A Primary Study on the Applicability of Gel-Type Filler Materials to Root Pass Welds of the Thick Plate
Article information
Abstract
Abstract
Welding generally uses filler material to create deposited weld metal. Filler materials play a significant role in welding by providing the material needed to fill and form the joint. Existing filler materials are limited in the workspace and welding position and need help responding flexibly to welds of complex shapes. This study conducted primary research to develop a gel-type filler material with adhesion and flexibility that can cope with limited workspace and location. We attempted to provide a new approach to welding by mixing metal powder and binder to create a gel-type filler material. Root pass welding of thick plates was performed using a laser heat source. The bead appearance and cross-section of the weld zone were observed according to the type and composition of the metal powder, the binder content, and the method of synthesizing the metal powder. The physical characteristics of the weld were analyzed through observation of the hardness and microstructure of the weld.
1. Introduction
In the modern manufacturing industry, welding is one of the essential processes and plays an essential role in rapidly and robustly combining various materials and shapes. Welding materials are used in various industries, such as building, construction, and offshore plants in addition to the shipbuilding and automobile sectors where welding is applied, and new welding materials are also under development due to the development of welding technology1-6). Bae1) developed a new welding material with high strength and high heat resistance for Cr-Mo-X steel through thermodynamic simulation. Park3) developed stainless steel-based flux- cored wire instead of conventional nickel-based welding materials to weld 9% nickel steel, and reported its physical properties and fracture mechanics performance. Kim4) and Ramachandran5) reported on the weldability of a new aluminum alloy wire (magnesium content: 7 wt.% or higher), which had higher mechanical properties than the commercial aluminum 5183 alloy wire due to the high magnesium content, for GMA welding of the aluminum 5083 alloy.
Conventional welding methods, however, are often limited by the workspace and welding position, making it difficult to flexibly respond to the weld zones of complex geometry. These problems are further noticeable during work in a limited space or when welding is required at various angles. Gel-type welding materials that use metal powder and binders have the potential to overcome such constraints. Gel-type materials are easily applicable even in narrow spaces due to high flexibility and variability, and can precisely respond to the weld zones of complex geometry. They can also minimize thermal deformation and residual stress problems that may occur in traditional welding methods. Gel- type welding materials made by mixing metal powder and binders enable welding at low temperature, which helps reduce thermal deformation and improve the quality of the weld. Therefore, development of new welding methods using such materials can significantly improve the efficiency and quality of the manufacturing process.
The gel-type welding material to be developed in this study has not been commercially developed. Research on paste or viscous materials that combine metal powder and binders has been mainly conducted in areas, such as brazing filler materials, metal injection molding (MIM) materials, and 3D printing materials. The most similar research to this study can be said to be research on making tape-type flexible filler materials related to brazing filler materials. Prado7) conducted research on the brazing process of nuclear reactor component joints made of tungsten. In this study, weldability was evaluated according to the binder content (5 to 30 wt.%) to secure the flexibility of the brazing filler tape used at joints. Izaguirre8) developed a flexible filler ribbon using the melt spinning technique for the brazing of W and CuCrZr materials applied to the heat sink. Sharma9) fabricated flexible Ni brazing foil by mixing an organic binder and Ni-alloy powder for the brazing of AISI304 steel, and reported that the maximum wettability of the brazing foil and higher bonding strength could be obtained when the ratio between the polymer and dispersant was 8:2 in the brazing paste. There are also studies on the effects of binders in brazing filler materials on the properties of joints. Sokolov10) conducted research on the effect of Sn on the metal binder used during the manufacture of diamond tools, and investigated the impacts of the content of Sn on the microstructure and hardness of the binder. It was reported that the Sn-containing binder exhibited higher mechanical properties and durability than the conventional binder because Sn improved wettability and bonding between diamond particles and the metal matrix. Elsener11) conducted research on the effects of the binder content in a Cu-based brazing filler material on the microstructures and properties of diamond and cubic boron nitride (cBN). Through an experiment that used Cu-based brazing filler materials with various binder contents, it was reported that the wettability and bonding strength of the filler material improved as the binder content increased.
