A method of butt welding irregular gaps

Abstract

The application provides a non-regular gap butt welding method, and relates to the technical field of welding.The non-regular gap butt welding method comprises the following steps: base layer welding, welding is carried out at the root of the gap by moving the welding gun from the small opening end of the gap to the large opening end, a base layer is formed, the size of the gap is judged according to the diameter of the welding crater, and the walking angle and the working angle of the welding gun are adjusted.In the base layer welding process, the change of the gap is determined by observing the size change of the welding crater, the walking angle and the working angle of the welding gun are adjusted, and thus the fusion of the weld at the root of the gap and the formation of the back weld are realized.The pose of the welding gun is continuously adjusted according to the gap, the small opening end of the gap is ensured to be penetrated, and the large opening end of the gap is ensured not to be welded, the back of the weld is ensured to be formed, the fusion of the weld at the root of the gap is ensured, and the weld quality and the flaw detection qualification rate are effectively ensured in the case that the gap of the bevel is uneven.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of welding, and in particular to a method for butt welding with irregular gaps. BACKGROUND

[0002] In the field of welding, butt welds in butt joint positions are a relatively simple form of welded joints.

[0003] In daily production, large-scale assembly welding parts are usually formed by assembling several small assembly welding parts. However, the small assembly welding parts are prone to welding deformation during welding, which causes changes in size. In the process of assembling large-scale assembly welding parts, in order to take into account the overall size of other positions, the gap of the butt weld is often uneven. In particular, when welding with uneven gap, welding defects such as incomplete penetration at the small gap end or back weld reinforcement at the large gap end are prone to occur, which makes it difficult to ensure the quality of the weld and the detection qualification rate, resulting in weld repair, which not only affects production efficiency, but also is not conducive to the fatigue resistance of the weld. SUMMARY

[0004] The present application provides a method for butt welding with irregular gaps, which improves the welding quality and welding production efficiency.

[0005] According to one aspect of the present application, a method for butt welding with irregular gaps is provided, the gap being in the shape of an "eight", and the method comprising the following steps:

[0006] Bottom layer welding: using a welding torch to move from the small end of the gap to the large end of the gap and weld at the root of the gap to form a bottom layer. According to the diameter of the welding crater, the size of the gap is determined, and the walking angle and working angle of the welding torch are adjusted.

[0007] In some embodiments, when the diameter of the welding crater is between 2.5mm and 3mm at the small end of the gap, the welding torch does not swing, the working angle of the welding torch is 90°, and the walking angle of the welding torch is 85°-90°.

[0008] In some embodiments, if the diameter of the welding crater is greater than or equal to 3mm, the welding torch is swung in a Z shape to swing the arc to both sides of the gap, and the walking angle of the welding torch is adjusted.

[0009] In some embodiments, as the gap gradually increases, the walking angle of the welding torch gradually decreases.

[0010] In some embodiments, the walking angle of the welding torch is 45°-90°.

[0011] In some implementations, the oscillation frequency of the welding torch gradually increases as the gap gradually increases.

[0012] In some implementations, if the weld pool is larger than the wire diameter, the welding torch is swung from the root of the gap toward the bevel side.

[0013] In some implementations, during the root pass welding, the welding torch performs the arc initiation and termination processes on the arc initiation plate.

[0014] In some embodiments, the following steps are included prior to the root layer soldering:

[0015] For tack welding, tack welding is performed at both ends of the gap, and the length of the tack weld is approximately 20mm to 40mm.

[0016] In some embodiments, the ends of the locating weld are ground and ground into a beveled structure.

[0017] In some embodiments, after the root pass soldering, the following steps are also included:

[0018] For filler layer welding, after arc initiation, the welding torch is oscillated and moved along the bevel side of the gap towards the root. After the arc reaches the root of the gap, the welding torch is oscillated laterally and held for a preset time on both bevel sides of the gap to form a filler layer on the top surface of the bottom layer.

[0019] In some embodiments, the distance between the weld seam of the filler layer and the surface of the workpiece to be welded is 1 mm to 1.5 mm.

