CN107350613B - Resistance spot welding process for steel workpiece with coating layer - Google Patents

Abstract

The invention discloses a resistance spot welding process method for steel workpieces with coatings. The steel workpieces to be welded are overlapped, and pressure is applied to the overlapped joints to maintain pressure; the first welding current is applied to the overlapped joints. position, and allow the first welding current to continue to act for a first time; after that, remove the first welding current and maintain it for a second time; after maintaining it for a second time, apply the second welding current to the overlap, and allow the second welding current to continue to act on the overlap. The second welding current continues to act for a third time; after continuing for the third time, the second welding current is slowly reduced to 0 in the fourth time; wherein, the second welding current is greater than the first welding current, and the first time and the second time The sum is greater than or equal to 600ms. The first welding current is output to destroy the coating, and the second welding current is output to complete the heating welding. A longer stop output sequence is set between the two to promote the discharge of the coating material, suppress the occurrence of spatter, and obtain a wider width. process window.

Description

Resistance spot welding process for coated steel workpieces

technical field

The invention belongs to the technical field of welding, and particularly relates to a resistance spot welding process method for a steel workpiece with a coating layer.

Background technique

In the resistance spot welding process, the molten base metal metal flies out from the joint surface of the lapped plate or from the contact surface of the plate and the electrode during welding, which is called "splash". After the splash phenomenon occurs, the sputtered metal adheres to the surface of the electrode, which affects the welding effect of the electrode, which in turn causes the electrode to be ground frequently and the service life of the electrode is shortened. Generally, the current when spatter occurs is called the spatter current or the upper limit current, and the current that guarantees the diameter of the reference nugget is called the lower limit current. The current range between the upper and lower current limits is an important criterion for evaluating the resistance spot welding process, which is called the process window.

Nowadays, in the manufacture and production of automobile body, the connection of body plates is mainly carried out by resistance spot welding. Due to the different roles of each part of the car body, the selection of plates is also different. For example, in order to meet the safety protection and anti-corrosion performance of passengers, the B-pillars of the car body are mostly made of hot stamped sheets with aluminum-silicon coating.

The occurrence of spatter during car body manufacturing and welding will affect the final welding joint performance, and cause frequent electrode grinding and reduction of electrode service life; the sputtered metal adheres to the car body surface, which affects the appearance of the car body surface. Clear processing, increase production cost. In the actual production line, due to the stability and consistency of the current of the welding machine, the actual current value often fluctuates around 10% of the set value. actual meaning.

SUMMARY OF THE INVENTION

One of the technical problems to be solved by the present invention is to suppress the occurrence of spatter and obtain a wider process window during the resistance spot welding process.

In order to solve the above-mentioned technical problems, an embodiment of the present invention proposes a resistance spot welding process method for a steel workpiece with a coating layer, comprising the following steps:

Lap the steel workpiece to be welded, and apply pressure to the lap joint to maintain the pressure;

applying the first welding current to the overlap, and making the first welding current continue to act for a first time;

after the first time, removing the first welding current for a second time;

After the second time is maintained, the second welding current is applied to the overlap, and the second welding current is continuously applied for a third time;

After lasting the third time, the second welding current is slowly reduced to 0 in the fourth time;

Wherein, the second welding current is greater than the first welding current, and the sum of the first time and the second time is greater than or equal to 600ms.

Preferably, the pressure maintained by the pressurization is 3-8KN.

Preferably, the pulse peak value of the first welding current is 3-5.5KA.

Preferably, the first time is 200-700ms.

Preferably, the second time is 300-600ms.

Preferably, the pulse peak value of the second welding current is 6-9.5 KA, and the third time is 300-600 ms.

Preferably, the fourth time is 80-240ms.

Preferably, it also includes:

After the second welding current is slowly reduced to 0, the lap joint is kept pressurized, and cooling water is supplied to the lap joint at the same time.

Preferably, the steel workpiece with a coating layer includes a zinc-coated steel workpiece and an aluminum-silicon-coated steel workpiece; wherein, the coating thickness of the zinc-coated steel workpiece is 10-35 μm, and the aluminum-silicon-coated steel workpiece has a thickness of 10-35 μm. The coating thickness of the workpiece is 20-50 μm; the sum of the thicknesses of the zinc-coated steel workpiece and the aluminum-silicon-coated steel workpiece is 2-4 mm.

