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The effect of TIG welding techniques on microstructure, properties and porosity of the welded joint of 2219 aluminum alloy
Accepted Manuscript The effect of TIG welding techniques on microstructure, properties and porosity of the welded joint of 2219 aluminum alloy Hui Li, Jiasheng Zou, Junshan Yao, Haoping Peng PII:
S0925-8388(17)32886-4
DOI:
10.1016/j.jallcom.2017.08.157
Reference:
JALCOM 42914
To appear in:
Journal of Alloys and Compounds
Received Date: 16 April 2017 Revised Date:
3 August 2017
Accepted Date: 16 August 2017
Please cite this article as: H. Li, J. Zou, J. Yao, H. Peng, The effect of TIG welding techniques on microstructure, properties and porosity of the welded joint of 2219 aluminum alloy, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.08.157. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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The effect of TIG welding techniques on microstructure, properties and porosity of the welded joint of 2219 aluminum alloy Li Hui 1, 2,
Zou Jiasheng 1,
Yao Junshan 3,
Peng Haoping 4
(1.Provincial Key Lab of Advanced Welding Technology, Jiangsu University of Science and Technology, Zhenjiang, 212003; 2. School of Mechanical and Vehicle Engineering, Changzhou Institute of Technology, Changzhou, 213002;
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3.Jiangsu ASWEMET Precision technology Co. Ltd, Pizhou, 221300;
4. Jiangsu Key Laboratory of Oil & Gas Storage and Transportation Technology, Changzhou University, 213164, Jiangsu, P.R. China)
Abstract: 2219 aluminum alloy was mainly used in the propellant tanks of domestic aerospace vehicles. Porosity was often observed during the VPTIG welding joint of the 2219 aluminum alloy. The new welding technology under condition of direct current electrode negative A-TIG (DCEN A-TIG) welding using a special active agent (AlF3, LiF, KF-AlF3, K2SiF6) had eliminated
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the welding porosity of 2219 aluminum alloy. DCEN A-TIG welding and VPTIG welding of 2219 aluminum alloy were performed. Compared with VPTIG welding, the current decrease of DCEN A-TIG welding had greatly reduced thermal effect on base metal and heat-affected zone. The DCEN A-TIG technology had prevented welding porosity, while VPTIG welding had contained numerous macro and micro porosity. The weakest part of the DCEN A-TIG welding joint was the fusion zone, however, the VPTIG joint
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mainly ruptured in the weld zone due to significant porosity defect. The grain sizes of the welding zone in VPTIG welding were coarser than those in DCEN A-TIG welding, which can reach the same mechanical performance index with VPTIG welding. Key words: 2219 aluminum alloy, porosity, DCEN A-TIG welding, VPTIG welding
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2219 high-strength aluminum alloy is an important structural material in the aerospace field[1,2]. It is the promising structural material used in space launch vehicles and other components[3,4]. However, there are several challenges in the welding of 2219 aluminum alloy[5,6]: high welding porosity, low joint strength coefficient, poor back of weld and large welding deformation. Conventional TIG welding of large thickness 2219 aluminum alloy plate requires large welding current as well as the use of multi-pass welding process. The welding joint strength coefficient of the 2219 aluminum alloy is seriously reduced and the welding deformation is increased. Several researchers have developed square wave AC TIG welding, VPTIG welding [7,8] and variable polarity plasma welding technology[9,10] to address these restrictions. However, these technologies failed to eliminate the instability process of the AC welding and poor back of weld. A good weld requires keyhole if variable polarity plasma welding technology is used. It suggests a very high need for welding current [11] ,when welding large thickness aluminum alloy plate. Although the welding current is less than the conventional AC TIG welding current, the current is still relatively large. In recent years, a new TIG welding technique A-TIG (active agent TIG) has attracted great attention worldwide [12-16].
Compared with conventional TIG welding, the area of the material surface is coated with a layer of active flux before using conventional TIG welding. The activated flux has no negative effect on the performance or life of the electrode. Residue is easily removed from the surface after welding. Under similar welding specification, A-TIG welding tremendously improves the weld penetration without increasing the width of weld seam. It also reduces welding distortion and production costs, while greatly improving productivity. A-TIG welding has wide-ranging applications. Currently, A-TIG welding is mainly used in stainless steel, carbon steel, nickel base alloy, and titanium alloys[17-21]. It is mainly focused on AC welding addition of active agent improving the penetration of the aluminum alloy through AC welding [22]. Active agent such as SiO2 exhibits the maximum impact on the depth of welding penetration. However, the oxide is bound to increase welding oxidation and affects the stability of the welding process and weld formation, which limits its application [22-23] . Although DCEP (Direct Current Electrode Positive) addition to weld aluminum alloy removes the oxide film on aluminum alloy surface, the calorific value of tungsten electrode is great and the burning is serious, thus it only uses a small
Received date: ******, 2017 Foundation item: NSFC(51671037) Corresponding author: Zou Jiasheng, Provincial Key Lab of Advanced Welding Technology, Jiangsu University of Science and Technology, Zhenjiang, 212003, Jiangsu, P.R. China, Tel: 0086-13775069549 , E-mail: [email protected]
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2 Results 2.1
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the experiment was conducted according to ASTM B557, five (including) or more samples were averaged as the final results. The fracture features were visualized by JSM-6460 scanning electron microscopy (SEM).The phases in the DCEN A-TIG welding joints were analyzed by the energy spectrum analysis and the X-ray diffraction (XRD, D/Max-2500PC) operated with an incident beam with ranging from 10◦to 90◦with a step size of 5◦and 3s in each step.
Comparison of welding current
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The DCEN A-TIG welding with current of 135A resulted in the implementation of 2219 aluminum alloy welding following single layer welding, reduced the welding layer and cost-effectively improved the production efficiency. The VPTIG welding technology was a two-layer welding system with backing weld current of 170A and cosmetic welding current of 140A. The primary difference between single and two layer welding technology was welding current level. Depending on the characteristics of the welding technology[27], welding current determined heat input, and then decided welding distortion, joint microstructure, directly affected the mechanical properties of the joint. Compared with conventional VPTIG welding, DCEN A-TIG welding technology greatly reduced welding current, which resulted in a decline in the welding heat input, thus reducing the welding distortion and thermal effect on the base material.
