Online analysis system for short-cycle arc stud welding

BAO Yefeng，LIAO Hailong，2，ZHANG Jinhui，ZHANG Jingzhang，YANG Ke，JIANG Yongfeng

（1.School of Mechanical and Electrical Engineering，Hohai University，Changzhou 213022，China；2.Wuhan National Laboratory of Optoelectronics，Huazhong University Of Science And Technology，Wuhan 430074，China）

Online analysis system for short-cycle arc stud welding

BAO Yefeng1，LIAO Hailong1，2，ZHANG Jinhui1，ZHANG Jingzhang1，YANG Ke1，JIANG Yongfeng1

（1.School of Mechanical and Electrical Engineering，Hohai University，Changzhou 213022，China；2.Wuhan National Laboratory of Optoelectronics，Huazhong University Of Science And Technology，Wuhan 430074，China）

Due to the high production efficiency and automation applications，short cycle arc stud welding is widely used in automotive industry.A system composed of signal acquisition unit and analysis software was developed in this paper.The signal acquisition unit consisted of PIC18F4580 and circuits of power supply，signal conditioning，external storage and RS485 communication，and the signals of welding current and voltage were gathered，stored，and transferred by software of the AD sampling，data storage and communication respectively based on the judgment of welding state.The interface of the system was programmed by Microsoft Visual Studio C#，which included the main window，welding process analysis window，penetration threshold settings window and alarm window.DP600 galvanized sheet trial showed that the short cycle arc stud welding analysis system could extract the characteristic values of the welding process，judge the stability of the arc. Penetration calculated by the system coincided well with actual values.Online monitor conclusion agreed well with the results of visual inspection，macro examination and torque test.The data collected by the system could be used to identify the causes of poor quality.

stud welding；signal acquisition；quality monitoring；online judgment

0 Introduction

On the eve of World War II，in the aircraft carrier construction process，a large number of wood planking needed to be fixed on the steel plate.The traditional method was to drill holes in the steel，then fixed with the studs and nuts，which was a very slow and labor job.In order to improve the efficiency，a new and cost-effective method should be invented.In this case，arc stud welding was arisen.Arc stud welding did not need to drill in the steel，the stud was quickly welded to the steel surface within 1～2 seconds.Since the invention of arc stud welding，it has been developed rapidly and used widely，such as in shipbuilding，automotive，steel structures，boilers and other industries[1-2].At present，there are two basic types of arc stud welding，which are differentiated by the source of welding power.The arc energy delivered by the capacitors is known as capacitor discharge stud welding（CDSW）.The three modes of the CDSW are initial-contact，initial-gap，and drawn-arc.The weld cycle for these processes is from 4 to 15 ms and the welding current is from several thousands to ten thousands amperage.As the high density of arc energy，the CDSW is especially suitable for welding studs on the thin gauge materials.It has been widely used in kitchenware，Chassis，cabinets and other manufacturing.Electric arc stud welding includes two kinds of method，which referred as short-cycle and draw-arc stud welding.For draw-arc method，the weld time can be adjusted from 100 ms to 2.5 s，and a dedicated ceramic ferrule is used to protect the weld pool against atmospheric oxidization and contain the molten liquid metal in order to center the arc.This method is suitable for thick gauge materials to weld studs，and has been widely applied in the steel structure，boiler and pressure vessel，food machinery.Weld time of the short-cycle stud welding（SCSW）is less than 100ms，and the explosion effect caused by high current is used to protect the welding area.Sometimes shielding gas is also applied to protect the welding.Generally，the ceramic ferrule is not needed in the SCSW，which benefit for automate welding.Along with other advantages，such as high production efficiency，thin plate application and quality weld，the SCSW has been widely applied in the automotive industry[3].Usually visual inspection，torque，tensile and bend test are used to evaluate the quality of stud weld according to relevant codes[4-7].Destructive tests such as tension and bend test，are generally used for pre-production to qualify the procedure and sampling during production，which are not suitable for evaluation of each stud weld during the production.Although the visual test can inspect each of the stud welds in the production，but the inspector must carry out special training and should be qualified.Moreover，the products passed by visual inspection，could not guarantee that the results of tensile and bend test are acceptable.In some applications where the large number of studs are required to be welded quickly and soundly，urgently need a system to analyze and judge the quality of welds on line.Therefore，a SCSW analysis system was developed in this paper，which could analyze the stability of the welding process，calculate the arc energy，molten amount of Zn layer，penetration of weld and other parameters，judge the quality of weld based on the signals of welding current and voltage on real time.

