Research on the latest welding thermal cycle test

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Research on a new welding thermal cycle test and analysis system

Abstract: a new welding thermal cycle test and analysis system is studied and developed by using computer software, hardware technology and data processing method. The system integrates the functions of welding thermal cycle temperature data acquisition, storage, processing and analysis. It can test and calculate welding thermal cycle parameters and phase change temperature platform in the process of welding cooling

key words: welding thermal cycle; Tester; Phase change temperature platform

welding thermal cycle curve contains important welding cooling process information such as welding joint temperature change and cooling phase change, which is of great significance for understanding welding cooling phase change process, joint structure, stress deformation and improving welding quality [1]. At the same time, welding thermal cycle parameters are important data for analyzing the microstructure and properties of welding heat affected zone (HAZ), and also an important basis for formulating, evaluating and optimizing welding processes. Therefore, the measurement, calculation and analysis of welding thermal cycle have important theoretical significance and practical value

the traditional method of testing welding thermal cycle is to use thermocouple and X-Y function recorder. This method has poor real-time performance and low accuracy. At the same time, it is difficult to obtain the thermal cycle curve with obvious characteristics of cooling phase change process due to the influence of mechanical inertia. In addition, the main parameters of welding thermal cycle can also be calculated by using the mathematical model derived from welding heat transfer, but this calculation process is cumbersome and has large errors, so it is difficult to obtain accurate data. The rapid development of computer software and hardware technology provides a new method for welding thermal cycle testing. In this study, computer software and hardware technology and data processing methods are applied to the development of welding thermal cycle test and analysis system, and the analysis and calculation of thermal cycle parameters are deeply studied. A variable length sliding window algorithm is proposed to find out the phase transformation point in the process of welding cooling. The system uses Borland c++builder and MS Access as development tools. The tool provides a friendly man-machine interface and has complete functions of collection, processing, calculation and analysis of welding thermal cycle parameters. It can perform operations such as thermoelectric potential data conversion, thermal cycle curve drawing, local curve amplification, primary and secondary difference analysis, numerical calculation of thermal cycle parameters, and also establishes a basic welding database, Meet the retrieval of relevant data

1 overall system design scheme

the system is divided into two parts: hardware system and software system. The overall system model is shown in Figure 1. The system has the following characteristics:

a. application oriented

the system collects the thermal cycle data from the welding site. Through theoretical analysis, the structure and performance of the joint can be judged. At the same time, by calculating the thermal cycle parameters, the welding process can be formulated and optimized to guide the welding production. The whole process is application-oriented, forming a closed loop of test → theoretical research → production application → test

b. it is open and extensible

the data acquisition part of the system is universal. As long as different data analysis program interfaces are designed for different fields, the system can be directly applied to collect digital signals and carry out correlation analysis and processing

1.1 system hardware design

the hardware part of the system consists of welding equipment, thermocouples, integrated amplification circuits, a/d interface boards of filter devices, microcomputers, etc. Welding equipment is the supplier of welding heat source; Thermocouples are selected according to the test accuracy and temperature range. Nickel chromium nickel silicon (K-type) thermocouples are selected for the system; The amplifier circuit has been integrated on the a/d board; A/d board selects a 12 bit a/d isa plug-in interface board; The microcomputer is a common compatible PC with a CPU frequency of 433MHz and a memory size of 64M

1.2 system software design

the system software includes four modules: Welding basic database module, welding thermal cycle data acquisition module, welding thermal cycle data processing module, and welding thermal cycle parameter simulation calculation module. The main interface and menu of the system are shown in Figure 2

the basic welding database module is composed of 7 data tables, including thermocouple, base metal type, base metal composition, welding material, welding method, test parameters, thermocouple temperature graduation, etc. The system connects with the database through ODBC interface. The main function of this module is to input, modify, delete and search data in each data table to obtain relevant welding information

the main function of the welding thermal cycle data acquisition module is to convert the thermoelectric potential difference analog signal of the thermocouple into digital signal and store it in the microcomputer hard disk. The module is in the pending service state after operation. Once the temperature reaches the set self starting value, it will immediately send the control word to start a/d conversion. The obtained thermoelectric potential data will be stored in the data after filtering and temperature compensation, and the working files of packaging waste treatment and resource recycling will be grasped. When the temperature is cooled to the self terminating value, the data acquisition program will be in the pending state again, and finally exit the program under the user's instruction

the welding thermal cycle data processing module is mainly used to display the welding thermal cycle curve, calculate the thermal cycle parameters and calculate the parameters of the cooling phase transformation process. It first obtains the test parameters of the data to be analyzed and the graduation table of the thermocouple from the database, then converts the thermoelectric potential difference into the temperature by looking up the table, calculates the primary difference and secondary difference of the curve according to the needs, and draws them together with the thermal cycle temperature curve, difference curve and data grid coordinate axis, and finally calculates the thermal cycle parameters such as T85, phase change temperature point and phase change temperature time, which are given in the form of test analysis report

the simulation calculation module of welding thermal cycle parameters is mainly used to calculate welding thermal cycle parameters by formula method, line diagram method or artificial neural network method according to the welding test parameters input by users. In this paper, the principle of data processing module and its application in the study of welding cooling phase transformation are introduced