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current for welding[24-26]. Therefore, the technology has no practical value. DCEN welding is an ideal method to weld 2219 aluminum alloy. Stable arc with the method of DCEN resolves unstable welding of aluminum alloy. The way of DCEN for aluminum alloy welding induces excessive heating in the base material, increases the depth of penetration and rate of heat utilization, reduces the welding current, and eventually saves energy. However, the key challenge is removal of the oxide film of aluminum alloy. Therefore, DCEN A-TIG facilitates welding of aluminum alloy, removes aluminum alloy oxide film and improves welding productivity using active agent under DCEN conditions. In order to verify the effectiveness of DCEN A-TIG, DCEN A-TIG and VPTIG welding of 2219 aluminum alloy with 4 mm thickness, were carried out. By comparing the welding current, welding formation, internal quality (porosity), microstructure and mechanical properties, the advantages and disadvantages of the DCEN A-TIG and VPTIG welding were summarized.
1 Materials and Methods
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The welding material included 2219-T6 aluminum alloy plate measuring 300 mm × 100 mm × 4 mm. The filler metal was ER2319 measuring 3.2 mm in diameter. Active agent was prepared using the four kinds of fluoride (AlF3, LiF, KF-AlF3, K2SiF6) packed in powdered form. Fronius MAGIC WAVE 2600 was used for welding in this study. Butt weld set-ups with zero gap were prepared for the DCEN A-TIG welding of 4mm 2219 aluminum alloy. First, all specimen surfaces were ground with an abrasive paper to remove impurities, followed by cleaning with acetone. Second, the active agent was dissolved in alcohol and stirred into a paste, and coated on the surface of the welding area. It was also used VPTIG welding technology for 4mm 2219 aluminum alloy. The welding parameters as shown in Table 1 were used for the current study. Table 1 Welding parameters
135A
Backing weld current (VPTIG)
170A
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Welding current(DCEN A-TIG) Face welding current (VPTIG)
140A
Travel speed
120mm/min
Wire feed rate
1300mm/min
Wire diameter
1.6mm
Shielding gas Argon
Argon (99.999% purity)
Gas flow rate
11 L/min
Electrode diameter
3.2 mm
After welding, the weld surface was observed and the internal quality was tested via X-ray detection. The cross sections of joints perpendicular to the welding direction were obtained for microstructural analysis. The polished specimens were etched with Keller reagent (1mlHF+1.5mlHCl+2.5ml HNO3 +95mlH2O). An optical microscope (OM) was used for the observations of the microstructural analysis. The sizes of the tensile samples were in accordance to ASTM E8M, and
Fig.1 Welding surface formation:(a)VPTIG welding, (b)DCEN A-TIG welding
2.2 Comparison of welding surface formation Fig.1(a) and Fig.1(b) were respectively illustrated the welding surface formation of 2219 aluminum alloy with VPTIG and DCEN A-TIG welding techniques. Well-formed and fish scales weld can be obtained when using VPTIG in contrast to DCEN A-TIG. VPTIG welding technique showed bright surface while DCEN A-TIG welding surface was gray, similar to helium arc welding surface. In addition, DCEN A-TIG welding technology resulted in seam width of 10 mm, while VPTIG welding resulted in seam width of 15 mm. It was obvious that VPTIG welding technology had a wider welding seam, suggesting that VPTIG welding technology had a greater HAZ and caused certain damage to the base material. 2.3 Comparison of welding internal quality The 2219 aluminum alloy is prone to manifest porosity defects in welding process. The welded joint qualified ac-
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2.4 Comparison of welding microstructure The welding microstructures of base metal, weld zone, fusion zone and heat affected zone of 4mm 2219 aluminum alloy using the VPTIG and the DCEN A-TIG welding technologies, were illustrated in Fig.3 and Fig.4 respectively. Fig.3(a) and Fig.4(a) presented images of base metal of two kinds of welding methods. With similar microstructures, base metals were composed of columnar crystals in the rolling direction and a large amount of granular precipitates dispersed. .
Fig.2 X-ray images: (a) VPTIG welding, (b) DCEN A-TIG welding As shown in Fig.3(b) and Fig.3(c), the top microstructure in weld zone consisted of coarse dendrites, strengthening phases and a small amount of porosity, yet the under microstructure in weld zone was composed of equiaxed grains, strengthening phases and more porosity. As shown in Fig.4(b),The DCEN A-TIG weld zone was composed of equiaxed grains and strengthening phases, while porosity was seldom seen. The grain size in Fig.4 (b) was significantly smaller than that in Fig.3(b) due to the larger welding current in the VPTIG welding technology, leading to coarser microstructure in weld zone, which had affected the performance and quality of the welding joint
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cording to I standard YS062097.Three pairs of 4mm 2219 aluminum alloy plates were welded using VPTIG technology. Fig. 2(a) demonstrated the X-ray image of VPTIG welding showing substantial macro porosity in the entire welding seam. Fig. 2(b) showed the X- ray image of DCEN A-TIG welding. One of the seven DCEN A-TIG welding seams showed excessive porosity. A large gap was observed in the front of welding seam due to scraping. Failure to adjust wire speed may generate excessive porosity in the arc position. No excessive porosity was observed in the other 6 welding seams suggesting that DCEN A-TIG welding technology can prevent porosity, partly due to the removal of oxide film and hydrogen removed.