[0020] In some embodiments, after the filler layer is welded, the following steps are also included:

[0021] For capping layer welding, after arc initiation, the welding torch is pushed at a constant speed from the small end to the large end along the welding direction, and the welding wire is always placed at the front 1 / 4 position of the molten pool to form a capping layer on the top surface of the filler layer.

[0022] One embodiment of the present invention has the following advantages or beneficial effects:

[0023] The irregular gap butt welding method provided in this embodiment, during the root pass welding process, determines the change in bevel gap by observing the changes in the size of the molten pool and the weld pool, and adjusts the walking and working angles of the welding torch to achieve real-time adjustment of the welding angle and welding technique. This ensures fusion of the weld at the root of the gap and formation of the weld on the back side. By continuously adjusting the position of the welding torch according to the bevel gap, it ensures penetration at the smaller end of the gap while preventing leakage at the larger end. It also ensures formation of the weld on the back side while guaranteeing fusion at the root of the gap. In cases of uneven bevel gaps, it effectively guarantees weld quality and flaw detection pass rate. Attached Figure Description

[0024] To better understand the present invention, reference may be made to the embodiments shown in the following drawings. Components in the drawings are not necessarily to scale, and related elements may be omitted to emphasize and clearly illustrate the technical features of the invention. Furthermore, related elements or components may have different arrangements as known in the art. Additionally, in the drawings, the same reference numerals denote the same or similar components in various figures. The above and other features and advantages of the present invention will become more apparent from a detailed description of exemplary embodiments thereof with reference to the accompanying drawings.

[0025] in:

[0026] Figure 1 The diagram shown illustrates the structure displaying the bevel gap in an irregular gap butt welding method according to an embodiment of the present invention. Figure 1 ;

[0027] Figure 2 The diagram shown illustrates the structure displaying the bevel gap in an irregular gap butt welding method according to an embodiment of the present invention. Figure 2 ;

[0028] Figure 3 The diagram shown illustrates the structure displaying the bevel gap in an irregular gap butt welding method according to an embodiment of the present invention. Figure 3 ;

[0029] Figure 4 The diagram shown is a schematic representation of the bevel gap after welding in an irregular gap butt welding method according to an embodiment of the present invention.

[0030] The reference numerals in the attached figures are explained as follows:

[0031] 100. Welding torch; 200. Workpiece to be welded;

[0032] 10. Gap; 11. Small end; 12. Large end;

[0033] 1. Apply the base coat; 2. Apply the filler coat; 3. Apply the top coat. Detailed Implementation

[0034] The technical solutions of the exemplary embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. The exemplary embodiments described herein are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Therefore, it should be understood that various modifications and changes can be made to the exemplary embodiments without departing from the scope of protection of the present invention.

[0035] In the description of this invention, unless otherwise expressly specified and limited, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance; the term "multiple" refers to two or more; and the term "and / or" includes any and all combinations of one or more of the associated listed items. In particular, references to "the / described" object or "an" object are also intended to indicate one of a possible plurality of such objects.

[0036] Unless otherwise specified or stated, the terms "connection," "fixed," etc., should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, an integral connection, an electrical connection, or a signal connection; "connection" can be a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0037] Furthermore, in the description of this invention, it should be understood that the directional terms such as "upper," "lower," "inner," and "outer" described in the exemplary embodiments of this invention are used to describe the angles shown in the accompanying drawings and should not be construed as limiting the exemplary embodiments of this invention. It should also be understood that, in the context of an element or feature being connected to another element (one or more) "upper," "lower," "inner," or "outer," it can be directly connected to the other element (one or more) "upper," "lower," "inner," or "outer," or indirectly connected to the other element (one or more) "upper," "lower," "inner," or "outer" through an intermediate element.

[0038] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the embodiments set forth herein; rather, they are provided so that the invention will be thorough and complete, and the concept of the exemplary embodiments will be fully conveyed to those skilled in the art. The same reference numerals in the drawings denote the same or similar structures, and therefore their detailed description will be omitted.

