Analysis and control research on micro porosity defects in butt welds of small diameter thick walled titanium alloy pipes
TA16 and TA17 group welding of small-diameter thick-walled titanium alloy tube butt joints for the causes and control of small porosity defects, respectively, from the “shielding gas,” “welding material hydrogen removal treatment,” “arc voltage, “butt joint bevel type” and “weld surface roughness” five aspects, through the use of automatic TIG welding process method to carry out comparative welding test research. Through in-depth analysis of the test results, that “improve the accuracy of the weld end roughness” is an effective measure to control the emergence of small porosity of such welded joints. The test results of this type of titanium alloy small diameter thick-walled pipe butt welding engineering applications have important reference significance and practical value.
Titanium alloy pipe is a new material; its low density, high strength, good room temperature, high temperature, and low-temperature mechanical properties, and excellent corrosion resistance in a variety of media, in addition to being widely used in aerospace, petrochemical and machinery manufacturing, etc., is now also used in the construction of the ship’s pipe system. TA16, TA17 titanium alloy is a chemically active TA16, TA17 titanium alloy is a chemically active metal; in the welding process is very easy to produce porosity defects, if not controlled, porosity defects will lead to product scrap. Sometimes in the use of TA16 and TA17, titanium alloy small diameter thick-walled pipe pair welding, for some special reasons, does not allow the weld within any size of porosity defects. But found, in the welding process, the weld seam of this welded joint very easily produces a diameter of less than 0.08mm micro porosity defects. Such micro porosity defects cannot be found in the 100% X-ray inspection, only 200 times the microscopic observation. There needs to be more research on this kind of microporosity in China, and there is no supporting literature and data for the mechanism and control method of microporosity. We have carried out experimental research on this topic to understand the root cause of microporosity in such joints and to find out the method of controlling microporosity. This test is mainly from the “shielding gas,” “welding material hydrogen treatment,” “arc voltage,” “butt joint bevel type,” and “welding end roughness,” five aspects of comparative welding test research. The study results will have important reference significance and practical value for the engineering application of small-diameter, thick-walled titanium alloy pipe butt welding.
1. Test materials and methods
Table of Contents
The subject of this study is the specifications of the TA16 and TA17 tube pairs for butt welding quasi-8mm × 1.5mm, welding filler material for titanium alloy wire.
Butt joint pair type set a total of three: I butt joint, see Figure 1; bevel open 45 ° (no straight section) butt joint, see Figure 2; bevel open 45 ° (with a straight section) butt joint, see Figure 3. Unless otherwise specified, the test is used in the I-butt joint (Figure 1).
Considering the titanium alloy welding requirements for cleanliness are very high, and to avoid the impact of cleaning before welding on each group of test porosity defects, the subject of all the tests carried out to take the same and strict cleaning process specifications for the cleaning of the welded parts to oil.
Figure.1 I type bevel pipe – pipe pair type schematic diagram
Figure.2 45 ° bevel (no straight section) pipe-pipe pair type schematic diagram
Figure.3 45° bevel (with straight section) pipe-pipe pair type schematic diagram
Each group of comparison tests are automatic TIG pulse welding process for self-melting priming and filler wire forming welding, welding equipment for the same I-arc400 automatic argon arc welding machine; welding parameters are the same. In the welding process, the argon gas protects and ensures pre-welding gas and post-welding hysteresis ventilation.
After welding, each group of test welded joints needs to be 100% X-ray inspection (sensitivity 0.08mm) and microscopic metallurgical testing (magnification 200 times).
2. Test results
2.1 The effect of shielding gas on microporosity
The specific comparison of this group of tests and test results is shown in Table 1. Test results show that: “shielding gas” on the “microporosity” generated no effective inhibition.
2.2 Welding material to remove the influence of hydrogen treatment on microporosity
It is generally believed that hydrogen is the main cause of porosity. To avoid the high hydrogen content of the welding material caused by porosity defects, the group of comparative welding tests. The group test comparison and test results are shown in Table 2. Test results show that: “weld material in addition to hydrogen treatment” on the “microporosity” generated no effective inhibition.
