/mol. When
k¼
rre l $ p2 $r 4 $Cp $rden vf
(14)
Thus
DT ¼ k$I2
(15)
As shown in Fig. 5, when the preheating temperature of hotwire reached 200 C or much more, the values of the coefficient k can be regarded as a constant. Fig. 6 shows that under the specified welding conditions, the preheating temperature of the welding wire increases almost parabolically with the heating current. The measurement errors of the temperature in Fig. 6 are ±2%, and the theoretical temperature on the wire endpoint tends to be higher than the measured values. This reason was primarily because the thermal radiation losses from the welding wire were neglected in the theoretical calculation.
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Table 2 Measured temperatures on the sample point of welding wire (wire feed rate vf ¼ 1.2 m/min). Heating current I (A) 0 20 30 40 50 60 70 80 90 Welding wire temperature T ( C) 15 25 45 76 100 150 580 780 1120
3.4. Energy efficiency evaluation The difference between hot-wire laser welding and cold-wire laser welding is the preheating of the filler wire. Electricallyaided heating of the welding wire provides an additional energy input to the welding process, and a suitable preheating temperature of the welding wire can improve the stability of the welding process and significantly reduce the input laser power as well. The input laser power that is reduced by the electrically-aided heating of the welding wire is defined as the equivalent laser power DP. The equivalent laser power characterizes the difference between the laser power needed for hot-wire laser welding and that required for cold-wire laser welding. The equivalent laser power depends on the geometric feature, thermo-physical properties of the welding wire, the wire feed rate and the preheating temperature. Using the definition of the equivalent laser power yields.
DP$t$Aabs ¼ Cp $M$DT
(16)
Where
M ¼ rden $p$r 2 $l Fig. 4. Schematic of welding-wire heating analysis.
And Aabs is the absorptivity of the welding wire for the laser. In this study, the welding wire had an absorptivity of 40% for the fiber laser (W.M. Steen, 1991). Substituting Equations (4), (10) and (17) into Equation (16) yields.
DP ¼
Fig. 5. Coefficient k as a function of the preheating temperature of the hot-wire.
Fig. 6. Plot of heating current versus wire endpoint preheating temperature.
(17)
Cp $rden $kd $vw $d $d$DT Aabs
(18)
Fig. 7 shows that the equivalent laser power DP depends linearly on the butt-gap d and the temperature rise DT. The temperature increasing on the wire endpoint due to the electrically-aided heating of the welding wire can effectively boost the equivalent laser power, thereby decreasing the input laser power for welding. Moreover, the larger the butt gap, the higher is the equivalent laser power required. The energy input into the hot-wire laser welding system is primarily in the form of electrical energy, whereas the energy output of the system is in the form of thermal energy. When the process parameters are appropriate, the energy is utilized totally to the welding process (Salminen, 2010). Therefore, it was assumed that the output laser energy was completely converted into the thermal energy provided for welding. And then the energy efficiency ratio h of the hot-wire laser welding system can be described
Fig. 7. Relationship of wire endpoint temperature to equivalent laser power (vw ¼ 1.2 m/min).
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7
as the ratio of the total thermal power output Po to the total electrical power input Pi. Additionally, the total thermal power output Po of the hot-wire laser welding system is the sum of laser power output Pol and the thermal power output Poc from the hot wire. Then, the total electrical power input Pi of the system is the sum of the electrical power input Pil needed to generate the laser energy and the electrical power input Pic to preheat the wire. Let hl and hc denote the energy transfer efficiency ratios for the laser and the hot wire, respectively; then Pil ¼ Pol/hl and Pic ¼ Poc/hc. Hence, the energy efficiency ratio h of the hot-wire laser welding system is given as follows:
Pol þ Poc
h¼
Pol hl
(19)
þ Phoc c
0
In the same way, the energy efficiency ratio h of the cold-wire laser welding system can be described as follows:
h0 ¼
Pol þ DP$Aabs Pol hl
(20)
þ DP h
h h0 h0
(21)
Substituting Equations (19) and (20) into Equation (21) and simultaneously considering the laser output yields a low optical-toelectrical conversion efficiency of approximately 10%e20%. Considering the energy consumption during the laser transmission process, the energy transfer efficiency ratio of the laser hl is generally less than 0.1. Hence, the hl $Aabs term can be neglected, i.e., hl $Aabs z0. As Poc ¼ DP$Aabs , the energy conservation ratio z can be simplified as follows:
z¼
4. Conclusions
l
Then, the energy conservation ratio z for hot-wire laser welding relative to cold-wire laser welding can be written as follows:
z¼
Fig. 8. Plot of energy conservation ratio versus wire endpoint temperature (vw ¼ 1.2 m/min).
DP Pol
(22)
The energy conservation ratio z reflects the improvement in the energy consumption efficiency in hot-wire laser welding compared to cold-wire laser welding. Table 3 shows the laser power needed for welding with different butt gaps, the welding speed is 20 mm/s, the defocus is 8 mm, and the other welding conditions are the same. Substituting Equation (18) into Equation (21) yields the dependence of the energy conservation ratio z on the temperature on the wire endpoint, which is shown in Fig. 8. Within the allowed range of the temperature on the wire endpoint (600e800 C) and for gap sizes ranging from 0.4 to 1.2 mm, a maximum energy savings of 6%e16% was realized for hot-wire laser welding over coldfiller-wire laser welding. The results show that electrically-aided heating during the hot-wire laser welding process effectively increased the energy efficiency ratio and significantly decreased energy consumption. The energy conservation ratio z significantly increased with the larger butt gap and the increasing temperature on the wire endpoint from the electrically-aided heating of the welding wire.