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BM
HAZ
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Fusion zone
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WZ
The top in WZ
The under in WZ
The top
The interface
HAZ
The under
Fig.3 Microstructures of VPTIG welding: (a)BM , (b) The top in WZ, (c) The under in WZ , (d) Fusion zone (e) The interface , (f) HAZ
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BM
grain size in Fig.4(c) was smaller than that in Fig.3(d) and Fig.3(e). The welding current of DCEN A-TIG was small leading to lower thermal effect on the material, resulting in fine microstructure. Fig.4 (d) and Fig.3(f) had showed the microstructures of heat affected zones using DCEN A-TIG and VPTIG welding. The microstructures were comprised the rolling grains. On account of the larger welding current, the grain sizes and microstructure of the heat affected zone in VPTIG welding were coarser than that in DCEN A-TIG welding. 2.5 Comparison of welding hardness Fig. 5 showed the micro hardness of the joint. The measurement of micro hardness can reflect the fine change of microstructure of the joint, especially the hardness of heat affected zone (HAZ). According to the hardness measurement results, the micro hardness change trend of the DCEN A-TIG welding and the VPTIG welding was the same. The hardness of HAZ of the joint was higher than that of weld zone (WZ). The nearer the fusion line was, the lower hardness was. However, on the contrary, with the distance from the fusion line, the peak temperature of welding thermal cycle decreased gradually, the influence of the welding thermal cycle on the HAZ decreased, the hardness increased. After solution treatment, the base metal (BM) has a lot of fine and dispersion strengthened phase, the hardness of BM was the highest. Therefore the weld zone near the fusion line of the joint was most soft.
WZ Fusion zone HAZ
HAZ
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WZ, (c) Fusion zone ,(d) HAZ
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Fig.4 Microstructures of DCEN A-TIG welding: (a) BM, (b)
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Fig.3(d) illustrated the fusion zone between the weld zone and the heat affected zone in the VPTIG welding, and Fig.3(e) represented the interface between the top and the under in the weld zone, with a large degree of porosity. The mechanism of welding porosity in the aluminum alloy was more complex. The porosity in the VPTIG welding was mostly due to hydrogen, which had a solubility of 0.69 mL/100g in the liquid aluminum alloy. The solubility of hydrogen in the aluminum alloy was only 0.036mL/100g. Altered hydrogen solubility and supersaturated hydrogen can possibly precipitate from the weld pool, resulting in porosity defects [28,29]. The molten pool of the solid-liquid interface was the fusion zone, which required substantially lower bubble nucleation energy compared with the energy in the center of molten pool. The fusion zone near the liquid metal interface was a short time, adversely affecting the porosity overflow in the fusion zone. Compared with the under in the weld zone, the molten pool crystallization in the top in the weld zone was faster than the bubble nucleation, growth, floatation and the residual bubbles in the weld were transformed into porosity as shown in Fig.3(e). As shown in Fig.4(c), the fusion zone in DCEN A-TIG welding was marked by elongated grains and strengthening phases, without porosity. By comparison, the
Fig.5 The joint hardness distribution
2.6Comparison of welding mechanical property Mechanical property test was carried out using VPTIG and DCEN A-TIG welding. This study showed that the average tensile strength of the welded joint by VPTIG was 263.1MPa compared with that of the joint with DCEN A-TIG at 276.4MPa, which was a litter higher than that of the VPTIG welded joint, however, the difference was not significant. The joint average elongation of the DCEN A-TIG welding reached 6.05 %, slightly lower than that of VPTIG welding (6.21%), mainly owing to the VPTIG using the two layers of welding process of backing weld and face welding, the softening zone
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analysis of components semi-quantitatively. In this study, chemical analysis was used to determine the content of the weld zone. Several chemical constituents occur in the 2219 aluminum alloy. Fe as impurity element was not measured and the level of Mg and V were too low to be determined. Therefore, the levels of Si, Cu, Mn, Zn, Ti and Zr were analyzed.
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of the joint extended. The DCEN A-TIG joint fracture performance of the 2219 aluminum alloy was mainly fracture to the weld zone near the fusion line, as shown in Fig.6(a). The hardness of WZ near the fusion line was the minimum and the tensile test was shown that the fracture of the joint was near the fusion line, the tensile property was the worst. The fusion zone between the weld zone and the heat affected zones was inhomogeneous in chemical composition and organization. The fusion zone was also located in the geometrical changes caused by stress. Therefore, the fusion zone was also prone to break. Fig.6(b) presented the tensile experimental results of the VPTIG joint. The results showed that the VPTIG joint mainly fracture in the weld zone was attributed to significant porosity in the weld zone, as shown in Fig.6(c).
Fig.7 Fracture morphologies: (a) and (b)VPTIG welding, (c) and (d)
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DCEN A-TIG welding
Fig.6 Tensile specimens:(a) DCEN A-TIG welding ,(b) VPTIG weld-
No significant differences in chemical composition were observed between VPTIG and DCEN A-TIG welding, as shown in Table 2. The content of Mn, Zn, Cu, Ti, Zr were not altered after welding, while the content of Si decreased slightly, the difference were small, mainly because the chemical components of 2219 aluminum alloy and ER2319 welding wire were unchanged. It can be summarized that the DCEN A-TIG welding technology had no great influence on the chemical composition of the weld zone.