2.3 Arc voltage on the impact of microporosity
Using automatic TIG welding and arc welding before the distance between the tungsten electrode and the workpiece is pre-set through the welding process will not change. For the welding characteristics of the welding machine and the welding needs of this type of butt joint, the tungsten electrode from the workpiece distance adjustment distance (i.e., the arc length during welding) can only be between 1.0 and 2.0 mm, less than 1.0 mm easy to stick wire, stick tungsten, higher than 2.0 mm is not easy to start the arc, and the arc is not stable. So, this type of test was carried out in the preset arc length of 1.0, 1.5, and 2.0mm, three groups of comparative tests to compare different arc voltages on the impact of porosity defects in this type of welded joints. Specific tests and results are shown in Table 3. Test results show that: in the adjustable “arc voltage” range, “arc voltage” on the “microporosity” generated no effective inhibition.
Table.1 “Protective gas” group test situation and results
| Serial number | Test type: three-way protective gas (gun gas, hood gas, and back protection gas) | Test piece number | X-ray results | Metallographic examination results |
| 1 | Test type: three-way protective gas (gun gas, hood gas, and back protection gas) | GCY-1 | No pores | 8 φ 0.010mm porosity |
| GCY-2 | No pores | No pores | ||
| GCY-3 | No pores | Weld angle, 0.006mm porosity | ||
| GCY-4 | No pores | φ 0.006mm porosity | ||
| GCY-5 | No pores | No pores | ||
| 2 | Connect two protective gases (close the back to maintain gas) | B1 | No pores | No pores |
| B2 | No pores | No pores | ||
| B3 | No pores | Angle on the inner side of the deviation tube, 1 small pore | ||
| B4 | No pores | No pores | ||
| B5 | No pores | Weld protrusion, 1 small pore | ||
| 3 | Connect two protective gases (close the drag cover gas) | TZ-1 | No pores | No pores |
| TZ-2 | No pores | Weld angle, 0.005mm porosity | ||
| TZ-3 | No pores | No pores | ||
| TZ-4 | No pores | Weld protrusion, 0.005mm porosity | ||
| TZ-5 | No pores | The included angle is inward convex, and the air hole is 0.005mm | ||
| 4 | Delayed pre ventilation with three protective gases extended to 1 minute | YTQ-1 | No pores | Internally convex, 3 pieces φ 0.005mm porosity |
| YTQ-2 | No pores | Internally convex, 2 pieces φ 0.005mm porosity | ||
| YTQ-3 | No pores | On the included angle, 2 φ 0.025mm porosity | ||
| YTQ-4 | No pores | Internally convex, 3 pieces φ 0.010mm porosity |
Note: the shielding gas for the purity of ≥ 99.99% pure argon gas; gas circuit pressure of 0.2MPa
Table.2 “Welding material in addition to hydrogen treatment” group test situation and results
| Serial number | Test type: three-way protective gas (gun gas, hood gas, and back protection gas) | Test piece number | X-ray results | Metallographic examination results |
| 1 | Welding wire hydrogen removal treatment | QQCL01 | No pores | The included angle is inward convex, and the air hole is 0.025mm |
| QQCL02 | No pores | No pores | ||
| QQCL03 | No pores | Weld protrusion, upper, 0.005mm porosity | ||
| QQCL04 | No pores | No pores | ||
| QQCL05 | No pores | Convex inside the weld seam with 0.005mm porosity | ||
| 2 | Welding wire without hydrogen removal treatment | CL01 | No pores | Weld protrusion, upper, 0.006mm porosity |
| CL02 | No pores | Weld seam angle, 0.010mm porosity | ||
| CLO3 | No pores | 2 in the weld seam φ 0.003mm porosity | ||
| CL04 | No pores | No pores | ||
| CL05 | No pores | Weld angle, 0.006mm porosity |
2.4 Butt joint bevel group type on the impact of microporosity
The details of this group of tests and test results are shown in Table 4. From the table 4 different bevel groups to type of welding test results can be seen: in the “light pipe” on the direct welding, no porosity is found; and the remaining 3 groups of bevel-type welding test joints are “microporosity.” The group test shows that the “butt bevel group type” on the “microporosity” generated no effective inhibitory effect. For the “light pipe,” welded joints without microporosity can be inferred in the welding process; the welding environment and other welding conditions are the same, and welding bevel end roughness accuracy may impact the production of microporosity.
