CN109570729B

CN109570729B - Method for dynamically detecting quality of friction stir welding seam based on torque

Info

Publication number: CN109570729B
Application number: CN201811429875.8A
Authority: CN
Inventors: 欧艳; 谢鑫宇; 钱锦文; 肖逸锋; 张明华; 许艳飞; 吴靓
Original assignee: Xiangtan University
Current assignee: Xiangtan University
Priority date: 2018-11-28
Filing date: 2018-11-28
Publication date: 2020-11-13
Anticipated expiration: 2038-11-28
Also published as: CN109570729A

Abstract

The invention discloses a method for dynamically monitoring the quality of a friction stir welding seam. Firstly, a mechanical testing device, a sampling device and an oscilloscope are used for carrying out high-frequency sampling on torque, the oscilloscope is used for displaying torque change by carrying out high-frequency sampling on the torque, then the torque between two wave crests is defined as a period, the sampling analysis is carried out on the change condition of each period of the torque, each period is divided into two parts, the time of the wave crest falling to the wave trough is A, the time of the wave trough rising to the wave crest is B, finally, the A/B is calculated and analyzed, the relation that A/B is more than 0.8 and less than 1.3 is analyzed, and therefore whether a welding seam generates defects or not is judged. The invention dynamically monitors the quality of the friction stir welding seam based on the torque change, and the method is simple; the cost is low, and large-scale monitoring equipment investment is not needed; no destructive experiments were performed.

Description

Method for dynamically detecting quality of friction stir welding seam based on torque

Technical Field

The invention belongs to the field of friction stir welding seam quality analysis, and particularly provides a method for monitoring the quality of a friction stir welding seam in real time based on torque change conditions.

Background

Friction Stir Welding (FSW) is a method of pressure Welding a part to be welded into a whole by using a special type of stirring head which advances while rotating, generating heat by Friction between the stirring head and the workpiece, and causing the metal at that portion to be in a thermoplastic state by the frictional heat and plastically flowing from the front end to the rear portion thereof under the pressure of the stirring head. Compared with The traditional Welding method, The friction stir Welding has The advantages of high joint quality, small Welding deformation, small residual stress, no pollution in The Welding process and The like, and is a preferred Welding method for light alloys such as aluminum, magnesium and The like, so The friction stir Welding is widely applied to The fields of aerospace, rail transportation, marine ships and The like since The solid phase Welding technology invented by The british Welding Institute (The Welding Institute) in 1991. Friction stir welding has the advantage of process stability that fusion welding is not comparable, but results in defects when errors occur in process parameters and the like.

At present, friction stir welding defects are commonly classified into surface defects and internal defects. The surface defects have the characteristics of intuition and easy observation, can be directly detected by naked eyes, and are more complicated to detect the internal defects. Currently, the detection of internal defects includes destructive detection and nondestructive detection, and the destructive detection needs to be carried out by damaging a welding piece, so that the detection is not advisable in actual production. Therefore, the nondestructive inspection is of great significance to the practical production application of the friction stir welding. The current non-destructive inspection means include visual inspection, ray inspection, eddy current inspection, penetration inspection and ultrasonic inspection. However, these detection methods have respective defects, and eddy current and penetration detection can effectively detect the surface defects of the weld joint, but are not suitable for detecting the internal defects of the weld joint (such as tunnel defects, porosity and the like). The ray detection has good detection effect on tunnel type defects with large size, but cannot detect fine loose, incomplete penetration and close type defects. Ultrasonic non-destructive inspection can detect tunnel defects, lack of penetration defects, and fine porosity, but the equipment is expensive (see lujia, xuwei, zhengxingwei. progress of weld defect inspection and study of friction stir welding [ J ] hot working process, 2017(13): 23-25.). Moreover, the above mentioned detection methods can only detect the weld after the welding is completed, and cannot perform a dynamic real-time monitoring on the welding process.

Therefore, the invention of the nondestructive testing method capable of monitoring the welding seam in real time in the welding process is of great significance in the current situation. The method monitors the Welding quality of the friction stir Welding in real time through the torque periodicity characteristic of the friction stir Welding (see Xiong J T, Zhang X C, Li P, et al. characteristics of periodic variation in torque cured in circumferential Welding process [ J ]. Science & Technology of Welding & Joining, 2014,19(4): 350-354).