The resistance spot welding process method of the present invention firstly uses the first welding current to destroy the coating layer, and then uses the second welding current to complete the heating welding, and sets the output stop for a long time between the two to promote the coating layer material The discharge reduces the influence of the coating material on the heating welding, suppresses the occurrence of spatter, and obtains a wider process window, which ensures the practicability of the welding process in actual production.

Other advantages, objects, and features of the present invention will be set forth in the specification to the extent that follows, and will be apparent to those skilled in the art based on a review of the following, or may be Teachings are gained from the practice of the present invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the description and claims hereof as well as the appended drawings.

Description of drawings

The accompanying drawings are used to provide a further understanding of the technical solutions or the prior art of the present application, and constitute a part of the specification. The drawings representing the embodiments of the present application together with the embodiments of the present application are used to explain the technical solutions of the present application, but do not constitute limitations on the technical solutions of the present application.

1 is a schematic flowchart of a resistance spot welding process method for a steel workpiece with a coating layer according to an embodiment of the present invention;

2 is a schematic illustration of a process implementation device of a resistance spot welding process method according to an embodiment of the present invention;

3 is a schematic diagram of a welding current output timing sequence of a resistance spot welding process method according to an embodiment of the present invention;

4 is a comparison diagram of the maximum tensile shear force of the welding spot applying the resistance spot welding process method of the embodiment of the present invention and the comparative case in Comparative Experiment 1;

FIG. 5 is a comparison diagram of the maximum tensile shear force of the solder joints in the comparative experiment 2 using the resistance spot welding process method of the embodiment of the present invention and the comparative case.

Detailed ways

The embodiments of the present invention will be described in detail below with reference to the accompanying drawings and examples, so as to fully understand and implement the implementation process of how the present invention applies technical means to solve technical problems and achieve corresponding technical effects. The embodiments of the present application and the various features in the embodiments can be combined with each other under the premise of no conflict, and the formed technical solutions all fall within the protection scope of the present invention.

In the production of automobile body, it involves resistance spot welding of coated steel workpieces, such as welding of DP950 galvanized coated plate and aluminum-silicon 1500MPa hot stamping plate. The inventors conducted research on the resistance spot welding performance of this welding relationship and found that:

In the existing welding process, after the welding current is turned on, the coatings of the two materials are melted successively to form a molten coating. Due to the differences in thickness, melting point, boiling point, resistivity, etc. between the zinc coating and the aluminum silicon coating, the zinc coating melts first, the aluminum silicon coating melts after, and under the pressure of the welding electrode, the partially molten state The coating is squeezed out.

Then, under the action of the welding current, the heat input increases continuously, the temperature reaches the boiling point of zinc, and the liquid zinc vaporizes. At this time, there are three kinds of coating materials: molten aluminum silicon, molten zinc, and vaporized zinc at the interface. The state coating material does not drain well. In the subsequent process of forming the weld nugget, the undischarged coating material affects the heat input of the welding current, resulting in uneven heat input, and then splashing occurs at a small current, affecting the performance of the final welded joint and causing the process The window is narrow.

Therefore, the present invention proposes a resistance spot welding process for steel workpieces with a coating layer, which adopts a double-pulse method, and a certain welding dwell time is set between the two pulses to promote the discharge of the coating material in the spot welding, thereby avoiding the occurrence of splashes. , and a wider process window can be obtained.

For example, in the manufacture of car bodies, the commonly used sheets are galvanized coated sheets and hot stamped aluminum-silicon coated sheets. For the hot stamping aluminum-silicon coated plate, the coating material reacts with the steel matrix, and it is easy to form intermetallic compounds and iron-based solid solutions, and oxides are formed on the surface. Zinc-based oxides are also formed on the surface of galvanized coated sheets. Such intermetallic compounds and oxides are used to improve the corrosion resistance of the sheet, but at the same time are prone to spatter during resistance spot welding.

In this embodiment, the workpiece overlapped body to be welded is composed of a first steel workpiece with a zinc coating and a second steel workpiece with an aluminum-silicon coating, and the resistance spot welding process proposed by the present invention is applied to the workpiece overlapped body. method, the present invention has no particular limitation on the strength level of the steel material constituting the lap joint.

The present invention is also not limited to the production process and coating process of the welded steel. For example, the steel workpiece with aluminum-silicon coating is formed from hot-rolled steel sheet or cold-rolled steel sheet by hot stamping, such as zinc-coated steel workpiece. The steel workpiece is a hot-rolled or cold-rolled steel sheet on which a metallic zinc layer is formed, eg, the zinc coating process is hot-dip galvanizing, galvannealing, zinc electroplating, or zinc-iron electroplating.

