CN114147333A

CN114147333A - Resistance spot welding method for high-strength galvanized steel workpiece

Info

Publication number: CN114147333A
Application number: CN202111531194.4A
Authority: CN
Inventors: 张彩东; 杨宗全; 熊自柳; 齐建军; 孙力; 卢岳; 倪凤涛; 袁新建; 彭建
Original assignee: Chongqing University; HBIS Co Ltd
Current assignee: Chongqing University; HBIS Co Ltd
Priority date: 2021-12-14
Filing date: 2021-12-14
Publication date: 2022-03-08

Abstract

The invention discloses a resistance spot welding method of a high-strength galvanized steel workpiece, which comprises the following steps: s1, removing surface impurities of the high-strength galvanized steel workpiece before welding; s2, introducing cooling liquid into a copper electrode of the power frequency resistance welding machine; and S3, setting the high-strength galvanized steel workpiece as a lap joint, and performing resistance spot welding by using the power frequency resistance spot welding machine. The invention can improve the metallurgical reaction of the welding interface of the high-strength steel workpiece and the high-strength aluminum alloy workpiece, eliminate the defects of shrinkage cavity and crack in a nugget, improve the quality of welding spots and improve the performance of joints; meanwhile, by optimizing the resistance spot welding process parameters of the power frequency resistance welding machine, the welding heat input and heat balance are optimized, and the splashing is reduced, so that the quality of a spot welding joint is further improved.

Description

Resistance spot welding method for high-strength galvanized steel workpiece

Technical Field

The invention relates to the technical field of automobile manufacturing, in particular to a resistance spot welding method for a high-strength galvanized steel workpiece.

Background

Along with the gradual aggravation of global warming and energy exhaustion problems, the tail gas emission and energy consumption of automobiles become more serious, experiments prove that the automobile quality is reduced by half, and the fuel consumption is reduced by nearly half. Among the ways to lighten the weight of automobiles, adding high-strength steel plates to the automobile body instead of conventional materials is one of the effective means to achieve this goal, such as adding ultra-high-strength steel having a tensile strength exceeding 780 Mpa. In order to provide good corrosion resistance to the steel plate, the surface of the steel plate is usually coated with a corrosion-resistant coating, and the coating is widely applied in industry.

In the resistance spot welding process, the phenomenon that molten base metal flies out from the joint surface of the lap plate or flies out from the contact surface of the plate and the electrode during welding is called as 'splashing', and sputtered metal is attached to the surface of the electrode to influence the welding effect of the electrode, so that the electrode is frequently polished, and the service life of the electrode is shortened; and spatter generation is accompanied by internal defects of the spot welded joint, affecting the strength of the welded joint.

Disclosure of Invention

Aiming at the defects in the prior art, the invention aims to provide a resistance spot welding method for a high-strength galvanized steel workpiece, so as to solve the problems that shrinkage cavities and crack defects are easy to occur after welding, the quality of welding spots is poor, and splashing phenomenon is easy to occur in the prior art.

In order to solve the technical problems, the invention adopts the following technical scheme:

a resistance spot welding method of a high-strength galvanized steel workpiece comprises the following steps:

s1, removing surface impurities of the high-strength galvanized steel workpiece before welding;

s2, introducing cooling liquid into a copper electrode of the power frequency resistance welding machine;

and S3, overlapping the ends of the high-strength galvanized steel workpieces to be welded, and carrying out resistance spot welding on the overlapped parts by using the power frequency resistance spot welding machine.

Compared with the prior art, the invention has the following beneficial effects:

the invention can improve the metallurgical reaction of the welding interface of the high-strength steel workpiece and the high-strength aluminum alloy workpiece, eliminate the defects of shrinkage cavity and crack in a nugget, improve the quality of welding spots and improve the performance of joints; meanwhile, by optimizing the resistance spot welding process parameters of the power frequency resistance welding machine, the welding heat input and heat balance are optimized, and the splashing is reduced, so that the quality of a spot welding joint is further improved.

Drawings

Fig. 1 is a feature point macro photograph.

FIG. 2 is a graph showing the effect of different process window points on nugget diameter, weld spot thickness, reduction rate and tensile and shearing forces.

FIG. 3 is a graph showing the results of cross stretching.

Fig. 4 is a photomicrograph of a G-point joint.

Fig. 5 is a low-magnification SEM photograph of the G-point joint.

Detailed Description

The invention will be further explained with reference to the drawings and the embodiments.