In this study, the contents of the basic experimental research conducted on the applicability of a gel-type filler material (weld gel) to root pass welds of the thick plate are introduced. After performing laser welding using weld gel with different metal powder types and compositions, binder contents, and metal powder synthesis methods, the bead appearance and the cross-section of the weld zone were observed and analyzed. In addition, the physical characteristics of the weld were analyzed by observing the hardness and microstructure of the weld joint generated by weld gel.
2. Experimental Method
The base material used in the experiment of this study is SM490 with a thickness of 15 mm, and its microstructure is composed of ferrite and pearlite. Its detailed chemical composition and microstructure are shown in Table 1 and Fig. 1. The base material is carbon steel containing elements, such as Fe, Mn, Si, and C. The composition of the weld gel for it was also designed to have Fe as a major component.
Chemical compositions (wt.%) of base material used in this study
Microstructure of base material used in this study. The white/gray colored areas represent the pearlite and the dark gray colored areas represent the ferrite matrix
The weld gel was fabricated by mixing the powder, binder, and distilled water of the design composition at 50 to 90°C. The fabricated weld gel with viscosity and adhesion was applied to the welding position using the doctor blade method. Fig. 2 shows the fabricating process, deposition, and laser welding step of the weld gel. As shown in Fig. 2(a), the weld gel was fabricated by mixing metal powder and a binding solution containing the binder for the manufacture of the weld gel with flexibility and viscosity. In this instance, the powder used for the weld gel had a size of 35 to 100 μm. The binder was added and stirred in the distilled water heated to 50 to 90°C, and additives (e.g., thickeners, deformer agents, freezing stabilizers, and preservatives) were selectively mixed and stirred to fabricate the primary composition. It was then added into the metal powder and stirred to fabricate the weld gel. After that, the weld gel was applied to the target position, and the doctor blade method was performed to meet the constant height (Fig. 2(b)). Welding was then performed by irradiating the laser heat source to the weld gel (Fig. 2(c)).
Welding process using gel-type filler material, (a) manufacturing, (b) deposition and (c) welding step
Fig. 3 shows the schematic diagram of the welding specimen to perform welding using the weld gel. V-groove with a groove angle of 60˚ was processed in the base material for the manufacture of the butt joint-shaped weld, and the weld gel was deposited in the groove by 6 mm. It will be mentioned later, but for the evaluation of the weld zone according to the weld gel deposition amount
Schematic diagram of weld joint and groove
In this study, a disk laser (Trudisk3002, Trumpf) was used as the welding heat source. The geometry of the beam was circular, and defocusing was applied so that the size of the beam could match the width of the applied weld gel. The welding speed was fixed at 1.5 mm/s, the laser power at 3 kW, and the focusing distance at 510 mm (Table 2). A welding experiment was performed according to the types and components of the metal powder and binder of the weld gel, powder mixing method, and weld gel deposition amount to examine the applicability of the weld gel as a welding material. Table 3 shows various experimental conditions to evaluate the weldability of the weld gel. The weld bead appearance from the top and bottom and the cross-section of the weld zone were observed to examine the characteristics of the weld according to the weld gel design conditions. In addition, the hardness of the weld was measured and its microstructure was analyzed using an optical microscope.