Table.3 “Arc voltage” group test situation and results
| Serial number | Test type: three-way protective gas (gun gas, hood gas, and back protection gas) | Test piece number | X-ray results | Metallographic examination results |
| 1 | Tungsten electrode elevation 1.0mm | WJ01 | No pores | nothing |
| WJ02 | No pores | nothing | ||
| WJ03 | No pores | Inner convex, 6 pieces φ 0.010mm porosity | ||
| WJ04 | No pores | Internal convex angle, 2 pieces φ 0.010mm porosity | ||
| WJ05. | No pores | nothing | ||
| 2 | Tungsten electrode lifting by 1.5 mm | WJ7-1 | No pores | Inner convex. Upper, 0.005mm porosity |
| WJ7-2. | No pores | Angle between both sides< φ 0.005mm porosity | ||
| WJ7-3. | No pores | Angle bar side< φ 0.005mm porosity | ||
| WJ7-4 | No pores | The included angle is inward convex< φ 0.005mm porosity | ||
| WJ7-5 | No pores | nothing | ||
| 3 | Tungsten electrode lifting 2.0 mm | WJ5-1 | No pores | Internal convexity_ Up, φ 0.01mm porosity |
| WJ5-2 | No pores | No pores | ||
| WJ5-3 | No pores | φ 0.01mm porosity, weld seam angle | ||
| WJ5-4 | No pores | φ 0.01mm porosity, weld seam angle | ||
| WJ5-5 | No pores | No pores |
Table.4 “Butt joint bevel type” group test situation and results
| Serial number | Test type: three-way protective gas (gun gas, hood gas, and back protection gas) | Test piece number | X-ray results | Metallographic examination results |
| 1 | “Smooth tube” non groove test | G1 | No pores | No pores |
| G2 | No pores | No pores | ||
| G3 | No pores | No pores | ||
| G4 | No pores | No pores | ||
| G5 | No pores | No pores | ||
| 2 | 45 ° slope straight section (see Figure 2) | VX1-1 | No pores | 0.005mm porosity in the weld seam |
| VX2-2 | No pores | Internally convex, 0.005mm porosity | ||
| VX2-3 | No pores | Weld seam angle, 0.015mm porosity | ||
| VX2-4 | No pores | Internally convex, 0.005mm porosity | ||
| VX2-5. | No pores | Weld angle, 0.005mm porosity | ||
| 7DJ45-1 | No pores | Weld angle, φ 0.005mm porosity | ||
| 7DJ45-2 | No pores | No pores | ||
| 3 | 45 ° groove with straight section (see Figure 3) | 7DJ45-3 | No pores | Weld angle, 0.006mm porosity |
| 8DJ45-4 | No pores | Weld seam angle, 0.015mm porosity | ||
| 8DJ45-5 | No pores | Weld angle, 0.005mm porosity | ||
| 4 | I-shaped groove, flat end face deburring (see Figure 1) | PDM-1 | No pores | There is one 0.015mm air hole protruding inside the weld seam, φ Two 0.01mm pores, less than φ Two 0.005mm air holes |
| PDM-2 | No pores | One 0.01mm air hole protruding from the weld seam< φ Two 0.005mm air holes | ||
| PDM-3 | No pores | Convex inside the weld seam, φ 0.005mm air hole 1 | ||
| PDM-4 | No pores | Inner protrusion of weld seam_ Two 0.015mm air holes on top, φ Two 0.01mm pores, less than φ Three 0.005mm air holes | ||
| PDM-5 | No pores | On the included angle of the rod side, φ Two 0.01mm air holes< φ 0.005mm porosity |
2.5 Bevel end roughness on the effect of microporosity
The specific situation and test results of this group of tests are shown in Table 5. According to the 2 groups of bevel welding test results of different end roughness, the higher the accuracy of the end roughness, “microporosity,” the lower the chance of appearing. To be welded bevel end roughness of 0.8μm welded joints only 1 group of “microporosity,” and to be welded bevel end close to the mirror requirements of the welded joints in 200 times the microstructure inspection, not found any “microporosity.”
Table.5 “Weld surface roughness” group test situation and results
| Serial number | Test type: three-way protective gas (gun gas, hood gas, and back protection gas) | Test piece number | X-ray results | Metallographic examination results |
| 1 | The accuracy of end face roughness ranges from 1.6 μ Increase m to 0.8 μm | YJ1 | No pores | No pores |
| YJ2 | No pores | Internally convex, 2 pieces φ 0.005 porosity | ||
| YJ3 | No pores | No pores | ||
| YJ4 | No pores | No pores | ||
| YJ5 | No pores | No pores | ||
| 2 | The roughness of the end face is 0.8 μ The end face of m is polished to meet the requirements of approaching the mirror surface | PG1 | No pores | No pores |
| PG2 | No pores | No pores | ||
| PG3 | No pores | No pores | ||
| PG4 | No pores | No pores | ||
| PG5 | No pores | No pores |
The group test shows that: improving the “bevel end roughness accuracy” to control the titanium alloy small diameter thick-walled pipe welded joints generated “microporosity” chances.