1 Short-cycle stud welding process

The process of short-cycle arc stud welding can be divided into four steps:short circuit，pilot arc，main arc and plunge.Firstly，the stud inserted in the chuck of the gun is positioned against the workpiece.Then the welding power outputs 20～50 amperes current as the gun trigger is depressed，and the solenoid coil within the gun is energized at the same time that lift the stud off the work piece and the pilot arc is ignited.After the preset pilot arc time which is set by control unit，the main arc，whose current varies from 300 to 1 800 A depending on the size of the stud，is ignited and melts the end of the stud as well as the work piece to form a molten pool. Timing out the preset main arc time，the solenoid is de-energized.The stud moves to the work by the force of the spring in the gun.As the end of the stud contacts with the surface of the molten pool，the arc is extinguished and the stud continues moving in the molten pool，pushing away the liquid metal and forming weld at the bottom of the molten pool.

The pilot arc has two functions.One is to prepare for the main welding arc initiation and the other is to clean the surface of workpiece.The pilot arc has small current and low energy，which is not enough to melt a lot of workpiece and the stud，but can form an ionization space between the end of the stud and the workpiece to ensure that the main arc can be ignited successfully.In the meantime，the pilot arc can gasify the oil，decompose oxide on the surface of workpiece which can improve the quality of the weld.For the galvanized sheet，due to the low boiling point of Zn and easy gasification，the Zn vapor is easy to be trapped in the molten pool to form the voids after solidification.The pilot arc melts and gasifies Zn layer，lets it escape from the arc space so as to reduce Zn evaporation porosities in the weld.According to researches，whether the Zn layer can be completely evaporated by the energy of the pilot arc is a key factor to effect the number of voids in the weld[8].Therefore，the occurrence of weld voids can be monitored by calculating the energy and the amount of the melted Zn layerduring the pilot arc step.Because of the short weld time，large welding current and high energy density，the SCSW is suitable for sheet metal，and is not enough to form a droplet in the end of the stud.As a result，the waveforms of the current and voltage of the SCSW is relatively steady because of no droplet transfer.The average of current and voltage，and main arc time determine the depth of the molten pool，and ultimately affect the strength of the welding joint[9].In the plunge step，main current can be shut off at different moment.The SCWS is suitable for the stud with small diameter，as the low heat input，main current is applied in plunge step，known as hot plunge，to overcome the incomplete fusion defect.Therefore，the welding quality can be monitored based on whether the main arc is extinction，the input energy，melting depth of the workpiece calculated by penetration model according to the information of current and voltage of the main arc.

2 Structure of the SCSW analysis system

The SCSW online analysis system included the signal acquisition unit and personal computer（PC），as shown in figure 1.The signal acquisition unit was connected with the stud welding machine.The signals of welding current and voltage were collected in real time，and then uploaded to the PC through the communication port.The information of welding current and voltage was further analyzed and processed by the PC.Signal acquisition unit consisted of PIC18F4580 microcontroller and circuits of power supply，signal conditioning，external memory and RS485 communication.Thyristor controlled rectifier power supply had big capacity，high resistance to current impact，good control performance，high reliability，which was most used for stud welding.The current period of the thyristor controlled rectifier power supply was 3.3 ms and the frequency was 300 Hz.According to Nyquist sampling theorem，the sampling rate must be more than twice the maximum frequency component of the signal being measured.However，in order to show the signal feature better，generally，5 to 10 times of the highest signal frequency was selected as the sampling frequency.For thyristor controlled rectifier power supply，the minimum sampling frequency should be 1.5～3 kHz.The maximum rate of AD conversion of PIC18F4580 was 100 kbps. Considering the performance of MCU，sampling frequency was set to 5 kHz.The main arc time of the SCSW was less than 100 ms，plus the pilot arc time and the plunge time，the total welding period was less than 250 ms.As the AD of PIC18F4580 was 12 bits，one AD converting needed 2 bytes.A maximum data acquisition of once welding is about 5 kB.Obviously，the microcontroller’s on chip memory could not meet the requirement，an external memory，25LC1024，was used to store the data，which with a 1 024 kB serial reprogrammable flash memory accessed via a 1 Mbit SPI bus.As perfect performance of transmission rate，transmission distance and anti-jamming，the RS-485 communication interface was used between signal acquisition unit and PC.