The invention provides a resistance spot welding method of a high-strength galvanized steel workpiece, which comprises the following steps:

In a specific implementation, the cooling liquid in S2 includes cooling water and cooling oil. Welding parameters of the power frequency resistance electric welding machine in the S3 are as follows: the welding current is 9-12 kA, the welding time is 14-20 cycles, and the electrode pressure is 2.2-3.4 kN. The high-strength galvanized steel workpiece is made of an H220YD + ZM steel plate, the thickness of the plate is 1.4mm, and the plate is cut into small plates with the size of 125mm multiplied by 40 mm. The industrial frequency resistance welding machine adopts a Japanese minimalist pneumatic resistance spot welding machine (OBARA, SIV21) device.

And carrying out embodiment verification according to the parameter range, and testing and analyzing the mechanical property of the joint.

TABLE 1

The surface quality of the spot-welded steel sheets of examples 1 to 16 is good, and no obvious defect exists.

TABLE 2 tensile-shear Strength Meter of welding sample

Examples	Empty column	Pulling and shearing force (kN)	Splash condition
				1	1	10.04	CCCC
2	2	11.17	BBBB
				3	3	10.44	BBCD
4	4	10.40	BBBB
				5	3	10.80	BBBB
6	4	11.29	BBBB
				7	1	9.90	BBBB
8	2	11.33	BBBB
				9	4	11.38	BBBB
10	3	11.54	BBBB
				11	2	11.62	BBBB
12	1	12.06	BBBB
				13	2	10.86	AAAA
14	1	11.91	AABB
				15	4	12.44	AABB
16	3	12.01	AAAA

Note: the splashing conditions of 'A', 'B', 'C' and 'D' respectively represent 'A-level splashing', namely, more splashing phenomena are generated; the 'B-level splashing', namely the splashing phenomenon is less; the C-level splashing phenomenon is very little; the 'D-level splashing' is no splashing phenomenon.

In the embodiment, as the welding current is increased, the pulling and shearing force has an ascending trend, and the splashing condition is more serious. The increase in tensile shear strength, weld time and electrode pressure exhibited a tendency to increase and decrease, with maximum tensile shear strength at 16cycles and 2.6kN respectively. The influence of the three factors on the pulling and shearing force is positively analyzed and obtained according to the range difference and the factor effect, and the influence of the three factors on the pulling and shearing force is sequenced into time when the current is larger than the pressure.

The heat input quantity is increased along with the increase of the current, the diameter of the nugget is increased along with the increase of the heat input quantity, the minimum value is when the current is 9kA, the current is small, the heat input quantity is low, the diameter of the nugget is 6.71mm, and the minimum requirement is larger than 5 mm. The thickness and the diameter of the welding spot are opposite, the minimum value of the thickness of the welding spot occurs when the current is 13kA, the thickness of the welding spot is 2.18mm, the corresponding reduction rate is 31.77 percent and is more than 30 percent, and the requirement is not met. The requirement is met when the current is lower than 12KA, the reduction rate is on the rise, and as the current becomes larger, mainly because the heat input increases, nuggets are gradually formed, and the maximum value appears at 11KA, and then gradually stabilizes. Comprehensively considering the splashing condition and the reduction rate requirement, when the optimal parameter is 11kA, the pulling and shearing force is 14.72 kN.

As the weld time increased, the heat input increased and the nugget diameter gradually increased, with the minimum occurring at 12 cycles. At the moment, the welding time is short, the heat input is low, the diameter of a nugget is 6.51mm, and the minimum requirement is more than 5 mm. The thickness of the welding spot is opposite to the diameter, the minimum value of the thickness of the welding spot occurs when the welding time is 18cycles, the thickness of the welding spot is 2.42mm, the corresponding reduction rate is 24.27 percent but still less than 30 percent, and the requirement is met. And with the increase of welding time, the pulling and shearing force shows the trend of increasing firstly and then decreasing in fluctuation, the nugget is gradually formed with the increase of heat input, the pulling and shearing force is increased, under the condition that the current is 16cycles, the pulling and shearing force is 11.75kN, when the current is further increased, the crystal grain grows up due to the welding heat input, the performance is reduced, but the formation of the nugget and the growth of the crystal grain act simultaneously, and the pulling and shearing force fluctuates.

Along with the increase of the electrode pressure, the nugget diameter and the welding spot thickness are stable, the minimum value is that when the electrode pressure is 2.8kN, the nugget diameter is 6.51mm, which is more than the minimum requirement of 5 mm. The minimum value of the thickness of the welding spot occurs when the electrode pressure is 2.2kN, the thickness of the welding spot is 2.26mm, and the corresponding reduction rate is 29.27 percent, so that the requirement of being less than 30 percent is met. Along with the increase of the electrode pressure, the change of the diameter of the nugget and the thickness of the welding spot is stable, and the reduction rate shows a descending trend. With increasing electrode pressure, the tensile shear force increases first and then decreases and reaches a maximum at 2.6 KN. Considering the spatter situation, a group with a welding time of 2.6kN was chosen, where the tensile shear was 11.75 kN.