The present invention will be discussed below with reference to the flowchart shown in FIG. 1 , the schematic illustration diagram of the implementation device for implementing the resistance spot welding process method according to the embodiment of the present invention shown in FIG. 2 , and the welding current output timing diagram shown in FIG. 3 . The resistance spot welding method of the Example is demonstrated.

First, as shown in step S110 in FIG. 1 , the steel workpieces to be welded are overlapped, and the workpiece overlapped body composed of the first steel workpiece 9 and the second steel workpiece 10 is placed between the spot welding electrodes 11a and 11b ( as shown in picture 2). The pressing device 12 in FIG. 2 provides pressure, and the spot welding electrodes 11a and 11b apply pressure to and maintain the overlapping parts of the workpiece overlapping bodies.

The first steel workpiece 9 and the second steel workpiece 10 constituting the workpiece overlap body are respectively galvanized coated plates and aluminum-silicon coated plates. The spot welding electrodes 11a and 11b are usually made of chrome copper, and the round tops thereof have flat end faces with a certain diameter, and the diameter of the flat end faces is preferably in the range of 4-8 mm. In this embodiment, the diameter of the flat end face of the spot welding electrode is 6 mm.

As shown in FIG. 2, the pressure is provided by a pressurizing device 12, and the pressure generation can be based on a servo motor or based on compressed air. The spot welding electrodes 11a and 11b are relatively clamped and pressurize the overlapped body of the workpiece, and the magnitude and holding time of the pressure output are controlled by the pressure controller 8 . Specifically, in this embodiment, the pressing pressure is 3-8KN.

The S110 process is set to make the welding electrode and the workpiece overlapping body, and the steel workpieces constituting the workpiece overlapping body closely adhere, so as to reduce the contact resistance. For example, the aluminum-silicon coating on the surface of the hot-stamping plate with aluminum-silicon coating has high hardness, and its surface is uneven on the microscopic level. Press to make the contact surfaces of the lap joints close to each other to reduce the contact resistance. And in the welding process, maintaining the pressure can also promote zinc removal and limit the role of molten metal splashing.

Further, in order to ensure the close contact effect between the welding electrode and the workpiece overlap body, and between the steel workpieces constituting the workpiece overlap body, before outputting the welding current, the pressure needs to continue for a certain period of time. It is called the preloading stage. Similar to the existing process, the duration of the preloading stage is generally 300-600ms. During welding after the preload phase, the pressure is also maintained at all times.

After the process S110, the process S120 shown in FIG. 1 is continued, the spot welding electrode outputs the first welding current, the first welding current acts on the lap joint of the workpiece overlapping body, and a current path is formed between the spot welding electrodes 11a and 11b, The coating layer of the steel workpiece to be welded is destroyed by the current heating effect.

FIG. 3 is a schematic diagram of the output sequence of the welding current. In the process S120, the first welding current is a power frequency alternating current, and the pulse peak intensity of the first welding current is I1, which lasts for a first time t1. In the process S120, The coating layer is melted and vaporized, and part of the molten coating layer is discharged under pressure.

After that, the process S130 shown in FIG. 1 is continued, and the welding current output state is shown in FIG. 3 , the spot welding electrode stops outputting the first welding current, and continues to maintain the second time t2. The coating is completely drained from the weld. To ensure the discharge effect, the sum of the first time t1 and the second time t2 is greater than or equal to 600ms.

It is not difficult to understand that, in the above steps S120 and S130, the specific selection of t1, t2, and I1 is related to the coating material and the coating thickness. In a specific application scenario, the thickness of the zinc coating is preferably in the range of 10-35 μm, and the thickness of the aluminum-silicon coating is preferably in the range of 20-50 μm. A preferred range for t1 is 200ms to 700ms. The preferred range of t2 is between 300ms and 600ms. The preferred range for I1 is 3-5.5KA.

For example, in a preferred embodiment, the thickness of the zinc coating is 28 μm, the thickness of the aluminum silicon coating is 35 μm, t1 is 300 ms, t2 is 300 ms, and I1 is 4KA.

After the process S130, the process S140 in FIG. 1 is continued. The output state of the welding current is shown in FIG. 3, and the spot welding electrode outputs the second welding current, which acts on the overlap of the workpiece overlapped body. The second welding current is a power frequency alternating current and lasts for the third time t3. The pulse peak intensity of the second welding current is I2, and the pulse peak intensity of the second welding current is greater than the pulse peak intensity of the second welding current, that is, I2 is greater than I1.