0.4 1210
0.6 1400
1) In this study, an equivalent laser power computation method was presented to describe the reduction of the input laser power by electrically-aided heating of the welding wire. An energy efficiency model was developed for hot-wire laser welding, and the energy conservation ratio of hot-wire laser welding to coldwire laser welding was evaluated. 2) In the hot-wire laser welding process, electrically-aided heating increased the energy efficiency ratio and significantly reduced energy consumption: in the hot-wire laser welding process, a maximum energy savings of 16% was realized over cold-wire laser welding. Moreover, the energy conservation ratio z was directly related to the endpoint temperature on the electricallyaided heated wire and to the butt gap. 3) The optimum preheating temperature range for the welding wire in hot-wire laser welding was measured to range from 600 to 800 C. Also, a model was formulated to calculate the preheating temperature of the welding wire. The preheating temperature of the welding wire increases almost parabolically with the heating current under the specified welding conditions. 4) The wire feed rate has a positive linear correlation with the butt gap and the welding speed under certain defined conditions in hot-wire laser welding. Acknowledgments The authors are grateful to the National Natural Science Foundation of China (No. 51175162) for financial support. Nomenclature
Table 3 Laser power in welding with different butt gaps. Butt gap d (mm) Laser power Pl (W)
Compared to cold-wire laser welding process, electrically-aided preheating is used for the welding filler wire in hot-wire laser welding process. Using a suitable preheating temperature for the filler wire increased the stability of the welding process and improves weld seam formation, while reducing the total energy input during the welding process and then increasing the efficiency of energy usage. The primary conclusions of this study from the energy efficiency evaluation of hot-wire laser welding are given below.
0.8 1510
1.0 1600
1.2 1800
Aabs Cp d
absorptivity of the welding wire for the laser specific heat of the welding wire at atmosphere pressure plate thickness
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I k kd l m M Pi Pic Pil Poc Pol DP Q r R t T Ta DT vf Vg Vw Vw W
d z h 0
h
hc hl rden rre
heating current coefficient form factor of weld determined by the reinforcement requirement heating extension of the welding wire mass of a segment of the heated welding wire mass of the heated welding wire total electrical power input of the system electrical power input sum of the electrical power input thermal power output from the hot wire sum of laser power output from the hot wire equivalent laser power quantity of heat radius of the welding wire resistance of the welding wire heating time temperature on the wire endpoint air temperature temperature rise of the welding wire wire feed rate welding speed volume of the plate gap volume of the filler wire work done butt gap energy conservation ratio for hot-wire laser welding energy efficiency ratio of the hot-wire laser welding system energy efficiency ratio of the cold-wire laser welding system energy transfer efficiency ratios for the hot wire energy transfer efficiency ratios for the laser material density of the welding wire resistivity of the welding wire
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€nsleben, P., Brülhart, M., Ernst, F.O., 2011. Integrating Bunse, K., Vodicka, M., Scho energy efficiency performance in production management gap analysis between industrial needs and scientific literature. J. Clean. Prod. 19, 667e679. Chase Jr., M.W., 1998. NIST-JANAF Themochemical Tables In: J. Phys. Chem. Ref. Data, Monograph, fourth ed., vol. 9 Retrieved 22/5/2014 from. www.webbook.nist. gov. Dahmen, M., Gudukkurt, O., Kaierle, S., 2010. The ecological footprint of laser beam welding. Phys. Procedia 5, 19e28. Daub, R., Wiedenmann, R., Mahrle, A., Duong, J., Zaeh, M.F., 2010. Influence on the efficiency of the heat conduction mode laser beam welding process regarding different laser spot geometries. In: International Congress on Applications of Lasers & Electro-optics (ICALEO). Anaheim Kalifornien/USA, vol. 26, p. 30. Dilthey, U., Fuest, D., Scheller, W., 1995. Laser welding with filler wire. Opt. Quantum Electron. 27 (12), 1181e1191. Duflou, J.R., Sutherland, J.W., Dornfeld, D., Herrmann, C., Jeswiet, J., Kara, S., Hauschild, M., Kellens, K., 2012. Towards energy and resource efficient manufacturing: a processes and systems approach. CIRP Annals-Manufacturing Technol. 