Process parameter values for weld gel test
Experimental conditions for weldability of weld gel
3. Experiment Results
3.1 Weldability evaluation according to the weld gel metal powder type
To manufacture weld gel, the type of the alloy powder, which is the main material that constitutes the weld gel, must be determined first. The purpose of this study is to develop the weld gel to be used in the root pass welds of carbon steel. Thus, three types of Fe-based metal powder (pure Fe, carbon steel, and stainless steel (STS316)), were selected and evaluated as candidates for weld gel metal powder. Fig. 4 shows the weldability evaluation results according to the type of weld gel metal powder. The appearance of the weld bead from the top and bottom and the cross-section of the weld zone can be seen. As can be seen from the figure, cracks occurred in the weld zone that applied the weld gel containing carbon steel powder in the results of observing the cross-section of the weld zone while no defect occurred when pure Fe powder and stainless steel (STS316) powder were used. It appears that cracks occurred when carbon steel powder was used because the high carbon equivalent developed the microstructure of martensite. The microstructure analysis results for this will be mentioned in detail in section 3.6. Based on these results, it was judged that carbon steel powder and stainless steel (STS316) powder were potential candidates for the alloy powder of weld gel. Considering the mechanical properties of the weld zone, stainless steel (STS316) powder was selected as the weld gel alloy powder candidate to be used in the root pass welds of carbon steel.
Weldability evaluation results according to type of metal powder for weld gel
3.2 Weldability evaluation according to the weld gel binder content
As mentioned above, stainless steel alloy powder was selected as the weld gel candidate for carbon steel. To secure the flexibility and adhesion of such weld gel, a binder must be applied to the alloy powder. Poly vinyl alcohol (PVA) was selected as the binder to be used in the weld gel. Water was used as a solvent because PVA is a water-soluble polymer. Since the binder content affects the applicability and adhesion of the weld gel and the fluidity of the melt pool, the weldability of the weld gel was evaluated according to the binder content. Fig. 5 shows the weldability evaluation result of the weld gel according to the PVA content. As the PVA content increases, the viscosity of the weld gel tends to increase and the content of carbon, which is the main component of PVA, increases in the weld. As can be seen from Fig. 5, more slag occurred on the top bead and the weld with insufficient fluidity was formed as the PVA content increased. These results indicate that it is desirable to control the PVA content at 10 wt.% or less compared to the distilled water weight during the manufacture of weld gel.
Weldability evaluation results according to PVA(binder) content
3.3 Weldability evaluation according to the stainless steel alloy composition ratio
As mentioned in section 3.1, stainless steel alloy powder was used as the alloy powder of weld gel in this study. In relation to this, weldability was evaluated for three types of stainless steel powder with different compositions and phases to examine the effect of the powder composition of stainless steel on weldability. Fig. 6 shows the weldability evaluation results for the weld gel fabricated with ferritic STS430, martensitic STS420, and austenitic STS316. As can be seen from the figure, the weld zone with no crack defect was formed only when the weld gel containing austenitic STS316 alloy powder was used. This appears to be due to the presence of Ni that has a significant impact on the toughness of the weld. The ferritic and martensitic stainless steel alloy powders tested in this study contain only Cr, and they do not contain Ni, which has a significant effect on the toughness of the weld. Therefore, when the general carbon steel-based thick plate base material is used as in this study, it is deemed desirable to use welding materials that contain a certain amount of Ni to secure the toughness of root pass welds and control cracks.
Weldability evaluation results according to stainless steel alloy composition ratio
3.4 Weldability evaluation according to the weld gel metal powder synthesis method
To examine the effect of the metal powder synthesis method on weldability, weldability was evaluated after manufacturing weld gel by preparing austenitic STS316 powder of the same composition using two methods (mechanical mixing and alloying). As can be seen from Fig. 7, humping occurred on the upper side of the bead and the bottom-side bead was not formed in the case of the mixed powder because the fluidity of the melt pool was not secured due to the non-uniform mixing of alloy components. On the other hand, when the alloying powder was used, good bead appearance was observed with no defect in the cross section.
Weldability evaluation results according to alloying method
3.5 Weldability evaluation according to the weld gel deposition amount
The amount of weld gel applied to the root pass is also very important for obtaining a sound root pass weld. To this end, when the weld gel was applied to the carbon steel base material, weldability was evaluated by varying the amount of the weld gel per unit length. The results are shown in Fig. 8. It can be seen that the bottom-side bead occurred and no crack was observed from the cross-section of the weld zone when the deposition amounts were 0.75 and 1.13 g/cm. When the deposition amount was 1.5 g/cm or higher, however, the bottom-side bead did not occur and pore defects were developed inside. Under the three deposition amount conditions, the welding speed and spot size were the same. It is judged that defects occurred only when the deposition amount was 1.5 g/cm or higher because the heat input was not sufficient due to the small spot size and thus all of the applied weld gel could not be melted. When the deposition amount was 1.5 g/cm or less, the effective thickness of the root pass increased as the deposition amount increased. When 1.13 g/cm was applied, a maximum effective thickness of 5.3 mm was obtained.