Fig.1Online analysis system for the SCWS

3 Design of control software of the acquisition unit

The control software of signal acquisition unit was consisted of four modules:welding state judgment module，AD sampling module，data storage module and communication module.The welding state judgment module was applied to check the start and end of welding.The function of AD sampling module was to collect the signals of welding current and voltage.Whether the AD sampling module worked or not was determined by the output of the welding state judgment module.Once the welding state judgment module found that welding was triggered，the AD sampling module worked immediately until the end of welding.The data storage module transferred data from the internal memory to the external memory.Writing date to the external memory was timeconsuming for the microcontroller.In order to solve this problem，two equal size data buffer A and B were set in the RAM of microcontroller.The data from AD conversion were first saved to the buffer A.When the buffer A was full filled，the data in buffer A was packaged and written to the external memory，and the subsequent data from AD conversion were saved to the buffer B.Before the buffer B was full filled，all of the data in buffer A had been written to the external memory.After the buffer B was full filled，the data was also packaged and written to the external memory，and so on.At the end of welding，the communication module worked，the data in external memory was uploaded to the PC immediately.When the communication was finished，signal acquisition unit entered a standby state.During once welding，there were five states for data acquisition unit:standby state(Zero)，judgment state（State），AD conversion state（AD），Store state（Store），communication state（RS-485）.Figure 2 showed the alternation between the five states，"AD"and"State"，"Store"and"AD"were bidirectional switching，"Zero"to"State"，"Store"to"RS-485"，"RS-485"to"Zero"were unidirectional switching.

Fig.2Relationship of different states in data acquisition for welding process

4 Design of the SCSW analysis software

The SCSW analysis software was programmed by Microsoft Visual Studio C#.The software consisted of serial port data receiving and pre-processing，voltage and current data separation，waveform drawing，display adjustment，waveform characteristic analysis，data storage and quality evaluation，as shown in figure 3.The interface was designed as Windows style.

Figure 4 showed the main window of the software.There were the menu zone，waveform display zone，characteristic value analysis zone，display adjustment zone and serial port settings zone.The waveform display zone was used to display the waveforms of current and voltage of the stud welding.The amplitude，width，horizontal and perpendicular position of the waveforms could be adjusted with the function button in the display adjustment zone.The characteristic data column showed the characteristic values of the waveform，including the pilot arc current，pilot arc time，average main arc current，average main arc voltage，main arc time and plungetime.The serial port setting zone was used to set the parameters required for serial communication，such as serial port number，baud rate，data bit，stop bit and on/off switch.The"Hidden Grid"button in the menu was to activate or deactivate the grids in the waveform zone.The"Tools"menu contained："Import Waveform Data"，"Save Waveforms"，"Setting Thresholds"and"Penetration analysis"."Import waveform data"was to save waveform data into excel file，"Save waveform"to save the waveform into GIF graphics file."Set Threshold"was used to activate the penetration settings window，modify the penetration depth value.The"Penetration analysis"was used to activate the function of judging the welding quality with penetration.When this function was activated，the system evaluated the welding quality at the end of each welding process.If the weld was suspected，the alarm window would be popped up."Clear Waveform"was used to clear the waveforms displayed in the display zone."Measure"menu could activate or deactivate the 2 cursors for the waveform data measurement in the waveform display zone.The cursors had been activated in figure 4，the values of the waveforms of the cursors were displayed.The cursor could be moved by the control bar.With this function it was possible to easily read the values of welding current and voltage at the two cursors position and the time between them."Process Evaluation"was used to pop up the weld process analysis window.