[0039] During the assembly process, the influence of materials can cause uneven gaps in butt welds, resulting in defects such as incomplete fusion at the root of the butt weld and weld beads. To solve this problem, such as... Figures 1-4As shown, this embodiment provides a method for non-irregular gap butt welding, where the gap 10 has an "eight" shaped structure. The non-irregular gap butt welding method includes the following steps: welding the root layer 1, using a welding torch 100 to move from the small end 11 to the large end 12 of the gap 10 and welding at the root of the gap 10 to form the root layer 1, determining the size of the gap 10 based on the diameter of the weld pore, and adjusting the travel angle and working angle of the welding torch 100.

[0040] It should be noted that during fusion welding, a molten pool with a certain geometric shape is formed in the liquid metal portion of the workpiece 200 to be welded. By controlling the length, position, shape, and temperature of the electric arc, a molten hole is formed at the root of the bevel gap 10, which is the weld molten hole.

[0041] It should be noted that the base material of the workpiece 200 to be welded is S500MC, with the following main chemical composition: C≤0.21%, Si≤0.30%, Mn≤1.5%, P≤0.030%, S≤0.025%. Its main mechanical properties are: yield strength ≥500MPa, tensile strength ≥550MPa~700MPa, and elongation ≥12%~17%.

[0042] It should be noted that this irregular gap butt welding method uses solid welding wire to butt weld box-type plates with bevel gap 10. It is suitable for box-type structures and irregular gaps 10, performing single-sided welding with double-sided forming. Since the box structure is a closed structure, the back-side weld formation cannot be inspected visually or repaired; it must be formed in one operation. Specifically, single-sided welding with double-sided forming means that molten metal flows through the welding pore to the back of the bevel gap 10 in the workpiece 200, and forms the back weld after cooling.

[0043] It should be noted that the bevel gap 10 is approximately 2mm to 8mm. For the entire weld, the bevel gap 10 has an "eight" shape, that is, one end of the weld is the small end 11, and the bevel gap 10 of the small end 11 is relatively small, approximately 1mm to 1.5mm, 2mm to 3mm, or 1.5mm to 2mm. The other end of the weld is the large end 12, and the bevel gap 10 of the other end is relatively large, approximately 4mm to 4.5mm, 6mm to 8mm, or 5mm to 7mm.

[0044] The irregular gap butt welding method provided in this embodiment, during the welding of the root pass 1, determines the change in the groove gap 10 by observing the changes in the size of the molten pool and the weld porosity, and adjusts the travel angle and working angle of the welding torch 100 to achieve the purpose of adjusting the welding angle and welding technique at any time, thereby achieving fusion of the weld at the root of the gap 10 and the formation of the back weld. By continuously adjusting the position of the welding torch 100 according to the groove gap 10, it ensures that the smaller end 11 of the gap 10 is fully penetrated while also preventing leakage at the larger end 12. This ensures the formation of the back weld while guaranteeing fusion at the root of the gap 10. Even with uneven groove gaps 10, this method effectively guarantees weld quality and flaw detection pass rate.

[0045] In one embodiment, at the small end 11 of the gap 10, if the diameter of the weld hole is between 2.5 mm and 3 mm, the welding torch 100 does not swing, the working angle of the welding torch 100 is 90°, and the travel angle of the welding torch 100 is 85° to 90°.

[0046] Because the gap 10 at the small end 11 of the gap 10 is relatively small, the welding torch 100 can weld the gap 10 without oscillation. At this time, the travel angle of the welding torch 100 is 85° to 90°, ensuring that the arc temperature is concentrated at the root of the gap 10 and that the root of the gap 10 is fully penetrated. When the working angle of the welding torch 100 is 90°, the temperature of the molten pool increases rapidly, and the weld pool can open smoothly, resulting in a relatively flat weld formation on the back side, thereby reducing the possibility of concavity at the joint point.

[0047] It should be noted that a straight-line welding method is used at the narrow end 11 of the gap 10, and continuous arc welding is employed. Specifically, continuous arc welding uses a smaller welding current at the root of the gap 10, and the welding electrode is moved with an extremely short arc length and almost no or minimal lateral oscillation, so that the heat generated by the arc can be concentrated at the root of the gap 10, ensuring that the root weld is fully penetrated and achieves good formation with the back weld.