ing, (c) Macroscopic fracture morphology of VPTIG welding
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2.7 Comparison of welding fracture morphology Fig.7(a) and Fig.7(b) illustrated the joint fracture morphologies following VPTIG and DCEN A-TIG welding of 2219 aluminum alloy, respectively. Under similar temperature and material condition, the dimple size, depth and quantity depend on the inclusions and second phase particle size, spacing and number in the tensile fracture process. Excessive levels of inclusions or secondary phase particles lead to poor plasticity. The fracture mode of two welding methods was ductile fracture with large fracture edges and clear contour profile. The welding process had generated enough plastic deformation. The points taken by the energy spectrum analysis were not all Al2Cu, as there were α-Al and Al2Cu, the second phase particles should be Al2Cu, as shown in the Fig.7(b) with Fig.7(d). Comparing Fig.7(a) with Fig.7(c), the DCEN A-TIG welding consisted of smaller equal-axis dimples. 2.8 Comparison of welding chemical composition Conventional analytical methods including electron probe and energy spectrum analysis were indicated for the
Table 2 The main elements in welding joint（wt.%） Si
Cu
Mn
Zn
Ti
Zr
2219
0.2
5.8~6.8
0.2~0.4
0.1
0.02~0.15
0.1~0.25
ER2319
0.2
5.8~6.8
0.2~0.4
0.1
0.1~0.2
0.1~0.25
VPTIG
0.084
6.25
0.27
0.012
0.11
0.12
0.068
6.32
0.27
0.013
0.10
0.12
DCEN A-TIG
2.9 Slag composition analysis The active agent in this study were four fluoride（AlF3, LiF, KF-AlF3, K2SiF6）mixed component, The active agent in the process of DCEN A-TIG welding not only reacted with arc, but also taken place a series of complex chemical metallurgy reaction at high temperature, and further affected the weld metal composition, performance and welding process. Active agent was directly coated on the parent metal, with low melting point and temperature chemical activity, the reactions generally happened at the weld zone and the forefront before the molten pool formation. For the study mechanism of active agent in the welding process, slag formed on weld surface after welding was studied by means of EDS
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B
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D A Fig.8 EDS analysis of slag composition
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Through the analysis of Fig.9, it was due to the effect of active agent on 2219 aluminum alloy oxidation film, causing Al2O3 peeling and existing in slag; Al phase was mainly on account of the scraping; Other compounds, such as Li3AlF6, K2LiAlF6 were the interaction between active agent composition under arc in consequence; The possibile chemical reactions between Al2O3 and active agent need to be further studied.
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ionization when current zero-crossing as well as the electron emission ability of the electrode, so the stability of the AC welding was poor, the prevalence of welding porosity must be greater than the DCEN A-TIG welding. Secondly, as is known, hydrogen is the main reason for porosity of aluminum and aluminum alloy welding. Generally, arc moisture derived from the atmosphere as well as the parent metal, welding material following water adsorption[30].. Moisture adsorption had an important influence on the weld porosity[31-32]. The generated surface oxide film at room temperature consisted of a small amount of crystal morphology of γ-Al2O3 and non-crystalline state of Al2O3, and oxide film on the surface of the aluminum alloy was not dense, there was lots of capillary porosity, under the condition of relative larger humidity environment, Oxide film layer thickening was accelerated and moisture absorption increased, surface oxide can be converted into water oxide or hydrated oxide[33-34]. In the process of welding, aluminum alloy and moisture reacted as follows: (1) Al2O3•H2O=Al2O3+H2O 3H2O+2Al=Al2O3+6[H] (2) Part of the atomic hydrogen in the formula (2) was absorbed by aluminum alloy, part of hydrogen reassembled for molecules into the aluminum alloy to form porosity. The balance equations (1) and (2) were strongly to the right, almost all of the hydrogen in the H2O entered into the aluminum alloy. Therefore, base metal and wire material absorption of trace moisture could make welding seam seriously hydrogen absorption in the welding process.
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analysis to determine the elements. As shown in Fig.8, various elements inside slag ,such as Al, O, F, K, Si ,had came out active agent or parent metal, there may be peeling Al2O3, newly generated and the rest of the fluoride, aluminum and aluminum compounds in slag. In order to determine possible phases, X-ray diffraction (XRD) analysis was carried out, as shown in Fig.8.
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Fig.9 XRD analysis of slag composition
3. Discussion
Generally, AC power was used to weld aluminum alloy because of the limitation of DCEP welding and failure of DCEN welding, which was difficult to remove the oxide film on aluminum alloy surface. In this study, the welding surface was coated with a layer of active agent to remove the oxide film on 2219 aluminum alloy surface, accomplishing for DCEN welding aluminum alloy. Compared with VPTIG welding, DCEN A-TIG can better remove the porosity of 2219 aluminum alloy joint, the reason as the following four aspects: Firstly, VPTIG welding was one of the AC TIG welding methods, due to AC existing the current zero-crossing point, the arc reignited when current zero-crossing. The stability of the welding arc depended mainly on the voltage and the gas
Fig.10 Process of oxide film removal As shown in Fig.8 EDS, the main constituents of active agent included AlF3, LiF, KF-AlF3, K2SiF6, which ionized a large number of F- ions under the action of the electric arc. The oxide film on the surface of aluminum alloy was also heated by arc and separated from aluminum alloy due to different expansion coefficients. These F- ions permeated into oxide film, shook the interface between oxide film and aluminum alloy and caused oxide film to spall. As we can see in Fig.10, oxide film was removed during the DCEN A-TIG welding process. Some studies [35,36] also suggested that these decomposition of F- ions under arc can squeeze into oxygen skeleton gaps of γ-Al2O3 octahedrons or tetrahedrons, resulting in oxide film structural expansion. In addition, the active agent make the oxide film tension broaden resulting in the de-
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connection zone between the covering layer and the base layer. (3) On account of the larger welding current, the grain sizes and organizations of the welding zone in VPTIG welding were coarser than that in DCEN A-TIG welding. The average tensile strength of the joint of DCEN A-TIG welding was 276.4MPa, DCEN A-TIG welding can reach the same mechanical performance index with VPTIG welding.
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crease of oxide film strength and easily being removed. The higher the concentration of F-, the faster the oxide film removed. Thirdly, F- ions can react with hydrogen in arc atmosphere to generate HF, which reduced the hydrogen partial pressure in the electric arc, decreased the hydrogen solubility in the molten pool, therefore, eliminated porosity. [H] in the molten pool can also react with F- ions, HF solubility in molten pool was very small and was easy to overflow from the welding pool[37]. The process of hydrogen removal was shown in Fig.11. In addition, Li+ can react with [H] to form LiH to remove hydrogen and prevent porosity. Due to rarely content of H, its compounds were difficult to detect.
Acknowledgement
This project is supported by a National Natural Science Foundation of China (Grant No. 51671037).