61, 587e609. Han, L., 2008. Application of Laser Welding Techniques in Yiqi-dazhong Magotan Auto-body Manufacturing, vol. 563, (08). Jinshu Jiagong (Rejiagong) [Metal Machining (Hot-Working)], 32e35þ40. Herring, H., 2006. Energy efficiency-a critical view. Energy 31, 10e20. IEA, 2011. Climate & Electricity Annual. Retrieved 7/8/2014 from. www.iea.org/ publications/freepublications/publication/Climate_Electricity_Annual2011.pdf. IEA, 2013. Key World Energy Statistics. Retrieved 7/8/2014 from. www.iea.org/ publications/freepublications/publication/KeyWorld2013.pdf. Ingarao, G., Ambrogio, G., Gagliardi, F., Di Lorenzo, R., 2012. A sustainability point of view on sheet metal forming operations: material wasting and energy consumption in incremental forming and stamping processes. J. Clean. Prod. 29-30, 255e268. Li, X., Xu, X., Bian, M., 2006. Applications of Laser Welding in the Automobile Industry, vol. 04. Dian Hanji [Electric Welders], pp. 47e49. Martin-Vega, L.A., 1999. DMI II: past, present, and future perspectives [R]. Key Report. In: 1999 NSF Design & Manufacturing Grantees, Los Angeles, USA, 1999. Musa, C., Licheri, R., Locci, A.M., Orrù, R., Cao, G., Rodriguez, M.A., Jaworska, L., 2009. Energy efficiency during conventional and novel sintering processes: the case of TieAl2O3eTiC composites. J. Clean. Prod. 17 (9), 877e882. Patterson, M.G., 1996. What is energy efficiency-concepts, indicators and methodological issues. Energy Policy 24 (5), 377e390. €tzer, I., 2005. Laser beam welding makes aircraft lighter. Fraunhofer Mag. 1, Ro 36e37. Retrieved 12/6/2014 from. www.archiv.fraunhofer.de/archiv/ magazin04-08/fhg/Images/magazine1-2005-36f_tcm6-14041.pdf. Salminen, A., 2010. The filler wire-laser beam interaction during laser welding with low alloyed steel filler wire. Mechanika 84 (4), 67e74. Available: Retrieved 10/6/ 2014 from. www.zurnalas.mechanika.ktu.lt/files/Mech484/Salminen484.pdf. Steen, W.M., 1991. Laser Material Processing, first ed. Springer-Verlag, London, p. 266. Sun, Z., Salminen, A.S., 1997. Current status of laser welding with wire feed. Mater. Manuf. Process. 12 (5), 759e777. Um, J., Stroud, I.A., 2013. Total energy estimation model for remote laser welding process. Procedia CIRP 7, 658e663. Wang, Q., Liu, F., Li, C., 2013. An integrated method for assessing the energy efficiency of machining workshop. J. Clean. Prod. 52, 122e133. Witik, R.A., Gaille, F., Teuscher, R., Ringwald, H., Michaud, V., M^ anson, J.E., 2012. Economic and environmental assessment of alternative production methods for composite aircraft components. J. Clean. Prod. 29-30, 91e102. Zhang, T.W., Boyd, S., Vijayaraghavan, A., Dornfeld, D., 2006. Energy use in nanoscale manufacturing. In: Presented at the IEEE International Symposium on Electronics and the Environment, San Francisco, CA, May 811, pp. 266e271. Zhao, B., Duan, K., Zhao, W., Li, C., Wang, Y., 2010. Experimental study on high power all-fiber laser. Chin. Opt. Laser 8 (4), 41e47.
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Contents lists available at ScienceDirect
Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Energy efficiency evaluation of hot-wire laser welding based on process characteristic and power consumption Haiying Wei*, Yi Zhang, Lipeng Tan, Zhihua Zhong State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, Hunan 410082, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 January 2014 Received in revised form 6 September 2014 Accepted 4 October 2014 Available online xxx
Energy conservation has become one of the priorities in manufacturing industries, while the energy efficiency at process level may provide deep understanding of the energy consumption during the manufacturing process. Hot-wire laser welding of double galvanized high-strength steel DP800 has a wide range of potential applications in the automobile manufacturing field. Based on the introduction of equivalent laser power, a mathematical model was developed to obtain the energy efficiency of hot-wire laser welding. The results of theoretical calculation and experimental verification indicate that the optimum pre-heating temperature range of the welding wire in hot-wire laser welding is from 600 to 800 C. The preheating temperature of the welding wire increases almost parabolically with the heating current. Furthermore, the developed technique can be used to realize a maximum energy savings of 16% over cold-wire laser welding. The conclusions allowed to formulate the energy efficiency evaluation as green manufacturing of novel process is concerned. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Energy efficiency evaluation Hot-wire laser welding Equivalent laser power Mathematical model
1. Introduction Energy conservation is an effective means of reducing the imbalance between energy supply and demand, while being necessary for the protection of the natural environment and ~ os et al., 2011; Bunse et al., 2011). Statisecological resources (Ban tical data from the International Energy Agency (IEA) shows that the energy consumption of the manufacturing industry accounts approximately one-third of global final energy use (IEA, 2011). IEA, 2011, global electricity consumption produced 1582 Mtons of CO2, 42.6% of which resulted from electricity consumption in the manufacturing industry (IEA, 2013). Hence, energy conservation and efficiency improvement have become a focal point and must be addressed by manufacturing industries. Especially, the energy consumption of laser welding is quite high. ‘A 1-kW laser requires 35 kW of electric power, and its energy availability is unusually quite low’ stated Zhao et al. (2010). And the measurement and management of energy efficiency represent the environmental and economic benefits of laser machining (Zhang et al., 2006). Just as the Director of the Division of Design & Manufacturing of the U.S. National Science Foundation (NSF), Dr. MartineVega stated in his keynote that by 2020, the manufacturing
* Corresponding author. E-mail address: [email protected] (H. Wei).