[0048] In one embodiment, if the diameter of the weld cavity is greater than or equal to 3 mm, the welding torch 100 adopts a Z-shaped oscillation to oscillate the arc to both sides of the bevel gap 10 and adjust the travel angle of the welding torch 100.

[0049] When the diameter of the weld pool exceeds 3mm, it indicates a significant change in the size of gap 10. In this case, the welding torch 100 continues to oscillate in a Z-shape, moving the arc towards both sides of the bevel. This oscillation of the welding torch 100 towards the bevel lowers the temperature of the weld pool, adapting to the larger gap 10 and thus achieving filling and welding of the larger gap. During this process, the travel angle of the welding torch 100 can be adjusted; for example, the welding torch 100 can tilt backward.

[0050] In one embodiment, such as Figures 1-4 As shown, as the gap 10 gradually increases, the travel angle of the welding torch 100 gradually decreases.

[0051] When other welding factors are the same, the travel angle of the welding torch 100 is different depending on the gap 10. As the gap 10 gradually increases, the travel angle of the welding torch 100 gradually decreases. By using the change in the angle of the welding torch 100, the temperature of the molten pool can be controlled, the arc can be dispersed, the temperature of the molten pool is lower, and the weld surface is thinner, so as to achieve the filling and welding of a larger gap 10.

[0052] In one embodiment, the travel angle of the welding torch 100 is 45° to 90°. The travel angle of the welding torch 100 is at least greater than 45° to avoid poor nozzle protection, which could lead to quality defects such as porosity.

[0053] In one embodiment, as the gap 10 gradually increases, the oscillation frequency of the welding torch 100 gradually increases.

[0054] The oscillation frequency of the welding torch 100 increases with the increase of the gap 10, so as to achieve rapid welding with a larger gap 10. For example: when the bevel gap 10 is 3mm, the oscillation frequency of the welding torch 100 is 2 strokes / second; when the bevel gap 10 is 3mm to 5mm, the oscillation frequency of the welding torch 100 is 4 strokes / second; when the bevel gap 10 is greater than 5mm and less than 8mm, the oscillation frequency of the welding torch 100 is 5 strokes / second.

[0055] In one embodiment, if the weld pore is larger than the diameter of the welding wire, the welding torch 100 is swung from the root of the gap 10 toward the bevel side.

[0056] During the oscillation of the welding torch 100, it is necessary to continuously observe the diameter of the weld molten hole. If the weld molten hole is larger than the diameter of the welding wire, the welding torch 100 needs to be oscillated from the root of the gap 10 towards the bevel side to avoid weld burn-through and quality defects such as weld beads.

[0057] In one embodiment, during the welding of the bottom layer 1, the welding torch 100 performs the arc initiation and arc termination processes on the arc initiation plate.

[0058] Arc ignition refers to the process of igniting the welding arc on the electrode or wire during arc welding. Arc termination refers to the interruption of the arc during welding and the end of the welding process, which can result in defects such as craters, porosity, cracks, air bubbles, and slag inclusions. By ensuring that arc ignition and termination occur on the arc-starting plate during the root pass (1), defects in arc ignition and termination can be avoided in the main weld body, thus improving weld quality. It is understood that the arc-starting plate is removed after the overall weld is completed.

[0059] In one embodiment, before welding the base layer 1, the step further includes: tack welding, where tack welding is performed at both ends of the gap 10.

[0060] Before welding the first layer, the plates of the box-shaped structure are first assembled to meet the process size requirements. Position welding is then performed at both ends of the bevel gap 10. Position welding can also be performed in other parts of the entire structure to meet the spot fixing requirements and play the role of initial pre-positioning, so as to avoid position deviation during the welding of the first layer.

[0061] In one embodiment, the weld length for tack welding is approximately 20mm to 40mm. Tack welding is performed strictly in accordance with the process qualification requirements to ensure that there are no welding defects in the tack welding.

[0062] In one embodiment, the ends of the weld seam of the positioning weld are ground and ground into a sloped structure.

[0063] By grinding the two ends of the tack weld, welding defects at the start and end of the arc are removed, and the ends are ground into a sloped structure to facilitate smooth welding in the later stages and prevent other quality defects.