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S0925-8388(17)32886-4
DOI:
10.1016/j.jallcom.2017.08.157
Reference:
JALCOM 42914
To appear in:
Journal of Alloys and Compounds
Received Date: 16 April 2017 Revised Date:
3 August 2017
Accepted Date: 16 August 2017
Please cite this article as: H. Li, J. Zou, J. Yao, H. Peng, The effect of TIG welding techniques on microstructure, properties and porosity of the welded joint of 2219 aluminum alloy, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.08.157. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
1
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The effect of TIG welding techniques on microstructure, properties and porosity of the welded joint of 2219 aluminum alloy Li Hui 1, 2,
Zou Jiasheng 1,
Yao Junshan 3,
Peng Haoping 4
(1.Provincial Key Lab of Advanced Welding Technology, Jiangsu University of Science and Technology, Zhenjiang, 212003; 2. School of Mechanical and Vehicle Engineering, Changzhou Institute of Technology, Changzhou, 213002;
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3.Jiangsu ASWEMET Precision technology Co. Ltd, Pizhou, 221300;
4. Jiangsu Key Laboratory of Oil & Gas Storage and Transportation Technology, Changzhou University, 213164, Jiangsu, P.R. China)
Abstract: 2219 aluminum alloy was mainly used in the propellant tanks of domestic aerospace vehicles. Porosity was often observed during the VPTIG welding joint of the 2219 aluminum alloy. The new welding technology under condition of direct current electrode negative A-TIG (DCEN A-TIG) welding using a special active agent (AlF3, LiF, KF-AlF3, K2SiF6) had eliminated
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the welding porosity of 2219 aluminum alloy. DCEN A-TIG welding and VPTIG welding of 2219 aluminum alloy were performed. Compared with VPTIG welding, the current decrease of DCEN A-TIG welding had greatly reduced thermal effect on base metal and heat-affected zone. The DCEN A-TIG technology had prevented welding porosity, while VPTIG welding had contained numerous macro and micro porosity. The weakest part of the DCEN A-TIG welding joint was the fusion zone, however, the VPTIG joint
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mainly ruptured in the weld zone due to significant porosity defect. The grain sizes of the welding zone in VPTIG welding were coarser than those in DCEN A-TIG welding, which can reach the same mechanical performance index with VPTIG welding. Key words: 2219 aluminum alloy, porosity, DCEN A-TIG welding, VPTIG welding
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2219 high-strength aluminum alloy is an important structural material in the aerospace field[1,2]. It is the promising structural material used in space launch vehicles and other components[3,4]. However, there are several challenges in the welding of 2219 aluminum alloy[5,6]: high welding porosity, low joint strength coefficient, poor back of weld and large welding deformation. Conventional TIG welding of large thickness 2219 aluminum alloy plate requires large welding current as well as the use of multi-pass welding process. The welding joint strength coefficient of the 2219 aluminum alloy is seriously reduced and the welding deformation is increased. Several researchers have developed square wave AC TIG welding, VPTIG welding [7,8] and variable polarity plasma welding technology[9,10] to address these restrictions. However, these technologies failed to eliminate the instability process of the AC welding and poor back of weld. A good weld requires keyhole if variable polarity plasma welding technology is used. It suggests a very high need for welding current [11] ,when welding large thickness aluminum alloy plate. Although the welding current is less than the conventional AC TIG welding current, the current is still relatively large. In recent years, a new TIG welding technique A-TIG (active agent TIG) has attracted great attention worldwide [12-16].
Compared with conventional TIG welding, the area of the material surface is coated with a layer of active flux before using conventional TIG welding. The activated flux has no negative effect on the performance or life of the electrode. Residue is easily removed from the surface after welding. Under similar welding specification, A-TIG welding tremendously improves the weld penetration without increasing the width of weld seam. It also reduces welding distortion and production costs, while greatly improving productivity. A-TIG welding has wide-ranging applications. Currently, A-TIG welding is mainly used in stainless steel, carbon steel, nickel base alloy, and titanium alloys[17-21]. It is mainly focused on AC welding addition of active agent improving the penetration of the aluminum alloy through AC welding [22]. Active agent such as SiO2 exhibits the maximum impact on the depth of welding penetration. However, the oxide is bound to increase welding oxidation and affects the stability of the welding process and weld formation, which limits its application [22-23] . Although DCEP (Direct Current Electrode Positive) addition to weld aluminum alloy removes the oxide film on aluminum alloy surface, the calorific value of tungsten electrode is great and the burning is serious, thus it only uses a small
Received date: ******, 2017 Foundation item: NSFC(51671037) Corresponding author: Zou Jiasheng, Provincial Key Lab of Advanced Welding Technology, Jiangsu University of Science and Technology, Zhenjiang, 212003, Jiangsu, P.R. China, Tel: 0086-13775069549 , E-mail: [email protected]
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2 Results 2.1
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the experiment was conducted according to ASTM B557, five (including) or more samples were averaged as the final results. The fracture features were visualized by JSM-6460 scanning electron microscopy (SEM).The phases in the DCEN A-TIG welding joints were analyzed by the energy spectrum analysis and the X-ray diffraction (XRD, D/Max-2500PC) operated with an incident beam with ranging from 10◦to 90◦with a step size of 5◦and 3s in each step.
Comparison of welding current
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The DCEN A-TIG welding with current of 135A resulted in the implementation of 2219 aluminum alloy welding following single layer welding, reduced the welding layer and cost-effectively improved the production efficiency. The VPTIG welding technology was a two-layer welding system with backing weld current of 170A and cosmetic welding current of 140A. The primary difference between single and two layer welding technology was welding current level. Depending on the characteristics of the welding technology[27], welding current determined heat input, and then decided welding distortion, joint microstructure, directly affected the mechanical properties of the joint. Compared with conventional VPTIG welding, DCEN A-TIG welding technology greatly reduced welding current, which resulted in a decline in the welding heat input, thus reducing the welding distortion and thermal effect on the base material.