Fig.3Diagram of the SCSW analysis software

Fig.4Main window of the SCSW analysis system

The SCSW process analysis window was shown in figure 5，which composed of the welding parameters，input parameters and results.The welding parameters included：current，voltage，time and energy of the pilot arc，whether the pilot extinction，current，voltage，time，energy and arc extinction of the main arc，and plunge time.Theseparameters were extracted and calculated from the waveforms of welding current and voltage.The input zone involved the diameter of the stud and the flange，the thickness of the Zn layer，the arc heating coefficient and the main arc heating coefficient.The data should be filled depended on the stud and sheet being used in welding. The heating factors of the pilot arc and the main arc had been preset to 0.42 and 0.65，which could be modified according to practice.The recommended main arc current and time shown in figure 5 were calculated from the empirical formulas based on the stud diameter and the flange diameter，as a reference for comparing with the actual welding parameters.The minimum energy required to melt the Zn layer was calculated from the preestablished thermo-mechanical model of the Zn layer melting，and the thickness of the Zn layer.The amount of the melted Zn was calculated by the pre-established formula，which included the measured energy of the pilot arc，the arc heating coefficient and the minimum energy required for the melting of Zn.The penetration depth was calculated from the pre-established thermodynamic model of penetration depth.The data of the SCSW process analysis could be saved in excel format.Press the"generate analysis report"button，the saved file name and path would be prompted.

Fig.5Stud welding process analysis interface

5 Trial of the SCSW analysis system with galvanized sheet

The studs were welded with ASW 850，which was developed by Changzhou Welding Technology Development Co.Ltd.for the drawn-arc and short-cycle stud welding modes.The rated current of the machine was 850 A.The closed loop circuitry regulated power output of the machine to adjust constant current for effects of cable heating and incoming power fluctuations，and infinitely adjustable time and current controls allowed for precise adjustment of weld parameters.A thermal-overload and phase fault detecting circuitry had been designed to protect system in extreme operating environments.The sheet used in the test was Zn galvanized DP600，which was a kind of advanced strength steel with tensile strength 600 MPa.Thickness of sheet was 1.5 mm，Thickness of Zn layer was about 25 μm.Before the test，the surface of the sheet was degreased.Copper plated Studs used in test was M6× 25 mm，with flange diameter 7 mm，material was 08A.Before welding，started the SCSW analysis system，the main interface shown in figure 4 would appear on PC screen.Set the serial port parameters，pressed the"Connect" button to activate the serial port.Opened the penetration threshold input window using"set threshold"menu under the“Tools”menu，input 0.7 mm as the minimum melting depth of the workpiece；Opened the stud welding process analysis window as shown in figure 5.Input stud diameter，flange diameter，galvanized layer thickness inthe parameter input zone.When welding，the signal acquisition unit would collect the data of welding current and voltage automatically.In the meantime，the data was stored in the external memory through the buffer.At the end of each welding，the data would be immediately transmitted to the PC，and used to display the waveform，extract feature of data，calculate the amount of melted galvanized layer and the penetration.The calculated penetration would be compared with the preset threshold of penetration.If it is less than the threshold，the alarm window would pop up.Figure 6 was the alarm window when the welding quality was suspect during the test.Figure 5 was the corresponding welding process analysis report.As shown the penetration depth calculated on-line was 0.5 mm，less than the set threshold 0.7 mm.It was judged that the quality of welding was not good.Figure 5 also indicated that there were extinctions both during the pilot arc and main arc，which agreed with the waveforms displayed in figure 6.