[0064] It should be noted that after the tack welding, subsequent welding is required to complete the overall welding process. The subsequent welding of the butt joint includes the welding of the root pass 1, the welding of the filler layer 2, and the welding of the cover layer 3.

[0065] In one embodiment, after welding the base layer 1, the method further includes the following steps: welding the filler layer 2. After arc ignition, the welding torch 100 is oscillated and moved along the bevel side of the gap 10 towards the root. After the arc reaches the root of the gap 10, the welding torch 100 is oscillated laterally and remains on both bevel sides of the gap 10 for a preset time to form the filler layer 2 on the top surface of the base layer 1. The preset time can be selected from 0.5 seconds to 1 second.

[0066] Using filler layer 2 for welding has several advantages. First, it can enhance the grinding between layers to eliminate defects in the upper weld and remove surface oxide scale. Second, it allows the weld to cool, controls the interlayer temperature, and reduces heat input. Third, it can grind the weld smooth, making the weld shape more aesthetically pleasing and reducing the risk of weld defects on the surface of filler layer 2.

[0067] The arc is initiated at the end of the bevel gap 10. The angle between the welding torch 100 and the workpiece 200 to be welded is 70° to 80°, that is, the working angle of the welding torch 100 is 70° to 80°, and the wire extension length is about 20mm to 25mm. The welding adopts the left welding method. After the arc is ignited, the arc cannot be pressed down. The welding torch 100 is moved along the bevel side of the gap 10 towards the root. After the arc is initiated, the welding torch 100 is oscillating. When the arc reaches the root of the bevel gap 10, it is oscillated laterally with a small amplitude. It stays on the two bevel sides of the gap 10 for a preset time. This ensures the smoothness of the weld and also facilitates the melting of the edges on both sides of the bevel, reducing the risk of defects such as slag inclusions.

[0068] It is important to note that electrode manipulation refers to the three basic movements of the welding electrode or wire during the normal welding stage: first, electrode feeding, i.e., feeding the electrode along its centerline into the molten pool; second, electrode movement forward, moving along the welding direction; and third, lateral oscillation. Depending on the method of lateral oscillation, various electrode manipulation techniques can be employed, such as linear reciprocating manipulation, crescent-shaped manipulation, oblique circular manipulation, triangular manipulation, and serrated manipulation.

[0069] It should be noted that the filler layer 2 can be a single layer or two layers, and a second filler layer 2 can be superimposed on the surface of one filler layer 2. Understandably, the welding quality of the top filler layer 2 will directly affect the aesthetic appearance of the cover layer 3.

[0070] In one embodiment, the distance between the weld of the filler layer 2 and the surface of the workpiece 200 to be welded is 1 mm to 1.5 mm. This reduces the sharp edges of the bevel. It is understood that this allowance can be appropriately adjusted according to the welding parameters of the cover layer 3, or according to the drawings, welding grade requirements, and weld appearance quality requirements.

[0071] In one embodiment, after the filler layer 2 is welded, the following steps are also included:

[0072] For the welding of the cover layer 3, after the arc is started, the welding torch 100 is pushed at a constant speed from the small end 11 to the large end 12 along the welding direction, and the welding wire is always placed at the 1 / 4 position of the front end of the molten pool, so as to form the cover layer 3 on the top surface of the filler layer 2.

[0073] It should be noted that the number of cover layers 3 can be single or double. A filling layer 2 can be superimposed on the surface of one of the cover layers 3, or the two cover layers 3 can at least partially overlap, or the two cover layers 3 can be simultaneously disposed on the top filling layer 2. This embodiment does not impose a specific limitation on the form and can be adjusted according to the actual production situation.

[0074] In one embodiment, before welding the cover layer 3, the slag and metal spatter on the surface of the filler layer 2 weld should be cleaned, and the uneven parts of the joint should be ground smooth with a grinding wheel. At this time, the horizontal swing amplitude of the welding gun 100 should not be too large, otherwise coarse weld waves and undercut will occur.