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current for welding[24-26]. Therefore, the technology has no practical value. DCEN welding is an ideal method to weld 2219 aluminum alloy. Stable arc with the method of DCEN resolves unstable welding of aluminum alloy. The way of DCEN for aluminum alloy welding induces excessive heating in the base material, increases the depth of penetration and rate of heat utilization, reduces the welding current, and eventually saves energy. However, the key challenge is removal of the oxide film of aluminum alloy. Therefore, DCEN A-TIG facilitates welding of aluminum alloy, removes aluminum alloy oxide film and improves welding productivity using active agent under DCEN conditions. In order to verify the effectiveness of DCEN A-TIG, DCEN A-TIG and VPTIG welding of 2219 aluminum alloy with 4 mm thickness, were carried out. By comparing the welding current, welding formation, internal quality (porosity), microstructure and mechanical properties, the advantages and disadvantages of the DCEN A-TIG and VPTIG welding were summarized.
1 Materials and Methods
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The welding material included 2219-T6 aluminum alloy plate measuring 300 mm × 100 mm × 4 mm. The filler metal was ER2319 measuring 3.2 mm in diameter. Active agent was prepared using the four kinds of fluoride (AlF3, LiF, KF-AlF3, K2SiF6) packed in powdered form. Fronius MAGIC WAVE 2600 was used for welding in this study. Butt weld set-ups with zero gap were prepared for the DCEN A-TIG welding of 4mm 2219 aluminum alloy. First, all specimen surfaces were ground with an abrasive paper to remove impurities, followed by cleaning with acetone. Second, the active agent was dissolved in alcohol and stirred into a paste, and coated on the surface of the welding area. It was also used VPTIG welding technology for 4mm 2219 aluminum alloy. The welding parameters as shown in Table 1 were used for the current study. Table 1 Welding parameters
135A
Backing weld current (VPTIG)
170A
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Welding current(DCEN A-TIG) Face welding current (VPTIG)
140A
Travel speed
120mm/min
Wire feed rate
1300mm/min
Wire diameter
1.6mm
Shielding gas Argon
Argon (99.999% purity)
Gas flow rate
11 L/min
Electrode diameter
3.2 mm
After welding, the weld surface was observed and the internal quality was tested via X-ray detection. The cross sections of joints perpendicular to the welding direction were obtained for microstructural analysis. The polished specimens were etched with Keller reagent (1mlHF+1.5mlHCl+2.5ml HNO3 +95mlH2O). An optical microscope (OM) was used for the observations of the microstructural analysis. The sizes of the tensile samples were in accordance to ASTM E8M, and
Fig.1 Welding surface formation:(a)VPTIG welding, (b)DCEN A-TIG welding
2.2 Comparison of welding surface formation Fig.1(a) and Fig.1(b) were respectively illustrated the welding surface formation of 2219 aluminum alloy with VPTIG and DCEN A-TIG welding techniques. Well-formed and fish scales weld can be obtained when using VPTIG in contrast to DCEN A-TIG. VPTIG welding technique showed bright surface while DCEN A-TIG welding surface was gray, similar to helium arc welding surface. In addition, DCEN A-TIG welding technology resulted in seam width of 10 mm, while VPTIG welding resulted in seam width of 15 mm. It was obvious that VPTIG welding technology had a wider welding seam, suggesting that VPTIG welding technology had a greater HAZ and caused certain damage to the base material. 2.3 Comparison of welding internal quality The 2219 aluminum alloy is prone to manifest porosity defects in welding process. The welded joint qualified ac-
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2.4 Comparison of welding microstructure The welding microstructures of base metal, weld zone, fusion zone and heat affected zone of 4mm 2219 aluminum alloy using the VPTIG and the DCEN A-TIG welding technologies, were illustrated in Fig.3 and Fig.4 respectively. Fig.3(a) and Fig.4(a) presented images of base metal of two kinds of welding methods. With similar microstructures, base metals were composed of columnar crystals in the rolling direction and a large amount of granular precipitates dispersed. .
Fig.2 X-ray images: (a) VPTIG welding, (b) DCEN A-TIG welding As shown in Fig.3(b) and Fig.3(c), the top microstructure in weld zone consisted of coarse dendrites, strengthening phases and a small amount of porosity, yet the under microstructure in weld zone was composed of equiaxed grains, strengthening phases and more porosity. As shown in Fig.4(b),The DCEN A-TIG weld zone was composed of equiaxed grains and strengthening phases, while porosity was seldom seen. The grain size in Fig.4 (b) was significantly smaller than that in Fig.3(b) due to the larger welding current in the VPTIG welding technology, leading to coarser microstructure in weld zone, which had affected the performance and quality of the welding joint
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cording to I standard YS062097.Three pairs of 4mm 2219 aluminum alloy plates were welded using VPTIG technology. Fig. 2(a) demonstrated the X-ray image of VPTIG welding showing substantial macro porosity in the entire welding seam. Fig. 2(b) showed the X- ray image of DCEN A-TIG welding. One of the seven DCEN A-TIG welding seams showed excessive porosity. A large gap was observed in the front of welding seam due to scraping. Failure to adjust wire speed may generate excessive porosity in the arc position. No excessive porosity was observed in the other 6 welding seams suggesting that DCEN A-TIG welding technology can prevent porosity, partly due to the removal of oxide film and hydrogen removed.
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3
BM
HAZ
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Fusion zone
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WZ
The top in WZ
The under in WZ
The top
The interface
HAZ
The under
Fig.3 Microstructures of VPTIG welding: (a)BM , (b) The top in WZ, (c) The under in WZ , (d) Fusion zone (e) The interface , (f) HAZ
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BM
grain size in Fig.4(c) was smaller than that in Fig.3(d) and Fig.3(e). The welding current of DCEN A-TIG was small leading to lower thermal effect on the material, resulting in fine microstructure. Fig.4 (d) and Fig.3(f) had showed the microstructures of heat affected zones using DCEN A-TIG and VPTIG welding. The microstructures were comprised the rolling grains. On account of the larger welding current, the grain sizes and microstructure of the heat affected zone in VPTIG welding were coarser than that in DCEN A-TIG welding. 2.5 Comparison of welding hardness Fig. 5 showed the micro hardness of the joint. The measurement of micro hardness can reflect the fine change of microstructure of the joint, especially the hardness of heat affected zone (HAZ). According to the hardness measurement results, the micro hardness change trend of the DCEN A-TIG welding and the VPTIG welding was the same. The hardness of HAZ of the joint was higher than that of weld zone (WZ). The nearer the fusion line was, the lower hardness was. However, on the contrary, with the distance from the fusion line, the peak temperature of welding thermal cycle decreased gradually, the influence of the welding thermal cycle on the HAZ decreased, the hardness increased. After solution treatment, the base metal (BM) has a lot of fine and dispersion strengthened phase, the hardness of BM was the highest. Therefore the weld zone near the fusion line of the joint was most soft.