industry will encounter six major challenges, one of which will be environmental compatibility (Martin-Vega, 1999). Meanwhile, green manufacturing may offer an effective and sustainable solution. Therefore, the need for improvements in energy efficiency of manufacturing process will double over the next 40 years (Allwood et al., 2011). Moreover, studies revealed that the assessment of energy consumption have become an urgent matter for manufacturing industry (Wang et al., 2013), while the energy efficiency at process level have been considered rather interesting and may provide the deep understanding of the energy transformation during the manufacturing process (Apostolos et al., 2013). And it is essential for the manufacturing industry to improve energy efficiency of both its current technological processes and novel manufacturing processes (Apostolos et al., 2013; Herring, 2006). Advanced manufacturing is being developed in the direction of using high-power lasers, which uses “optical energy sources” and offers the significant advantages of high energy density focusing, ease of operation, high flexibility, efficiency, and high quality. In manufacturing processes, lasers have a wide range of applications. Some types of laser welding can replace traditional connection techniques and have gradually been applied in the fields of transportation, aerospace, and nuclear power. In aircraft manufacturing, for example, laser welding techniques have been applied to form the entire connection of the inner bulkheads and reinforce the ribs of the aircraft fuselages and wings of the Airbus A380 (Witik et al., € tzer, 2005). Laser welding greatly reduces the aircraft 2012; Ro
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weight, fuel consumption, and exhaust and produces significant economic benefits in the form of lower operating costs. In the auto manufacturing field, laser welding technologies have been widely used to create connections for new materials for a higher strength and a lower weight of body-in-white. By 2000, 50% of the resistance spot welding production lines of the Big Three automakers in the United States had been replaced by laser welding production lines (Li et al., 2006). The FAW-Volkswagen Automotive Company in China has employed laser welding in the manufacturing of almost all of its brand-name auto models (Han, 2008). Advocates of the use of laser welding of body-in-white acknowledge that it offers many advantages. But the strict machining precision and assembly accuracy requirements of autobody parts have limited the applications of laser welding to some extent. For example, in butt-joint welding, the butt gap cannot exceed 10%e15% of the plate thickness (Dilthey et al., 1995). Thus, laser welding with a filler wire can considerably reduce the requirements on the butt gap for parts. The filler-wire laser welding technique decreases the fitting precision requirements on parts, generates a smaller heat-affected zone than conventional welding techniques, and produces a weld seam whose performance is similar to that using a conventional laser welding. In body-in-white welding, especially, butt-style filler-wire laser welding has the potential to replace the lapped-style resistance spot welding since it can increase the strength of the weld seam and lower the autobody weight. On the other hand, another hitch that might limit the applications of laser welding is low efficiencies in the energy conservation. To increase the laser welding energy efficiency, the characteristics of laser welding and the methods of proper improvements have been recently discussed in the studies (Apostolos et al., 2012; Dahmen et al., 2010; Daub et al., 2010). Apostolos et al. (2012) discussed that laser machining is a thermal process with relatively low energy efficiency because materials machining and cleaning primarily rely on melting or vaporization mechanisms. By measurement of separated boundaries laser systems, this research revealed that higher laser power and higher pulsing frequency improved the energy efficiency. By compared technical aspects of various laser welding, Dahmen et al. (2010) proposed heat input and different laser system had influence on the energy efficiency, which could be increased by using laser light sources with high energy efficiencies, as using fiber lasers or disk lasers. Daub et al. (2010) studied the relationship between the beam spot geometry and the laser energy efficiency. Despite these methods discussed, at process level, the laser welding are still not well documented in terms of energy efficiency, especially lack of analysis of process energy efficiency in hot-wire laser welding when replacing coldwire laser welding. Also, energy efficiency, as an important driver for manufacturing industry, has been debated and rethought recently. Herring (2006) specified the terms ‘energy efficiency’ as the ratio of energy services out to energy input, and by the review of literature, this research pointed out that efficiency gains meant higher levels of energy service. Patterson (1996) classified the operational indicators of energy efficiency into four groups: thermodynamic indicator, physical-thermodynamic indicator, economic-thermodynamic indicator and economic indicator. And Apostolos et al. (2013) divided energy efficiency into four elemental level: process level, machine level, line level and factory level. By comparative analysis of different system scale levels of discrete part manufactory, Duflou et al. (2012) proposed that while at unit process level, redesign of machine tools and optimization of process parameter settings allow improving the energy efficiency. Ingarao et al. (2012) focused on the machine tool architecture at process level and investigated the energy consumption by comparative analysis of the different
load capacities. When the methods and models to evaluate the energy efficiency are concerned, some quantitative analysis have been reported in the studies (Bhushan, 2013; Musa et al., 2009). Bhushan (2013) performed experiments to study the effect of the process parameters on energy consumption and tool life. This research present a statistical methodology based model for selecting optimum machining conditions, and the result showed that the energy consumption was reduced by 13.55% and the tool life was boosted by 22.12%. Musa et al. (2009) quantitatively compared two sintering techniques, the hot pressing and spark plasma sintering, in terms of energy efficiency with experimental study and focused on operating conditions and end-product characteristics. More recently, some quantitative analysis of the energy efficiency of laser welding, a typical example of non-conventional processes, have been made in the studies (Um and Stroud, 2013; Apostolos et al., 2012). Um and Stroud (2013) provided an estimation model for remote laser welding based on the analysis of robot motion and laser source, and the results showed that laser power and process time were important factors in energy efficiency estimation. Apostolos et al. (2012) investigated laser machining energy efficiency under two different boundaries with using an index Eef1 to measure the energy efficiency of the entire laser machine system and using an index Eef for only the workpiece and laser beam interaction respectively. This research efforts produced that the process parameters and processing strategies influenced the energy efficiency. From the literature review in the domain of energy efficiency of manufacturing processes, the laser welding process, which were mainly defined as the thermal efficiency and whose efficiency vastly depended on the machining conditions, required to accurately evaluate the result, especially, the mathematical models based on the characteristics of the process. As pointed out by Apostolos et al. (2012), the laser based processes energy efficiency might be difficult to be analytically modeled. In this paper, a quantitative evaluation of the energy consumption and energy efficiency during hot-wire laser welding of double galvanized high-strength steel, which has a wide range of applications in the auto manufacturing field, was obtained in this work based on the essence of energy transformation and technological characteristics of the laser welding process. The results were theoretically evaluated and experimentally verified. The paper offered a method to formulate the energy efficiency evaluation as green manufacturing of novel process is concerned. The paper proceeds as follows: Section 2 describes the process characteristics and parameters for hot-wire laser welding, and provides such experimental details as methods and materials. Section 3 develops a model to calculate and analyze the wire feed rate, preheating temperature, and energy efficiency in hot-wire laser welding based on the measurement results of the wire preheating temperature and the introduction of the equivalent laser power. Section 4 offers the conclusions of the study. 2. Experiments 2.1. Process features and parameters The process parameters of laser welding include the laser power, the welding speed, the defocusing distance, and the characteristics of gas protection such as the gas type, the blowing mode (coaxial or side-blown) and the flow rate. In laser welding with a hot filler wire, process control in joining materials is much more complicated by the preheating and the addition of the welding wire. Various process parameters such as the direction, the angle, the feed placement and the wire extension cause such four hot filler wire transfer modes in the hot-wire laser welding as drop transfer, fusing transfer, continuous transfer and top thread transfer.