The second welding current is used for heating and welding, so that the welding point of the overlapped joint of the workpieces is in a molten state. Due to the discharge of the coating material in the previous process and the additional preheating effect of the first welding current, the heat affected zone of the heating welding in this process is large, which is beneficial to improve the mechanical properties of the final welded joint.

It is not difficult to understand that in step S140, the selection of I2 and t3 is related to the thickness and material of the plate. In a specific application scenario, the thickness of the steel sheet with zinc coating is preferably in the range of 0.8-1.8 mm, the thickness of the steel sheet with aluminum-silicon coating is preferably in the range of 1-2 mm, and the sum of the two thicknesses is 2-4 mm; correspondingly , the preferred range of t3 is 300ms to 600ms, and the preferred range of I2 is 6-9.5KA.

For example, in a preferred embodiment, the thickness of the steel sheet with zinc coating is 1.2 mm, the thickness of the steel sheet with aluminum silicon coating is 1.8 mm, t3 is 400ms, and I2 is 9KA.

After the process S140, the process S150 in FIG. 1 is continued. After the second welding current lasts for the third time, the second welding current output by the spot welding electrode slowly decreases to 0 in the fourth time t4, as shown in the timing sequence shown in FIG. 3 for details. . The slowly decreasing welding current in the process S150 can slow down the cooling rate of the welding spot at the lap joint, and then the molten welding spot is gradually cooled, so as to promote the uniform diffusion of the nugget area, which is beneficial to improve the mechanical properties of the final welded joint. The preferred range for t4 is 80-240ms. For example, take t4 as 200ms.

Finally, as with the existing process, after the output of the resistance welding machine drops to zero, the lap joint is kept pressurized for a period of time and cooled to form a nugget at the welding point.

Further, while maintaining the pressure, through 14 in FIG. 2, circulating cooling water is provided and flows out from the spot welding electrode to cool the welding spot, thereby increasing the cooling rate of the welding spot and improving the mechanical properties of the final nugget. . In this embodiment, the flow rate of cooling water is 2 cm ³ /s.

After maintaining the pressure for a period of time, the pressure is removed, and the welding operation of the welding point of the workpiece overlap body is completed.

In addition, in the implementation device of the resistance spot welding process method shown in FIG. 2 , 7 is an AC power source for supplying welding current. 13 is a current controller for controlling the magnitude and timing of the welding current output from the spot welding electrode. It can be understood that, 7, 8, 11a, 11b, 12, 13, and 14 in FIG. 2 can be realized by corresponding functional components of the existing resistance welding machine. For example, the single-phase AC resistance welding machine used in the process implementation in this embodiment.

In the spot welding process method proposed by the present invention, a first welding current with a relatively small current and a long duration is firstly used to ensure that the coating material at the contact interface of the overlapping body of the workpiece is melted and partially removed under a small heat input. , and then set a longer duration stop output sequence to ensure that the melted and vaporized coating material is completely discharged. Through the above steps, a wider current path is formed on the contact surface, and the occurrence of spatter is suppressed under the larger second welding current. Further, a second welding current with a relatively large current and a short energization time is used to obtain a weld nugget of sufficient size to meet the welding performance. After the second welding current ends, the slow-down current ensures the cooling rate, promotes the diffusion of carbon in the fusion zone formed by different matrix structures, makes it tend to be uniform, and improves the toughness of the welded joint.

The resistance spot welding process method of the steel workpiece with the coating layer provided by the present invention can completely suppress the spatter, improve the appearance quality of the workpiece, further omit the process of cleaning the surface burr of the workpiece, and improve the production efficiency. In addition, by adopting the resistance spot welding process method of the steel workpiece with the coating layer proposed by the present invention, and setting the output stop process for a long time, a better process window can be obtained under the condition of ensuring the specified welding strength. The process window is defined as the difference between the current at which spatter occurs in welding and the current at which the reference nugget diameter is guaranteed.

The beneficial effects of obtaining a better process window are further described below through two sets of comparative experiments.