WZ Fusion zone HAZ
HAZ
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WZ, (c) Fusion zone ,(d) HAZ
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Fig.4 Microstructures of DCEN A-TIG welding: (a) BM, (b)
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Fig.3(d) illustrated the fusion zone between the weld zone and the heat affected zone in the VPTIG welding, and Fig.3(e) represented the interface between the top and the under in the weld zone, with a large degree of porosity. The mechanism of welding porosity in the aluminum alloy was more complex. The porosity in the VPTIG welding was mostly due to hydrogen, which had a solubility of 0.69 mL/100g in the liquid aluminum alloy. The solubility of hydrogen in the aluminum alloy was only 0.036mL/100g. Altered hydrogen solubility and supersaturated hydrogen can possibly precipitate from the weld pool, resulting in porosity defects [28,29]. The molten pool of the solid-liquid interface was the fusion zone, which required substantially lower bubble nucleation energy compared with the energy in the center of molten pool. The fusion zone near the liquid metal interface was a short time, adversely affecting the porosity overflow in the fusion zone. Compared with the under in the weld zone, the molten pool crystallization in the top in the weld zone was faster than the bubble nucleation, growth, floatation and the residual bubbles in the weld were transformed into porosity as shown in Fig.3(e). As shown in Fig.4(c), the fusion zone in DCEN A-TIG welding was marked by elongated grains and strengthening phases, without porosity. By comparison, the
Fig.5 The joint hardness distribution
2.6Comparison of welding mechanical property Mechanical property test was carried out using VPTIG and DCEN A-TIG welding. This study showed that the average tensile strength of the welded joint by VPTIG was 263.1MPa compared with that of the joint with DCEN A-TIG at 276.4MPa, which was a litter higher than that of the VPTIG welded joint, however, the difference was not significant. The joint average elongation of the DCEN A-TIG welding reached 6.05 %, slightly lower than that of VPTIG welding (6.21%), mainly owing to the VPTIG using the two layers of welding process of backing weld and face welding, the softening zone
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analysis of components semi-quantitatively. In this study, chemical analysis was used to determine the content of the weld zone. Several chemical constituents occur in the 2219 aluminum alloy. Fe as impurity element was not measured and the level of Mg and V were too low to be determined. Therefore, the levels of Si, Cu, Mn, Zn, Ti and Zr were analyzed.
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of the joint extended. The DCEN A-TIG joint fracture performance of the 2219 aluminum alloy was mainly fracture to the weld zone near the fusion line, as shown in Fig.6(a). The hardness of WZ near the fusion line was the minimum and the tensile test was shown that the fracture of the joint was near the fusion line, the tensile property was the worst. The fusion zone between the weld zone and the heat affected zones was inhomogeneous in chemical composition and organization. The fusion zone was also located in the geometrical changes caused by stress. Therefore, the fusion zone was also prone to break. Fig.6(b) presented the tensile experimental results of the VPTIG joint. The results showed that the VPTIG joint mainly fracture in the weld zone was attributed to significant porosity in the weld zone, as shown in Fig.6(c).
Fig.7 Fracture morphologies: (a) and (b)VPTIG welding, (c) and (d)
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DCEN A-TIG welding
Fig.6 Tensile specimens:(a) DCEN A-TIG welding ,(b) VPTIG weld-
No significant differences in chemical composition were observed between VPTIG and DCEN A-TIG welding, as shown in Table 2. The content of Mn, Zn, Cu, Ti, Zr were not altered after welding, while the content of Si decreased slightly, the difference were small, mainly because the chemical components of 2219 aluminum alloy and ER2319 welding wire were unchanged. It can be summarized that the DCEN A-TIG welding technology had no great influence on the chemical composition of the weld zone.
ing, (c) Macroscopic fracture morphology of VPTIG welding
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2.7 Comparison of welding fracture morphology Fig.7(a) and Fig.7(b) illustrated the joint fracture morphologies following VPTIG and DCEN A-TIG welding of 2219 aluminum alloy, respectively. Under similar temperature and material condition, the dimple size, depth and quantity depend on the inclusions and second phase particle size, spacing and number in the tensile fracture process. Excessive levels of inclusions or secondary phase particles lead to poor plasticity. The fracture mode of two welding methods was ductile fracture with large fracture edges and clear contour profile. The welding process had generated enough plastic deformation. The points taken by the energy spectrum analysis were not all Al2Cu, as there were α-Al and Al2Cu, the second phase particles should be Al2Cu, as shown in the Fig.7(b) with Fig.7(d). Comparing Fig.7(a) with Fig.7(c), the DCEN A-TIG welding consisted of smaller equal-axis dimples. 2.8 Comparison of welding chemical composition Conventional analytical methods including electron probe and energy spectrum analysis were indicated for the
Table 2 The main elements in welding joint（wt.%） Si
Cu
Mn
Zn
Ti
Zr
2219
0.2
5.8~6.8
0.2~0.4
0.1
0.02~0.15
0.1~0.25
ER2319
0.2
5.8~6.8
0.2~0.4
0.1
0.1~0.2
0.1~0.25
VPTIG
0.084
6.25
0.27
0.012
0.11
0.12
0.068
6.32
0.27
0.013
0.10
0.12
DCEN A-TIG
2.9 Slag composition analysis The active agent in this study were four fluoride（AlF3, LiF, KF-AlF3, K2SiF6）mixed component, The active agent in the process of DCEN A-TIG welding not only reacted with arc, but also taken place a series of complex chemical metallurgy reaction at high temperature, and further affected the weld metal composition, performance and welding process. Active agent was directly coated on the parent metal, with low melting point and temperature chemical activity, the reactions generally happened at the weld zone and the forefront before the molten pool formation. For the study mechanism of active agent in the welding process, slag formed on weld surface after welding was studied by means of EDS
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Through the analysis of Fig.9, it was due to the effect of active agent on 2219 aluminum alloy oxidation film, causing Al2O3 peeling and existing in slag; Al phase was mainly on account of the scraping; Other compounds, such as Li3AlF6, K2LiAlF6 were the interaction between active agent composition under arc in consequence; The possibile chemical reactions between Al2O3 and active agent need to be further studied.