Disclosure of Invention

Aiming at the defects of the prior art, the invention is based on the periodic characteristic of friction stir welding and has the characteristic of high consistency with the periodicity of the surface arc lines, the onion rings and other tissues according to physical parameters such as torque and the like. The change condition of the torque is analyzed in the friction stir welding process, so that the welding seam quality of the friction stir welding is monitored in real time, and the torque is a quantity convenient to measure in numerous physical quantities in the friction stir welding process. This is detected based on the nature of the generation of bulk (hole) defects. Because the change condition of the torque of the friction stir welding is under high-frequency sampling, the change rule is similar to a periodic function graph, and the monitoring method is provided based on the condition

The technical scheme of the invention is as follows: the method comprises the steps of placing a workbench on a mechanical testing device during friction stir welding, connecting an oscilloscope with the mechanical testing device to show the torque change condition of a stirring head, analyzing the torque change condition in the oscilloscope, defining an area between two wave crests as a period, dividing each period into a part A from a peak value to a valley value and a part B from the valley value to the peak value, calculating and analyzing the ratio of A/B, and determining that the weld quality is qualified when the A/B is more than 0.8 and less than 1.3, thereby monitoring the weld quality in real time.

The principle for the analysis of torque is that the process of one cycle can be broken down into two phases (as in fig. 3): from time t₀-t₂Is stage A, t₂-t₄Is stage B, t₀The moment when the stirring pin just contacts the base material, the stage A is increased with the temperature, the advancing acting force is increased with the increasing advancing trend of the stirring pin, and further the torque is increased with the time. t is t₂The B stage is a stage in which the base material in the viscoplastic flow is moved to the rear side of the probe at the same speed as the probe is driven by the probe, and the friction between the probe and the base material disappears, but the base material is still heated by shear deformation at a high strain rate, and therefore the torque decreases as the material is softened by heating. t is t₄At the moment of stirringThe needle has rotated through one cycle and has advanced a distance forward, i.e. the next cycle is to begin.

Specifically, the method for monitoring the quality of the friction stir welding seam in real time based on the torque change condition comprises the following concrete implementation steps:

(1) the friction stir welding workbench is arranged on a mechanical testing device, the mechanical testing device utilizes a piezoelectric transducer to measure, and a torque bridge circuit is used for measuring torque

(2) The period is predicted by the rotating speed omega, and T is 1/omega, so that the change situation of the torque in one period is clearly shown.

(3) The torque is sampled at high frequency by a torque sampling device, and the sampling frequency is 1000 Hz.

(4) Extracting a torque high-frequency sampling signal Y through an oscilloscope, and dividing the signal Y into intervals with the interval length of T₁Interval of T, T<T₁<1.5T。

(5) And analyzing each interval, and searching a peak value and a valley value of each interval, wherein the time taken for the peak value to fall to the valley value is A, and the time taken for the valley value to reach the peak value is B.

(6) And calculating the ratio of A to B in an interval, and when the ratio of A to B meets the relation that 0.8< A/B <1.3, determining that no defect exists in the interval, otherwise, when A and B do not meet the relation, determining that the defect exists in the interval and recording.

(7) Entering the next interval and judging whether the Y signals are completely searched, finishing data to obtain a result if the Y signals are completely searched, and entering the step 5 if the Y signals are not completely searched.

The method is suitable for a normal constant-speed welding stabilization stage, in a friction stir welding starting stage, a stirring head is just contacted with a base metal in an insertion-retention process, so that a torque signal is unstable, and when the friction stir welding is about to finish, a key hole is formed in a welding line, so that the torque also becomes unstable, and the detection method cannot be used in the two stages.

Different from other detection methods, the method can detect in the friction stir welding process without destructive experiments, so that the degree of real-time dynamic monitoring can be achieved, and even welding parameters can be adjusted in real time through the detection result.

The reason why the period is predicted in the step 2 is that the time period of the torque change is consistent with the time period 1/omega of the tissue and the displacement change, and the period is predicted one by one, so that the oscilloscope can be adjusted more conveniently and the division can be performed rapidly.

The high frequency sampling of the torque in the step 3 is to show the periodicity of the torque, and if the low frequency sampling is carried out, the periodicity of the torque cannot be shown.

The 4 th step is divided into sections with a width T₁，T<T₁<1.5T can enable two peaks and valleys in one period to be completely contained, and omission or incomplete data does not occur. Processing the ratio of A to B in the 5 th step and the 6 th step, judging the relationship between the ratio and 0.8 and 1.3, and taking the relationship as 0.8<A/B<1.3 when satisfying then think the welding seam is qualified to carry out the same calculation and judge to every interval, this relational expression is through judging hole type defect production essence, can be more accurate and quick judgement defect's production, thereby can carry out real-time supervision to whole friction stir welding seam consequently.