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For the wire feed direction in hot-wire laser welding, the wire can be fed in such two directions as a dragging feed and a trailing feed relative to the direction of the welding speed. The smooth transitions in the wire melting process can be easily obtained, resulting in an expected stable welding process under dragging feed mode. However, even small vibration in the trailing feed process could cause the adhesion between the wire and the weld bead and destabilizing the welding process. Also, the hot filler wire in trailing feed mode can only get the energy from the melting pool for being further heated and melted. Size limitations imposed by the wire feed nozzle and the laser head result in a wire feed angle between the wire and the laser beam. When the wire feed angle is fairly large, wire extension must be increased to prevent interference between the wires feed nozzle and the workpiece, easily leading to vibration at the wire tip. However, when the wire feed angle is fairly small, the feed placement of the hot filler wire is difficult to adjust. The reason was that a fairly small deviation in the position of the welding wire produces a sharp increase in the deviation of the beam-wire coupling along the beam propagation direction. The vibration at the wire tip and the deviation in the beam-wire coupling can both destabilize and even interrupt the welding process. On the other hand, the wire feed angle also affects the area of the heat affected zone (HAZ). The heat input to the welding wire under the laser irradiation varies with the wire feed position. The deviation of the welding wire from the laser light in the direction of cross section should be minimized to obtain a symmetric weld seam and higher melting efficiency of the welding wire. The higher the laser power and the wider the weld bead formed, the larger is the allowable deviation of the wire feed placement in the direction of cross section. If there is zero deviation in the direction of cross section for the wire feed placement, the wire feed placement can also be described in terms of the beam-wire distance, which corresponds to the deviation of the center of the welding wire tip from the center of the laser beam in the welding direction. In the dragging feed mode, the placement of the welding wire usually coincides with the center of the light spot such that the beam-wire distance is zero, and the hot filler wire heats up and melts under the irradiation of the laser. At the same time, in thin-plate hot-wire laser welding, the welding wire is usually placed on the workpiece surface, i.e., the placement deviation in the direction of the weld depth is zero. The wire heating extension is the distance between the wire nozzle and the workpiece along the feeding direction. The wire heating extension affects the rigidity and the directivity of the welding wire. Under normal conditions, the wire heating extension should not be too long because the wire heating extension can affect the preheating time of the welding wire, thereby affecting the preheating temperature of the welding wire. For the same heating current, the larger the wire heating extension is, the longer the preheating time and the higher the preheating temperature are. Therefore, the dragging feed mode was used in the following experiments with a wire feed angle of 45 , a zero beam-wire distance, and wire heating extension of 17 mm. 2.2. Experimental details Experiments were performed on high-strength automobile steel sheets DP800 with double zinc coating and dual phase. This test
3
material has a tensile strength Rm 800 MPa, a yield strength Rp0.2 450 MPa, and test-piece dimensions of 60 mm 35 mm 1.2 mm. The edges of the test-piece were sanded before testing to ensure that the butt gap was even, and acetone was used to clean the butt position. The welding filler wire was high-strength steel as well and had a 1.0-mm diameter. The chemical constituents of the DP800 and the high-strength-steel welding wire are listed in Table 1. Fig. 1 is a schematic of the hot-wire laser welding test apparatus. Welding tests were carried out with a continuous wave fiber laser (IPG YLR 4000) with a wavelength of 1070 nm, TEM00 mode, and an optical fiber core diameter of 0.3 mm. The welding head was a Precitec YW50, which was fitted with a coaxial air blow protection device; the alignment and focusing system was composed of an alignment lens with a 150 mm focal length and a focusing lens with a 200 mm focal length, which produced a light spot with a 0.4-mm diameter at the laser focal spot. The hot wire equipment was a Fronius TransPuls Synergic 2700 hot-wire machine: the heating current of the welding wire ranged from 0 to 270 A, and the wire feed rate was could be regulated from 0.5 to 22 m/min. The welding head and the wire feed head were grasped by a welding robot (ABB IRB 2400). Controllers could be used to set the process parameters, including the laser power, the welding speed, the defocus, and the heating current. The optimal parameters were obtained by orthogonal testing. The hot-wire anode of the wire heating source was connected to the welding wire through the wire feed head, and the other side of cathode was connected to the workpiece. The heating circuit connection of the welding wire was maintained during welding, and coaxial argon (Ar) gas protection was used during welding at a flow rate of 15 L/min. The orthogonal test results for hot-wire laser welding produced the following optimum welding process parameters for a butt gap of 0.6 mm: laser power, 1400 W; heating current, 70e80 A; welding speed, 20 mm/s; and defocusing distance, þ8 mm. Under these conditions, good weld seams were formed, and the welding wire fused and mixed uniformly with the parent material in the weld seams. The weld seam zone consisted primarily of lath martensite with a small amount of bainite and austenite; the crystalline grains were quite fine and had a uniform elements distribution. The tensile test showed that the welded joint had a higher tensile strength than that of the basement material. The laser power and the welding speed determined the energy input for the welding process and thus affected the heat input and the width of the weld seam. The maximum width of the weld seam was obtained at a welding speed of 15 mm/s; however, a welding speed of 30 mm/s was excessive and caused the heat input insufficient, resulting in a narrow weld seam and lack of fusion. The defocusing distance affects the size of the laser light spot and the laser energy density distribution. When defocusing distance was zero, the welding wire did not melt, and it was difficult to maintain the stability of the welding process. The small diameter of the laser light spot placed a strict precision requirement on the placement of the feed wire. When there was a slight vibration at the wire tip or a small placement deviation between the wire and the laser beam, the welding wire was not heated well for absence of the irradiation of the full laser, and then lack of fusion occurred. Increasing the defocusing distance could improve the stability of the welding process. However, increasing the defocusing distance enlarged the
Table 1 Chemical constituents of DP800 and welding wire. Element
C
Si
Mn
P
S
Al
Ni
Mo
Ti
Fe
DP800 (%) Welding wire (%)
0.13 0.028
0.17 0.49
1.96 1.48
0.012 0.0094
<0.0050 0.0090
0.050 e
e 2.36
e 0.24
e 0.12
Bal. Bal.