Weldability evaluation results according to alloying method
3.6 Weld hardness and microstructure analysis
Fig. 9 shows the weld hardness analysis results according to the metal powder type of weld gel. As seen be seen from the figure, the hardness of SM490, which is carbon steel, was 200 HV or less, and the weld zone of the weld gel that used pure Fe powder was at a level similar to that of the base material. The weld zone of the weld gel that utilized carbon steel powder showed a high hardness value of 600 HV. It appears that this high hardness caused cracks in the weld zone as mentioned in section 3.1. The weld zone of the weld gel that used stainless steel powder showed a hardness of 450 HV, which is higher than the hardness of conventional stainless steel itself. This appears to be due to the formation of hard phases caused by dilution with the base material. To support these hardness analysis results, the microstructure of the weld was observed.
Measured hardness profiles of the weld metal according to type of metal powder for weld gel, (a) base metal (BM) (b) weld metal (elemental Fe) (c) weld metal (stainless steel) (d) weld metal (carbon steel)
Fig. 10 shows the microstructure observation results for the base material, heat affected zone (HAZ), and weld zone. Fig. 10(a) is the microstructure of the base metal (BM) corresponding to a hardness of 200 HV. Ferrite is the main microstructure, and the intermediate pearlite microstructure is observed. Pearlite is mainly located at the end of the ferrite crystal or at the ferrite grain boundary. Fig. 10(b) is the microstructure of HAZ. It is judged that grain refinement occurred due to recrystallization during welding. Fig. 10(c) is the microstructure of the weld metal (WM) by the weld gel that used pure Fe powder. It can be seen that the ferrite phase is dominant in the microstructure. Fig. 10(d) is the microstructure of the WM generated by the weld gel that used carbon steel powder. The martensite phase is dominant. Fig. 10(e) is the microstructure of the WM by the weld gel that used stainless steel powder. It is judged that a mixture of the martensite and austenite phases exists.
Comparison of microstructure (a) base metal (BM) (b) HAZ (c) weld metal (pure Fe) (d) weld metal (carbon steel) (e) weld metal (stainless steel)
4. Conclusion
This study aimed to conduct research on the method of making a gel-type welding material using metal powder and binders and to evaluate its applicability to the actual welding process. Basic research was conducted to develop a welding material that is not limited by the workspace and welding position and to present a new welding method that can flexibly respond to various weld shapes. The results are as follows.
1) Stainless steel powder was selected as the weld gel alloy powder candidate to be used in the root pass welds of carbon steel.
2) Since defects occur in the weld joint presented in this study when the poly vinyl alcohol (PVA) content is high, it is desirable to adjust the PVA content below 10 wt.% during the manufacture of weld gel.
3) When the general carbon steel-based thick plate base material is used, it is deemed desirable to use weld gel that contains a certain amount of Ni to secure the toughness of root pass welds and control crack.
4) It is desirable to use alloying powder rather than the mixed powder of metal elements.
5) If the weld gel deposition amount exceeds 1.5g/cm, the bottom-side bead does not occur and pore defects occur inside the weld zone.
It is expected that weld gel that is likely to flexibly respond to various weld shapes will be applied to shipbuilding and plant industries with many weld joints for large component and thick plates through further research and development.
Acknowledgement
This work was supported by the Technology Innovation Program(RS-2024-00442314, Field test demonstration of developed high-speed SPOT welding system for Aluminum BIW) funded By the Ministry of Trade Industry & Energy(MOTIE, Korea)