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ionization when current zero-crossing as well as the electron emission ability of the electrode, so the stability of the AC welding was poor, the prevalence of welding porosity must be greater than the DCEN A-TIG welding. Secondly, as is known, hydrogen is the main reason for porosity of aluminum and aluminum alloy welding. Generally, arc moisture derived from the atmosphere as well as the parent metal, welding material following water adsorption[30].. Moisture adsorption had an important influence on the weld porosity[31-32]. The generated surface oxide film at room temperature consisted of a small amount of crystal morphology of γ-Al2O3 and non-crystalline state of Al2O3, and oxide film on the surface of the aluminum alloy was not dense, there was lots of capillary porosity, under the condition of relative larger humidity environment, Oxide film layer thickening was accelerated and moisture absorption increased, surface oxide can be converted into water oxide or hydrated oxide[33-34]. In the process of welding, aluminum alloy and moisture reacted as follows: (1) Al2O3•H2O=Al2O3+H2O 3H2O+2Al=Al2O3+6[H] (2) Part of the atomic hydrogen in the formula (2) was absorbed by aluminum alloy, part of hydrogen reassembled for molecules into the aluminum alloy to form porosity. The balance equations (1) and (2) were strongly to the right, almost all of the hydrogen in the H2O entered into the aluminum alloy. Therefore, base metal and wire material absorption of trace moisture could make welding seam seriously hydrogen absorption in the welding process.
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analysis to determine the elements. As shown in Fig.8, various elements inside slag ,such as Al, O, F, K, Si ,had came out active agent or parent metal, there may be peeling Al2O3, newly generated and the rest of the fluoride, aluminum and aluminum compounds in slag. In order to determine possible phases, X-ray diffraction (XRD) analysis was carried out, as shown in Fig.8.
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Fig.9 XRD analysis of slag composition
3. Discussion
Generally, AC power was used to weld aluminum alloy because of the limitation of DCEP welding and failure of DCEN welding, which was difficult to remove the oxide film on aluminum alloy surface. In this study, the welding surface was coated with a layer of active agent to remove the oxide film on 2219 aluminum alloy surface, accomplishing for DCEN welding aluminum alloy. Compared with VPTIG welding, DCEN A-TIG can better remove the porosity of 2219 aluminum alloy joint, the reason as the following four aspects: Firstly, VPTIG welding was one of the AC TIG welding methods, due to AC existing the current zero-crossing point, the arc reignited when current zero-crossing. The stability of the welding arc depended mainly on the voltage and the gas
Fig.10 Process of oxide film removal As shown in Fig.8 EDS, the main constituents of active agent included AlF3, LiF, KF-AlF3, K2SiF6, which ionized a large number of F- ions under the action of the electric arc. The oxide film on the surface of aluminum alloy was also heated by arc and separated from aluminum alloy due to different expansion coefficients. These F- ions permeated into oxide film, shook the interface between oxide film and aluminum alloy and caused oxide film to spall. As we can see in Fig.10, oxide film was removed during the DCEN A-TIG welding process. Some studies [35,36] also suggested that these decomposition of F- ions under arc can squeeze into oxygen skeleton gaps of γ-Al2O3 octahedrons or tetrahedrons, resulting in oxide film structural expansion. In addition, the active agent make the oxide film tension broaden resulting in the de-
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connection zone between the covering layer and the base layer. (3) On account of the larger welding current, the grain sizes and organizations of the welding zone in VPTIG welding were coarser than that in DCEN A-TIG welding. The average tensile strength of the joint of DCEN A-TIG welding was 276.4MPa, DCEN A-TIG welding can reach the same mechanical performance index with VPTIG welding.
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crease of oxide film strength and easily being removed. The higher the concentration of F-, the faster the oxide film removed. Thirdly, F- ions can react with hydrogen in arc atmosphere to generate HF, which reduced the hydrogen partial pressure in the electric arc, decreased the hydrogen solubility in the molten pool, therefore, eliminated porosity. [H] in the molten pool can also react with F- ions, HF solubility in molten pool was very small and was easy to overflow from the welding pool[37]. The process of hydrogen removal was shown in Fig.11. In addition, Li+ can react with [H] to form LiH to remove hydrogen and prevent porosity. Due to rarely content of H, its compounds were difficult to detect.
Acknowledgement
This project is supported by a National Natural Science Foundation of China (Grant No. 51671037).
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Fig.11Process of hydrogen removal Lastly, the active agent covered the surface of weld segregated water to enter into the pool, as a result, DCEN A-TIG welding was not easy to produce porosity.
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This paper proposed a creative welding way of 2219 aluminum alloy, the welding area was coated on active agent in advance to remove the oxide film and porosity, direct current electrode negative (DCEN) TIG welding method was realized. By comparing welding currents, surface formation, internal quality (porosity), microstructures and mechanical properties of DCEN A-TIG and VPTIG welding, It was found that: (1) In case of 2219 aluminum alloy, the DCEN A-TIG welding current was reduced. The decrease of welding current greatly had reduced the thermal effect on weld zone and heat effect zone. The welding seam width of DCEN A-TIG welding was significantly less than that of VPTIG welding . (2) The 2219 aluminum alloy welding joint using the VPTIG welding technology contained numerous macro and micro porosity. The DCEN A-TIG technology had prevented welding porosity. The weakest part of the DCEN A-TIG weld was the fusion zone, however, the VPTIG joint mainly fracture in the weld zone was attributed to significant porosity in the
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