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the welding speed, in mm/s; d is the plate thickness, in mm; and d is the butt gap, in mm. Hence, substituting Equations (1) and (2) into Equation (3) yields the wire feed rate:
vf ¼
Fig. 1. Experimental schematic of hot-wire laser welding.
diameter of the laser light spot, resulting in the laser energy density being dropped. The defocusing distance with þ8 mm can obtain the beam diameter of the laser on the workpiece surface with 1.2 mm. At this moment, the diameter of the laser irradiation was slightly larger than the diameter of the welding wire, which maintained the stability of the welding process and produced good weld seams. Meanwhile, the heating current of the welding wire considerably affected the stability of the welding process as well. A FLIR A615 infrared (IR) thermal imaging system was used to measure the preheating temperature under different heating currents. The IR thermal imaging system was connected via a data line to a computer, and the temperature measurement process was controlled by a computer program. Before the temperature measurements were conducted, the parameters of the radiation rate, the air temperature, and the air humidity were set, followed by selecting a suitable image frame frequency and temperature measurement range. And the accuracy of the thermal imaging system is ±2% of the measured value. When the heating current was too low, the wire fusion was insufficient and then top thread occurred on the side of the weld seam. On the other side, too high heating current was apt to form molten drops at the tip of the welding wire, spattering and wire feed suspending occurred, and the welding process was discontinuous. For a suitable heating current, the welding wire formed transient molten drops at the tip of the welding wire. On this condition the welding process became stable, and then the optimum preheating temperature was obtained. 3. Results and discussion
kd $d $d$vw p$r 2
(4)
Equation (4) shows that under certain defined welding conditions, the wire feed rate has a positive linear correlation with the butt gap and the welding speed. At a fixed laser power, the value of the welding speed determines the energy input. The higher the welding speed, the lower the energy input. Hence, in hot-wire laser welding; the welding speed also has a fairly large influence on the stability of the wire feed. If the welding speed is too high, the wire feed rate must be increased to match the volume of plate gap. Then the feed wire will be easily led to vibration at the wire tip, destabilizing the welding process. Fig. 2 shows the wire feed rate as a function of the butt gap for a welding wire radius r of 0.5 mm, a plate thickness d of 1.2 mm, and a kd value of 1.1. 3.2. Measurement of wire preheating temperature A suitable preheating temperature of the welding wire is critical to obtain continuous transfer behavior of the welding wire and to ensure that the hot-wire laser welding process is stable. The experiment results showed that the preheating temperature of the welding wire was determined by several factors such as the heating current, the wire feed rate, etc. The results demonstrated that for a butt gap of 0.6 mm, the optimal heating current range was 70e80 A, and the heating temperature at these currents were in the optimal preheating temperature range for the welding wire. Fig. 3 shows the heat distribution on the welding wire, which had a heating extension of 17 mm. The temperature was measured at the sample point of the intersection between the welding wire and the workpiece. Actually, the measured position was the wire endpoint in the welding process, which was contacted with the melting pool. To minimize the measurement error, the temperature was recorded at 5 different times, and the mean value was used as the measured result. The wire feed rate was taken to be vf ¼ 1.2 m/min. Table 2 shows the measured preheating temperatures of the welding wire at different heating currents. When the heating current was in the 70e80 A range, the measured preheating temperature of the welding wire was 600e800 C, which was the optimal preheating temperature range of the welding wire for the welding process.
3.1. Calculation of wire feed rate During laser butt welding with the hot filler wire, the required feed amount of the welding wire is determined by the volume of the gap to be filled (Sun and Salminen, 1997):
Vw ¼ kd $Vg
(1)
where:
Vw ¼ vf $p$r 2 $t
(2)
Vg ¼ vw $d$d$t
(3)
Vw is the required volume of the filler wire, in mm3; Vg is the volume of the plate gap, in mm3; kd is the form factor of weld that is determined by the reinforcement requirement and is usually selected from 1.1 to 1.15; vf is the wire feed rate, in mm/s; r is the radius of the welding wire, in mm; t is the time, in seconds; vw is
Fig. 2. Wire feed rate versus butt gap under different welding speed.
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5
Then, the resistance R is given as follows:
R¼
rre $Dx p$r 2
(9)
where rre is the resistivity of the welding wire, in U m. The heating time to preheat the welding wire is the time period which takes the welding wire to make contact with the workpiece after the welding wire exits the wire feed nozzle. Hence, this time can be written as follows:
t ¼ l vf
(10)
where l is the heating extension of the welding wire, in m, and vf is the wire feed speed, in m/s. Substituting Equations (6)e(10) into Equation (5) yields:
Fig. 3. Thermal image of welding wire.
rre l$I 2 $ p2 $r 4 $Cp $rden vf
3.3. Preheating temperature analysis
DT ¼
The preheating temperature of the welding wire depends primarily on the heating current as well as the wire feed rate, the heating extension, the geometric feature and thermo-physical characteristics of the welding wire. Current heating during preheating of the welding wire produces a temperature rise of the welding wire. Considering conduction as the main action of the heat transfer in preheating the wire, two thermodynamic assumptions were simplified as following to establish a model to calculate the preheating temperature.