2 design of data processing module

2.1 differential analysis of thermal cycle curve

theoretically, the welding cooling phase transformation point, that is, the phase transformation temperature platform, should be visible on the welding thermal cycle curve. Because in the continuous cooling process, the phase change causes the change of heat, which is shown as the change of temperature platform or curve slope on the thermal cycle curve. However, for the transient phase transition, the phase transition temperature platform is not obvious because of the small amount of heat change and the accuracy limitation of traditional test instruments. If the temperature time curve t=f (T) is differentiated, a better result can be obtained. For the continuous time signal ya (T), the difference form is as follows:

for the discrete time series y (n), the value interval of N and N is 1, and the first-order backward difference form is:

y (n). The second-order backward difference form is:

according to equations (2) and (3), the first-order and second-order difference calculation of the collected discrete welding thermal cycle temperature data series can obtain the curves dt/dt=f'(T) and d2t/dt2=fn (T)

2.2 calculation of phase change temperature platform by variable length sliding window algorithm

at present, most of the research on the influence of welding thermal cycle on weld microstructure is carried out on the thermal simulator [3]. On the thermal simulator, the thermal cycle curve drawn by X-Y recorder makes the temperature platform not obvious, sometimes even not at all, because of accuracy, noise, mechanical inertia lag and other reasons. Using the current computer software and hardware technology, not only the temperature data of welding cooling process with sufficient accuracy can be collected, but also the implicit phase change information can be found by analyzing the temperature curve

in this system, for "$$curve and dt/dt=f'(T) and d2t/dt2=fn (T) calculated by software system According to the inflection point on the curve and the change of curve slope, the welding cooling transformation process can be determined. In order to deeply and carefully analyze the solid-state phase change process of welding cooling, the system has designed a variable length sliding window algorithm to calculate the phase change temperature platform parameters. Its principle is shown in Figure 3

the temperature values are compared point by point from the peak temperature Tm of the welding thermal cycle. If there are k consecutive temperature data (k is called the length of the sliding window, corresponding to the width of the sliding window in the figure) that remain the same or within a certain allowable range (set the threshold of this temperature range as R, corresponding to the height of the sliding window in the figure), it can be preliminarily judged that the k data is a temperature platform. Then continue to increase the K value until the temperature difference of the k data exceeds the threshold R, so as to determine that the temperature is the phase change temperature TX, and the time length occupied by the k data is the phase change time TX. Accordingly, the TB in Figure 3 and the upward increase in the reliability level of one cycle compared with one cycle product is the phase transition temperature, (te-ts) is the phase transition time (TS is the phase transition start time, Te is the phase transition end time)

3 system test test

in order to verify the functions of the hardware and software of the system, the welding HAZ thermal cycle test was carried out. 250mmx150mmx30mm 15MnMoVN low alloy steel plate is selected for plate surfacing in the test. The main test parameters are set as follows: welding current 170a; Welding voltage 27V; Welding speed: 2.5m/min; The sampling frequency is 10Hz. This system is used to process the test data. The results are shown in Table 1. The obtained thermal cycle curves are shown in Figure 4 and figure 5:

from the calculation results in Table 1, the upper limit temperature can be set. It can be seen that this system can not only accurately calculate the main parameters of the welding thermal cycle, but also calculate the temperature and phase change time of the phase change temperature platform, which is of great significance to the study of the phase change process of welding cooling

in Figure 4, there is no obvious difference between the thermal cycle curve measured by the system and the curve measured by the traditional X-Y recorder. However, in Figure 5, after analyzing the obtained curve with the variable length sliding window algorithm and local amplification, it can be observed that there are two very obvious temperature platforms in the medium temperature stage during the welding cooling process (i.e. at 400 ℃ and 340 ℃), However, it is difficult to find such a temperature platform with the traditional thermal cycle test method. The system can also be used to analyze other temperature sections of interest on the curve, so as to further obtain other useful information implied in the curve. This provides a new test method and approach for studying the phase transformation mechanism during welding cooling process, studying the microstructure and properties of HAZ, and developing steels and alloys with good weldability

5 conclusion

a. the new welding thermal cycle test and analysis system integrates the functions of welding thermal cycle temperature data acquisition, storage, processing and analysis. It is not only convenient for in-depth and detailed analysis of the data, but also can reveal the phenomena that are difficult to observe with traditional test methods

b. design a variable length sliding window algorithm to analyze and calculate the welding thermal cycle temperature data. For the first time, a welding cooling phase change temperature platform with obvious characteristics is obtained and the parameters of the phase change temperature platform are calculated automatically. This provides a new method and basis for analyzing the welding cooling phase change process and the microstructure and properties of HAZ. (end)

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