The invention has the following advantages:

1) the friction stir welding process can be monitored in real time.

2) The torque change condition is monitored, and the method is simple, convenient and accurate.

3) The method does not require destructive testing of the weld.

Drawings

FIG. 1 is a flow chart of the present invention

FIG. 2 shows the periodic variation of torque in the friction stir welding process at a forward speed v of 300mm/min and a rotational speed ω of 600rpm

FIG. 3 shows the change of torque in an interval during friction stir welding at a forward speed v of 300mm/min and a rotational speed ω of 600rpm

Detailed description of the invention

The invention is further illustrated in the following description and drawings and examples.

The rotation speed ω of the stirring head is 600rpm, the advancing speed v is 300mm/min, and the welding material is AA1104 aluminum alloy, for example, the detection flow is shown in fig. 1.

Strp1 friction stir welding bench was placed on the mechanical testing apparatus.

Strp2, period T1/omega 1/(600/60). times.0.001 100ms according to preset rotation speed of stirring head

Strp3 high frequency sampling of torque with a torque high frequency sampling device at 1000Hz

Strp4 extraction of Torque high frequency sampling Signal Y by oscilloscope as shown in FIG. 2

Strp5 processing signal Y with partition width T₁Interval of T

Strp6, searching peak and valley values of each interval, wherein the time for the peak value to fall to the valley value is A, and the time for the valley value to rise to the peak value is B, as shown in fig. 3.

Strp7, judging whether the ratio of A and B satisfies 0.8< A/B <1.3, if so, entering the next interval, otherwise, recording the defect, and entering the next interval.

Strp8, judging whether all the signals Y are searched, if so, sorting the previous defect generation condition and evaluating the whole defect; and if the search is not completed, jumping to the step 6 and circulating so on until the search is finished.

The detection method particularly performs a high-frequency sampling operation on the torque, judges the nature of the defect generation, can truly and comprehensively reflect the characteristics of the defect, and is simple and easy to implement.

Claims

1. The method for dynamically detecting the quality of the friction stir welding seam based on the torque is characterized in that the torque is measured through a mechanical testing device, then the torque condition of the torque is subjected to high-frequency sampling, then the torque change condition of a stirring head is displayed through an oscilloscope, and finally the quality of the friction stir welding seam is monitored in real time through the analysis of the torque change condition, and the method is characterized by comprising the following steps of:

(1) placing a workbench for friction stir welding on a mechanical testing device, wherein the mechanical testing device utilizes a piezoelectric transducer to measure and measures torque through a torque bridge circuit;

(2) passing rotational speedωPredicting a periodT=1/ωThe change situation of the torque in one period is clearly shown;

(3) carrying out high-frequency sampling on the torque by a torque sampling device, wherein the sampling frequency is 1000Hz;

(4) extracting a torque high-frequency sampling signal Y through an oscilloscope, and dividing the signal Y into intervals with the interval length of T₁At intervals ofT，T<T₁<1.5T;

(5) Analyzing each interval, and searching a peak value and a valley value of each interval, wherein the time taken for the peak value to fall to the valley value is A, and the time taken for the valley value to reach the peak value is B;

(6) calculating the ratio of A to B in an interval, and when the ratio of A to B meets the relation that 0.8< A/B <1.3, determining that no defect exists in the interval, otherwise, when A and B do not meet the relation, determining that the defect exists in the interval and recording;

(7) and entering the next interval to judge whether the signal Y is completely searched, finishing data to obtain a result after the signal Y is completely searched, and entering the step 5 again if the signal Y is not completely searched.

2. A method for dynamically detecting the quality of a friction stir welding seam based on torque is characterized by comprising the following steps: and calculating the ratio of the A to the B in a torque interval, wherein the time taken for the peak value to fall to the valley value in the torque interval is A, the time taken for the valley value to fall to the peak value is B, and when the ratio of the A to the B meets the relation that 0.8< A/B <1.3, the interval is considered to have no defect, otherwise, when the A and the B do not meet the relation, the interval is considered to have the defect.

3. The method for dynamically detecting the quality of the friction stir welding seam based on the torque as claimed in claim 1, wherein the method is only suitable for welding in a constant-speed stable welding state.