Equation (11) shows that the temperature rise of the welding wire. DT is directly proportional to the heating extension of the welding wire and the square of heating current I and inversely proportional to the wire feed rate vf. Here,
rre ¼ r0 ð1 þ aTÞ
(11)
(12)
T þ 273 T þ 273 2 T þ 273 3 þ C$ þ D$ 1000 1000 1000 , 2 !, T þ 273 þE MFe 1000
Cp ¼ 1000$ A þ B$ 1) Ignore the heat loss from thermal radiation in welding process. 2) Ignore the heat loss from heat convection in welding process. Thus, the principle of energy conservation can be used to equate the work done by the electric current on the welding wire to the quantity of heat produced by the welding wire:
W¼Q
(5)
where
W ¼ I 2 $R$t
(6)
Q ¼ Cp $m$DT
(7)
W is the work done, in J, by the current on the welding wire; Q is the quantity of heat, in J, produced by the welding wire; I is the heating current, in A; R is the resistance of the welding wire, in U; t is the heating time, in s; Cp is the specific heat of the welding wire at atmosphere pressure, in J/kg C; m is the mass of a segment of the heated welding wire, in kg; and DT, which is the temperature rise of the welding wire, in C, can be described as DT ¼ T Ta, where T is the temperature on the wire endpoint, in C, and Ta is the air temperature, in C. Let us consider a small length Dx along the welding wire (as shown in Fig. 4) to investigate the factors that cause a temperature rise in a segment of the welding wire. The mass m of the Dx segment of welding wire is given as follows:
m ¼ rden $p$r 2 $Dx
(8)
where rden is the material density of the welding wire, in kg/m3, and r is the radius of the welding wire.
(13) where T is the temperature of the welding wire in C. It is known that s ¼ 17 mm, r ¼ 0.5 mm, and vf ¼ 1.2 m/min. A query of the National Institute of Standards and Technology (NIST) database (Chase, 1998) shows that at 0 C, the welding wire has a resistivity r0 ¼ 9.7 108 U m, a temperature coefficient of resistance a ¼ 6.51 103 /Co, a density rden ¼ 7.87 103 kg/m3, specific heat coefficients at atmosphere pressure A ¼ 23.97, B ¼ 8.368, C ¼ 2.77 104, D ¼ 8.6 105 , and E ¼ 5 106, and that the molar mass of iron (Fe) MFe ¼ 55.85 g/mol. When
k¼
rre l $ p2 $r 4 $Cp $rden vf
(14)
Thus
DT ¼ k$I2
(15)
As shown in Fig. 5, when the preheating temperature of hotwire reached 200 C or much more, the values of the coefficient k can be regarded as a constant. Fig. 6 shows that under the specified welding conditions, the preheating temperature of the welding wire increases almost parabolically with the heating current. The measurement errors of the temperature in Fig. 6 are ±2%, and the theoretical temperature on the wire endpoint tends to be higher than the measured values. This reason was primarily because the thermal radiation losses from the welding wire were neglected in the theoretical calculation.
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Table 2 Measured temperatures on the sample point of welding wire (wire feed rate vf ¼ 1.2 m/min). Heating current I (A) 0 20 30 40 50 60 70 80 90 Welding wire temperature T ( C) 15 25 45 76 100 150 580 780 1120
3.4. Energy efficiency evaluation The difference between hot-wire laser welding and cold-wire laser welding is the preheating of the filler wire. Electricallyaided heating of the welding wire provides an additional energy input to the welding process, and a suitable preheating temperature of the welding wire can improve the stability of the welding process and significantly reduce the input laser power as well. The input laser power that is reduced by the electrically-aided heating of the welding wire is defined as the equivalent laser power DP. The equivalent laser power characterizes the difference between the laser power needed for hot-wire laser welding and that required for cold-wire laser welding. The equivalent laser power depends on the geometric feature, thermo-physical properties of the welding wire, the wire feed rate and the preheating temperature. Using the definition of the equivalent laser power yields.
DP$t$Aabs ¼ Cp $M$DT
(16)
Where
M ¼ rden $p$r 2 $l Fig. 4. Schematic of welding-wire heating analysis.
And Aabs is the absorptivity of the welding wire for the laser. In this study, the welding wire had an absorptivity of 40% for the fiber laser (W.M. Steen, 1991). Substituting Equations (4), (10) and (17) into Equation (16) yields.
DP ¼
Fig. 5. Coefficient k as a function of the preheating temperature of the hot-wire.
Fig. 6. Plot of heating current versus wire endpoint preheating temperature.
(17)
Cp $rden $kd $vw $d $d$DT Aabs
(18)
Fig. 7 shows that the equivalent laser power DP depends linearly on the butt-gap d and the temperature rise DT. The temperature increasing on the wire endpoint due to the electrically-aided heating of the welding wire can effectively boost the equivalent laser power, thereby decreasing the input laser power for welding. Moreover, the larger the butt gap, the higher is the equivalent laser power required. The energy input into the hot-wire laser welding system is primarily in the form of electrical energy, whereas the energy output of the system is in the form of thermal energy. When the process parameters are appropriate, the energy is utilized totally to the welding process (Salminen, 2010). Therefore, it was assumed that the output laser energy was completely converted into the thermal energy provided for welding. And then the energy efficiency ratio h of the hot-wire laser welding system can be described
Fig. 7. Relationship of wire endpoint temperature to equivalent laser power (vw ¼ 1.2 m/min).