Fig.6Alarm window of rejected welding

The appearance of the welding specimen，which corresponds to figure 6，was shown in figure 7a.It indicated that the weld flash did not wet enter periphery of the stud.The macro section of the specimen was shown in figure 7b.There were several large voids in the weld.The penetration depth was 0.56 mm，which was close to the calculated value.Analyzing the waveforms in figure 6，the large voids in the weld should be deduced by extinction of the pilot arc.Because arc extinction would reduce arc energy to vaporize the galvanized layer completely，a lot of Zn vapor was trapped in the weld to form voids.The proportion of melted Zn by pilot arc was shown in figure 5.The less penetration was due to the main arc extinction caused by short circuit.Although the current did not change because of constant current output of the welding machine，but the voltage was reduced almost to zero when short circuit happened.This caused the low the heat input and the increasing heat loss through wire and workpiece.The consequence was that the molten pool become smaller than normal and got a shallower depth.The average fracture torque of specimens in the same group was 19.8 N·m，and the fracture located at the welded，which was rejected according to the standard[10].Obviously，the conclusion of the SCSW analysis system was supported by these tests.

As weld quality was monitored on-line by the SCSW analysis system，the signals for all weld in production were saved permanently at the same time，which could be used to identify the concrete reasons of poor quality. Figure 8 showed four kinds of waveforms while bad welds were detected by the SCSW analysis system.Figure 8ashowed that the pilot arc was extinct just after 10 ms，the man arc was not ignited at all.This may be caused that the lift was set larger than the distance that the pilot arc could maintain.Figure 8b showed that both the pilot arc and the main arc had extinction because of short circuit.This may be due to the stud was not inserted in position，or the stud was instable in operation.As shown in figure 8c，short circuit was observed in the pilot arc，and the main arc was ignited as the stud contacted with the workpiece.So the actual main arc time was less than the set time，and the arc energy was not enough to melt enough metal.The reason for this was that the lift is too small.In figure 8d，welding voltage was dropped about 8 ms，but the voltage was not reduced to zero.The voltage dropped，which reduced the energy of main arc and affect the quality of weld，should be caused by the droplet transfer.

Fig.7Appearance of the welding specimen，which corresponds to figure 6

Fig.8Four kinds of waveforms while bad welds were detected by the SCSW analysis system

6 Conclusion

（1）The SCSW has been widely used in the automotive industry as the features of short main arc time，large current，high energy density，suitable for sheet welding，high production efficiency，no ferrule needed，and easy to automate.The weld quality of the SCSW can be monitored at real time by analyzing signals of welding current and voltage.

（2）The signal acquisition unit of the SCSW analysis system consisted of PIC18F4580 and circuits of power supply，signal conditioning，external memory and RS485 communication.The sampling frequency was 5 kHz and the external memory capacity was 1 024 kB.The software of the unit consisted of four modules:welding state judgment，AD sampling，data storage and communication.According to the different states，the corresponding modules activated to collect，store or transmit the signals of welding current and voltage respectively.

（3）The software of SCSW analysis was programmed by Microsoft Visual Studio C#.The interface involved the main window，welding process analysis window，threshold setting window and alarm window.The waveforms of welding current and voltage were displayed.The average current and time of pilot arc and main arc，as well as plunge time were extracted from the signals.In the meantime，the energy of pilot arc and main arc，amount of melted Zn layer and the penetration depth were calculated.The quality of welding was monitored on the real time.The signals for all weld in production were saved permanently.

（4）Trial of DP600 galvanized sheet showed that the SCSW analysis system could extract the characteristic parameters from the welding signals and judge whether the welding process was regular.The penetration depth calculated by the SCSW analysis agreed with the value of the macro section.The results of visual inspection，macro examination and torque test of the unqualified samples detected on line were also rejected.The signals for all weld in production，which were saved permanently，could be used to identify the concrete reasons of poor quality.

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TG409；TG438

A

1001－2303（2017）01－0008-09

献

BAOYF，LIAOH L，ZHANGJ H，et al.Online analysis systemfor short-cycle arc stud welding[J].电焊机，2017，47（1）：8-16.

2016-12-21

包晔峰（1966—），男，江苏常州人，博士，教授，焊接设备及自动控制和焊接工艺与组织性能的研究工作。

10.7512/j.issn.1001-2303.2017.01.02