The process window is a descriptive means for the image of the qualified joint generated by the combination of the welding current and the welding time under a certain condition. The process window determination provides a data reference for actual production, and the present invention performs the weld window study and determination for H220YD + ZM.

When the electrode pressure is 2.6kN, a process window experiment is carried out, and the welding current is small, so that the nugget diameter is small, and the color is grey white. When the current value reaches the DEF point, spatter is generated at this time and the color of the solder joint changes from gray to yellow. The morphology of the G point is closer to that of the E point.

TABLE 3 Process Window characteristic points

Numbering	Current (kA)	Time (cycles)	Pressure (kN)
				A	6.2	18	2.6
B	6.4	16	2.6
				C	6.4	14	2.6
D	8.8	18	2.6
				E	8.8	16	2.6
F	8.8	14	2.6
				G	8.6	16	2.6

Referring to SMTC5111003-2014 (V1) industry standard, to determine the H220LAD + ZM weldability window, the electrode pressure was fixed at 2.6 KN.

Maximum weld time 18 cycles: the welding current starts from 5.8KA, the step length is 200A, the point A is determined until the diameter of a welding spot reaches 5.0mm, the welding current starts from 9KA, the step length is 200A, and the corresponding current corresponds to the point D when the spot welding joint splashes.

Medium weld time 16 cycles: the welding current starts from the point A, the step length is 200A, the point B is determined until the diameter of the welding spot reaches 5.0mm, the welding current starts from the point D KA, the step length is 200A, and the corresponding current corresponds to the point E when the spot welding joint splashes.

Minimum weld time 14 cycles: and the welding current starts from the point B, the welding current is increased by 200A in a step length mode until the diameter of the welding spot reaches 5.0mm, the point C is determined, the welding current starts from the point E, the welding current is increased by 200A in a step length mode, and the corresponding current corresponds to the point F when the spot-welded joint is splashed. Generally, the first point to generate spatter is considered to be generating spatter.

When the diameter of the point A reaches 5mm, the point A is taken as the left boundary of the process window, and the point D is taken as the splash generating point, and the point D is taken as the right boundary of the process window. The left and right boundaries of the three different time levels were selected to determine the process window for H220LAD + ZM. As shown in table 3.

Therefore, under the condition of three selected welding times and the same electrode pressure, the suitable welding current range is 6.2-6.4 kA and 8.6-8.8 kA. The current range for the maximum welding time is 2.2kA, the current range for the minimum welding time is 2.2kA, and the optimal welding current in fig. 1 is the welding current at point G, i.e. point E minus 200 mA. According to the SMTC5111003-2014 (V1) standard, the G point is the optimal welding parameter of the welding window.

FIG. 2 shows the effect of different window points on nugget diameter, weld thickness, reduction rate and shear force. The evaluation standard refers to GB-T39167-2020 tensile shear test method of resistance spot welding and projection welding joint, and it can be seen that compared with DEF point, the influence effect of ABC is the same with the change of regular experimental current due to different currents. As the current increases, the nugget diameter also increases. The thickness of the welding spot is reduced, so that the reduction rate is correspondingly increased. The pulling and shearing force is increased and then reduced, the maximum value is under the condition of a point D, the pulling and shearing force is 11.03kN, and the rolling reduction meets the requirement of being less than or equal to 30 percent. The welding performance in the process range meets the standard requirements, and in combination with the results, the optimal welding parameters of the H220YD + ZM steel are as follows: the welding current is 8.6kA, the welding time is 16cycles, and the electrode pressure is 2.6 KN.

The method for measuring the mechanical properties of the spot-welded joint mainly comprises the steps of testing the tensile-shear strength, the cross tensile strength and the welding spot hardness of the welded joint. The corresponding shear test is carried out aiming at six characteristic points A-G of a welding process window of an H220YD + ZM galvanized plate, the cross stretching result is shown in figure 3, the average stretching force is 5.40kN, and the requirements of GB/T39081-.