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H. Wei et al. / Journal of Cleaner Production xxx (2014) 1e8
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as the ratio of the total thermal power output Po to the total electrical power input Pi. Additionally, the total thermal power output Po of the hot-wire laser welding system is the sum of laser power output Pol and the thermal power output Poc from the hot wire. Then, the total electrical power input Pi of the system is the sum of the electrical power input Pil needed to generate the laser energy and the electrical power input Pic to preheat the wire. Let hl and hc denote the energy transfer efficiency ratios for the laser and the hot wire, respectively; then Pil ¼ Pol/hl and Pic ¼ Poc/hc. Hence, the energy efficiency ratio h of the hot-wire laser welding system is given as follows:
Pol þ Poc
h¼
Pol hl
(19)
þ Phoc c
0
In the same way, the energy efficiency ratio h of the cold-wire laser welding system can be described as follows:
h0 ¼
Pol þ DP$Aabs Pol hl
(20)
þ DP h
h h0 h0
(21)
Substituting Equations (19) and (20) into Equation (21) and simultaneously considering the laser output yields a low optical-toelectrical conversion efficiency of approximately 10%e20%. Considering the energy consumption during the laser transmission process, the energy transfer efficiency ratio of the laser hl is generally less than 0.1. Hence, the hl $Aabs term can be neglected, i.e., hl $Aabs z0. As Poc ¼ DP$Aabs , the energy conservation ratio z can be simplified as follows:
z¼
4. Conclusions
l
Then, the energy conservation ratio z for hot-wire laser welding relative to cold-wire laser welding can be written as follows:
z¼
Fig. 8. Plot of energy conservation ratio versus wire endpoint temperature (vw ¼ 1.2 m/min).
DP Pol
(22)
The energy conservation ratio z reflects the improvement in the energy consumption efficiency in hot-wire laser welding compared to cold-wire laser welding. Table 3 shows the laser power needed for welding with different butt gaps, the welding speed is 20 mm/s, the defocus is 8 mm, and the other welding conditions are the same. Substituting Equation (18) into Equation (21) yields the dependence of the energy conservation ratio z on the temperature on the wire endpoint, which is shown in Fig. 8. Within the allowed range of the temperature on the wire endpoint (600e800 C) and for gap sizes ranging from 0.4 to 1.2 mm, a maximum energy savings of 6%e16% was realized for hot-wire laser welding over coldfiller-wire laser welding. The results show that electrically-aided heating during the hot-wire laser welding process effectively increased the energy efficiency ratio and significantly decreased energy consumption. The energy conservation ratio z significantly increased with the larger butt gap and the increasing temperature on the wire endpoint from the electrically-aided heating of the welding wire.
0.4 1210
0.6 1400
1) In this study, an equivalent laser power computation method was presented to describe the reduction of the input laser power by electrically-aided heating of the welding wire. An energy efficiency model was developed for hot-wire laser welding, and the energy conservation ratio of hot-wire laser welding to coldwire laser welding was evaluated. 2) In the hot-wire laser welding process, electrically-aided heating increased the energy efficiency ratio and significantly reduced energy consumption: in the hot-wire laser welding process, a maximum energy savings of 16% was realized over cold-wire laser welding. Moreover, the energy conservation ratio z was directly related to the endpoint temperature on the electricallyaided heated wire and to the butt gap. 3) The optimum preheating temperature range for the welding wire in hot-wire laser welding was measured to range from 600 to 800 C. Also, a model was formulated to calculate the preheating temperature of the welding wire. The preheating temperature of the welding wire increases almost parabolically with the heating current under the specified welding conditions. 4) The wire feed rate has a positive linear correlation with the butt gap and the welding speed under certain defined conditions in hot-wire laser welding. Acknowledgments The authors are grateful to the National Natural Science Foundation of China (No. 51175162) for financial support. Nomenclature
Table 3 Laser power in welding with different butt gaps. Butt gap d (mm) Laser power Pl (W)
Compared to cold-wire laser welding process, electrically-aided preheating is used for the welding filler wire in hot-wire laser welding process. Using a suitable preheating temperature for the filler wire increased the stability of the welding process and improves weld seam formation, while reducing the total energy input during the welding process and then increasing the efficiency of energy usage. The primary conclusions of this study from the energy efficiency evaluation of hot-wire laser welding are given below.
0.8 1510
1.0 1600
1.2 1800
Aabs Cp d
absorptivity of the welding wire for the laser specific heat of the welding wire at atmosphere pressure plate thickness
Please cite this article in press as: Wei, H., et al., Energy efficiency evaluation of hot-wire laser welding based on process characteristic and power consumption, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.10.009
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I k kd l m M Pi Pic Pil Poc Pol DP Q r R t T Ta DT vf Vg Vw Vw W
d z h 0
h
hc hl rden rre
heating current coefficient form factor of weld determined by the reinforcement requirement heating extension of the welding wire mass of a segment of the heated welding wire mass of the heated welding wire total electrical power input of the system electrical power input sum of the electrical power input thermal power output from the hot wire sum of laser power output from the hot wire equivalent laser power quantity of heat radius of the welding wire resistance of the welding wire heating time temperature on the wire endpoint air temperature temperature rise of the welding wire wire feed rate welding speed volume of the plate gap volume of the filler wire work done butt gap energy conservation ratio for hot-wire laser welding energy efficiency ratio of the hot-wire laser welding system energy efficiency ratio of the cold-wire laser welding system energy transfer efficiency ratios for the hot wire energy transfer efficiency ratios for the laser material density of the welding wire resistivity of the welding wire
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Please cite this article in press as: Wei, H., et al., Energy efficiency evaluation of hot-wire laser welding based on process characteristic and power consumption, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.10.009