3. Analysis and discussion
3.1 “Microporosity” cause analysis
From the morphology of the pores, there are surface pores; there are also pores inside the weld, sometimes with a single distribution, sometimes into a heap of dense, sometimes throughout the weld section, with diffuse distribution inside the weld. Generally speaking, the formation of pores generally goes through bubble nucleation, bubble growth, and bubble floating, three processes, and finally, the formation of pores. We believe that the test welded joints after 200 times the microscopic metallurgical examination found that the “micro-pore” may be the pore in the growth of the composite and not completely “eat” the micro-bubbles. In the welding metallurgy process, the melt pool is filled with a complex composition of various gases, these gas molecules in the arc under the action of high temperature and constantly to the liquid melt pool inside the diffusion and dissolution, and the temperature is getting higher and higher, the amount of dissolved gas in the metal is also getting more and more. If the gas precipitates, the bubble grows and floats faster; it can be aggregated into a large bubble escape. If the number of these micro bubbles, bubble radius, and volume is very small, they float slowly, too late to escape and remain in the weld, forming the so-called “microporosity.”
Statistically, the test produced “microporosity” defects mainly in the weld convex edge (occurrence rate: 51.0%) and weld nip (occurrence rate: 40.9%) on the remaining about 8.1% of the “microporosity” appeared in the middle of the weld (shown in Figure 4). Figure 5 lists this test’s typical “microporosity” defect shape and location distribution.
Figure 4 and Figure 5 shows that 200 times the microscopic observation of “micro-pore” shapes are spherical pores, black around the middle of the bright. And most of the “microporosity” appear in the weld convex edge and weld nip; at the boundary of the weld fusion zone, a smooth inner wall surrounds the pore.
Figure.4 Diagram of the microscopic morphology of the welded joint area
Usually, the impact of titanium alloy welded joints with porosity defects have many factors, such as environmental humidity, environmental cleanliness, the state of the workpiece surface, shielding gas, welding process, etc. The “microporosity” may be due to the unsatisfactory bevel state in the weld fusion zone at the boundary, the base material bevel near the oxide film is not completely dissolved and residual, the moisture in the oxide film due to thermal decomposition, and the formation of microbubbles in the oxide film, the melt pool crystallization after the formation of such “microporosity. “
3.2 Discussion of the influencing factors for effective control of “micro-pore” generation
Through the development of “shielding gas,” “welding material hydrogen treatment,” “arc voltage,” and “butt joint bevel type “four experimental studies, the results show that: 200 times the microscopic metallurgical examination found that each type of welded test joints is “microporosity” defects, “microporosity” defect incidence of up to 52.7%. The above four factors describe that “microporosity” generated no effective inhibitory effect. The results of the experimental study of “improving the bevel end roughness accuracy” show that “micro-pore” is reduced, and when the bevel end roughness accuracy is increased from 1.6μm to 0.8μm, the incidence of “micro-pore “The incidence is reduced to 10%; when the bevel end roughness accuracy close to the mirror requirements, are not found in the welded joint there is a case of “microporosity” defects.
As we all know, the surface state of the welded parts, especially the surface state of the butt end face, has a vital impact on the formation of porosity. The surface state of the butt end and the cleaning process and processing technology is closely related to the quality of end processing and constraints on the quality of cleaning; the lower the accuracy of the end roughness, the more difficult to clean, the higher the accuracy of the end roughness, the easier it is to clean the end impurities. From this group of test results can be considered, to be welded bevel end roughness precision is the main reason for the impact of “microporosity” defects.
Therefore, under special conditions of use, if the weld requirements cannot exist any “microporosity,” it is required to weld the workpiece to be welded bevel end roughness accuracy to the higher, the better.
Figure.5 Typical microporosity and location distribution map
4. Conclusion
- (1) “Shielding gas,” “welding material hydrogen removal,” “arc voltage,” and “different bevel type group” The quadrilateral side of the “microporosity” generated no effective inhibitory effect.
- (2) “Bevel end roughness accuracy” is the main cause of “microporosity” Improving the “bevel end roughness accuracy” can effectively control the titanium alloy small Diameter thick-walled pipe welded joints produced by the “microporosity” of the odds.
- (3) The results of this welding test research on small diameter thick-walled titanium alloy tube butt welding engineering applications have important reference significance and practical value.
Author: Song Yiyang