[0075] It should be noted that the arc is not interrupted throughout the welding process. The size of the groove gap 10 is determined by observing the size of the molten pool and the size of the weld pore. Various welding defects caused by uneven gap 10 are resolved by changing the electrode manipulation method and the angle of the welding torch 100.

[0076] It should be noted that the welding machine used in this invention is an S500 gas-shielded welding machine, connected via DC reverse polarity. The welding machine must be in good working order and possess the characteristics necessary to meet welding requirements, at least ensuring a stable arc, penetration depth, and minimal spatter. Before welding, the welding area must be clean and free of contaminants. Welding parameters are a major factor in ensuring weld quality, playing a crucial role not only in weld formation but also in the internal microstructure and grain size. Parameters must be adjusted on a test plate before welding to ensure optimal matching.

[0077] It should be noted that the irregular gap butt welding method shown in the accompanying drawings and described in this specification is merely one example of the application of the principles of the invention. Those skilled in the art will clearly understand that the principles of the invention are not limited to any details or components of the apparatus shown in the accompanying drawings or described in the specification.

[0078] It should be understood that the application of this invention is not limited to the detailed structure and arrangement of the components presented in this specification. The invention can have other embodiments and can be implemented and performed in various ways. The foregoing variations and modifications fall within the scope of this invention. It should be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more individual features mentioned or apparent in the text and / or drawings. All these different combinations constitute multiple alternative aspects of the invention. The embodiments described in this specification illustrate the best known mode for carrying out the invention and will enable those skilled in the art to utilize the invention.

[0079] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and exemplary embodiments are to be considered as exemplary only, and the true scope and spirit of the invention are indicated by the appended claims.

[0080] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of protection of the present invention is limited only by the appended claims.

Claims

1. A method of butt welding of irregular gap, the gap is "8" shape, characterized in that, The irregular gap butt welding method comprises the following steps: The backing layer welding is performed by moving the welding gun from the small opening end to the large opening end of the gap and welding at the root of the gap to form a backing layer, and the gap size is determined according to the diameter of the welding crater, and the walking angle and working angle of the welding gun are adjusted; As the gap gradually increases, the walking angle of the welding gun gradually decreases, and the walking angle of the welding gun is 45°-90°; As the gap gradually increases, the swing frequency of the welding gun gradually increases; If the diameter of the welding crater is greater than or equal to 3mm, the welding gun adopts Z-shaped swing to swing the arc to both sides of the gap, and the walking angle of the welding gun is adjusted; If the diameter of the welding crater is between 2.5mm and 3mm at the small opening end of the gap, the welding gun does not swing, the working angle of the welding gun is 90°, and the walking angle of the welding gun is 85°-90°.

2. The irregular gap butt welding method according to claim 1, characterized by, If the welding crater is larger than the diameter of the welding wire, the welding gun is swung from the root of the gap to the groove side.

3. The irregular gap butt welding method according to claim 1, characterized by, During the backing layer welding, the welding gun is used for arc striking and arc collecting on the arc striking plate.

4. The method according to any one of claims 1 to 3, characterized in that Before the backing layer welding, the method further comprises the following steps: The positioning welding is performed at both ends of the gap, and the length of the weld of the positioning welding is 20mm-40mm. The end of the weld of the positioning welding is polished and formed into a slope structure.

5. The method of irregular gap butt welding of claim 4, wherein, After the backing layer welding, the method further comprises the following steps:

6. The method of irregular gap butt welding according to any one of claims 1 to 3, wherein The filler layer welding is performed by swinging the welding gun after striking the arc, moving the welding gun along the groove side of the gap to the root, swinging the welding gun horizontally after the arc runs to the root of the gap, and stopping at both sides of the groove for a predetermined time to form a filler layer on the top surface of the backing layer. The distance between the weld of the filler layer and the surface of the workpiece to be welded is 1mm-1.5mm.

7. The method of irregular gap butt welding of claim 6, wherein, After the filler layer welding, the method further comprises the following steps:

8. The method of claim 6, wherein, The cover layer welding is performed by uniformly advancing the welding gun from the small opening end to the large opening end along the welding direction after striking the arc, and the welding gun is always located at the 1 / 4 position of the front end of the molten pool to form a cover layer on the top surface of the filler layer. ​

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