(t为被焊接工件中较厚板材厚度)。点焊接头焊核直径通过在LEICA显微镜下的微观组织测得，点焊时通过目测确认飞溅是否存在。In comparative experiments 1 and 2, by fixing some parameters and setting the peak current of the second welding current as a variable to carry out resistance spot welding, the lower limit current value that ensures the welding joint as the reference nugget diameter and the upper limit current that does not generate spatter are obtained. value, where the reference nugget diameter is

(t is the thickness of the thicker plate in the welded workpiece). The diameter of the weld nugget of the spot welding joint was measured by the microstructure under the LEICA microscope, and the presence of spatter was confirmed by visual inspection during spot welding.

The spot welding equipment in Comparative Experiments 1 and 2 is a single-phase AC resistance welding machine based on compressed air pressurization, and the diameter of the flat end face of the spot welding electrode is 6 mm. The workpieces to be welded are respectively selected from hot-dip galvanized coated DP590 industrial plate and aluminum-silicon coated 1500MPa hot stamped industrial plate. The thickness of the zinc layer and its alloying is about 28um, the thickness of the aluminum-silicon coating and its alloying is about 35um, and the size is 30mm×100mm. The pressurization pressure was 3.5 KN, and the cooling water rate was 2 cm ³ /s.

In the comparative experiments 1 and 2, the implementation case of the test number 1 adopts the process method of the present invention, and the comparative case of the test number 2 is used as a comparison, using a shorter residence time, and the thickness of the plate in the comparative experiment 1 is 1.2mm, and the obtained experimental data is as shown in the table. 1 shown.

The experimental data of Table 1 Comparative Experiment 1

In Comparative Experiment 2, the thickness of the DP590 plate was 1.2 mm, and the thickness of the aluminum-silicon hot stamping plate was 1.8 mm. The experimental data obtained are shown in Table 2.

Table 2 The experimental data of comparative experiment 2

From the comparative experimental data shown in Tables 1 and 2, it can be seen that the implementation case of Test No. 1 has a process window that is approximately doubled compared to the Comparative Case of Test No. 2.

In addition, the ratio of the maximum tensile shear force (in KN) to the total thickness of the workpiece in the implementation case of test number 1 in Table 1 and Table 2 are all greater than 5, and the failure modes are all nugget pull-out and the tensile shear force is almost close to that of galvanizing. Strength limit of coated panels. And as shown in Fig. 4 and Fig. 5, under the same second welding current, the maximum tensile shear force of the welding point of the implementation case is about 10% higher than that of the comparative case.

The above is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited to this. Any person familiar with the technology can easily think of changes or substitutions within the technical scope disclosed by the present invention. , all should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims (8)

1. A resistance spot welding process method with a coated steel workpiece, comprising the following steps: Lap the steel workpiece to be welded, and apply pressure to the lap joint to maintain the pressure; applying the first welding current to the overlap, and making the first welding current continue to act for a first time; after the first time, removing the first welding current for a second time; After the second time is maintained, the second welding current is applied to the overlap, and the second welding current is continuously applied for a third time; After lasting the third time, the second welding current is slowly reduced to 0 in the fourth time; Wherein, the steel workpiece with the coating layer includes a zinc-coated steel workpiece and an aluminum-silicon coated steel workpiece, and the second welding current is greater than the first welding current, and the first time is the same as the second welding current. The sum of the times is set to be greater than or equal to 600ms; the second time is set to be 300-600ms, and wherein the duration of the second time can cause the coating material to be completely discharged within the second time. 2 . The resistance spot welding method according to claim 1 , wherein the pressure maintained by the pressurization is 3-8KN. 3 . 3 . The resistance spot welding method according to claim 1 , wherein the pulse peak value of the first welding current is 3-5.5 KA. 4 . 4 . The resistance spot welding method according to claim 1 , wherein the first time is 200-700 ms. 5 . 5 . The resistance spot welding method according to claim 1 , wherein the pulse peak value of the second welding current is 6-9.5 KA, and the third time is 300-600 ms. 6 . 6 . The resistance spot welding method according to claim 1 , wherein the fourth time is 80-240 ms. 7 . 7. The resistance spot welding process method according to claim 1, characterized in that, further comprising: After the second welding current is slowly reduced to 0, the lap joint is kept pressurized, and cooling water is supplied to the lap joint at the same time. 8. The resistance spot welding method according to any one of claims 1 to 7, wherein the coating thickness of the zinc-coated steel workpiece is 10-35 μm, and the coating of the aluminum-silicon-coated steel workpiece has a thickness of 10-35 μm. The thickness is 20-50 μm; the sum of the thicknesses of the zinc-coated steel workpiece and the aluminum-silicon-coated steel workpiece is 2-4 mm.

Images