After microscopic observation of the points A to G, the fibrous tissue of the welding spot can be divided into three areas: a nugget zone (FZ), a Heat Affected Zone (HAZ), and a parent metal zone (BM). When the current exceeded 9kA, the weld spot changed color from off-white to yellow. And gradually deepens as the current increases. As the current increases, the nugget diameter also increases. The thickness of the welding spot is reduced, so that the reduction rate is correspondingly increased. The tensile shear force tends to increase first and then decrease, and the optimal parameter is that under the condition of 10kA of current, the tensile shear force is 14.72 kN. Under the rapid cooling speed, the structure of the molten core area is granular bainite and a small amount of low-carbon martensite. The heat affected zone is coarse ferrite and partial bainite. When the welding current is 9kA, the welding time is 16cycles, and the electrode pressure is 2.6kN, the nugget is completely formed, and the bainite is generated in the nugget region. When the welding current is 13kA, the heat input is increased, nuggets are completely formed, and a hole appears in the welding spot due to the excessively high heat input. The horizontal microhardness value is about 120HV in the parent metal area, about 130-160HV in the heat affected area and about 200HV in the nugget area. The microhardness value of the heat affected zone increases from the zone close to the base material to the fusion zone. Presenting a "W" type distribution. The vertical microhardness value is low at two sides and high at the center, and because the nugget area is heated to austenitize in the welding process and then undergoes bainite transformation in the rapid cooling process, the bainite content is obviously increased, so the microhardness value of the nugget area is far higher than that of the base material area.

Electrode wear was studied, and resistance spot welding electrode life was the number of welded spots when the spot strength decreased below a predetermined strength in the case of continuous welding of a new pair of electrodes. The observation of the electrode mainly comprises the steps of observing the changes of an electrode cap and a welding spot when 50 points are welded at intervals, namely respectively welding for 50 times, 100 times and 150 times till 500 times and then stopping welding, and comparing the macroscopic morphology, the metallographic phase and the mechanical property of the welding spot when the welding spot is welded for different times.

Referring to the SMTC5111003-2014 industry standard, taking a G point parameter as a standard, dotting and welding 1 group of metallographic samples by taking a small piece on carbon paper after 1 group of 50 welding points are finished, and drawing and shearing 3 groups of metallographic samples after 2 group of 50 welding points are finished, wherein the current of the G point is E point current-200A, time is 16cycles and pressure is 2.6 KN. With the progress of spot welding, the quality of the electrode is continuously reduced, and the size of the corresponding welding spot also fluctuates greatly. The welding spot spattering condition is gradually serious. When the number of welding times exceeds 250, spatter occurs. Along with the increase of the welding times, the electrode cap is gradually worn, the diameter of the welding point on the white paper is gradually increased, and a small amount of adhesion is generated. 500 welding spots meet the requirement, no point with the diameter of the welding spot nucleation smaller than 5.0mm appears, the reduction rate meets the requirement of being less than or equal to 30 percent, and 500 times of electrode abrasion experiments meet the standard requirement of SMTC5111003-2014 under the process window.

The invention can improve the metallurgical reaction of the welding interface of the high-strength steel workpiece and the high-strength aluminum alloy workpiece, eliminate the defects of shrinkage cavity and crack in a nugget, improve the quality of welding spots and improve the performance of joints; meanwhile, by optimizing the resistance spot welding process parameters of the power frequency resistance welding machine, the welding heat input and heat balance are optimized, and the splashing is reduced, so that the quality of a spot welding joint is further improved. The invention enables the welded material of the H220YD + ZM galvanized steel plate to be more optimized, and the optimal parameters of the tension-shear performance can reach 14.72 kN; after the HC340LAD + ZAM zinc-aluminum-magnesium coating steel plate is welded, the optimal parameter of the pulling and shearing force can reach 14.75 KN.

Finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention and not for limiting the technical solutions, and those skilled in the art should understand that modifications or equivalent substitutions can be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions, and all that should be covered by the claims of the present invention.

Claims

1. A resistance spot welding method of a high-strength galvanized steel workpiece is characterized by comprising the following steps:

2. The method for resistance spot welding of a high strength galvanized steel workpiece according to claim 1, wherein the coolant in S2 includes cooling water and cooling oil.

3. The resistance spot welding method for the high-strength galvanized steel workpiece according to claim 1, wherein the welding parameters of the power frequency resistance welder in S3 are as follows: the welding current is 9-12 kA, the welding time is 14-20 cycles, and the electrode pressure is 2.2-3.4 kN.

4. The resistance spot welding method of a high strength galvanized steel workpiece according to claim 1, characterized in that surface impurities in S1 include rust, oxide film and dirt.

5. The resistance spot welding method for a high-strength galvanized steel workpiece according to claim 1, characterized in that the resistance spot welding method according to any one of claims 1 to 4 is applied to H220YD + ZM steel and HC340LAD + ZAM.