The reliability of resistance welding quality is due to the precise control of equipment

The reliability of resistance welding quality is due to the precise control of equipment
Zhang Huajian Zhou Liangyu

(Beijing Lianxinbang Machinery Co., Ltd., Room 102, Room 706, No. 10, Baifuquan Road, Changping Science and Technology Park, Beijing 102200)

Abstract: This paper outlines the new requirements for the quality control of electric resistance welding in modern aircraft. Taking the products of BRITISH FEDERAL as an example, the method of precision control of electric resistance welding is discussed. Some results of some welder process tests are exemplified.

Summary: Summary on the new requirements of resistance welding quality control for modern aero components; Detailed descriptions has been given to the means of precision resistance welding quality control based on the features of British Federal aero type DC machines; Typical welding sample lab test results will Be given in this literature.

Keywords: aircraft resistance welding quality precision control

0 Preface

With the rapid development of space technology, materials science, microelectronics and computer technology, military and civil aviation and spacecraft, such as satellite and Manned Spacecraft vehicles - rockets and propulsion rocket engines, The performance of missiles, aircraft and their respective engines (hereinafter collectively referred to as aircrafts) for various purposes has made great progress, and the technical standards for structures, materials and their adoption have undergone major changes. One of them is to improve the maneuverability, try to reduce the take-off weight, increase the range, replace the conventional materials with high-strength super materials, reduce the self-weight, and increase the push (force) weight (quantity) ratio. Today's advanced aircraft, without exception, widely used alloy components of high-strength aluminum alloy, titanium alloy super high-alloy, martensitic stainless steel, high-strength alloy structural steel is a composite material. As a thin-walled aircraft, resistance welding is still one of its main methods of connection. However, due to the thinning of the wall thickness, the weldability of the material is deteriorated, so that the range of process parameters available for resistance welding is small, and the quality standard of acceptance is greatly improved, which requires precise control and monitoring of the process to ensure the quality of welding. For the purpose of reliability and safety of welding parts, BRITISH FEDERAL, Sciaky (USA) and other world-renowned resistance welding machine manufacturers have developed low-inertial wear-resistant linear rolling guides used in aviation and aerospace fields at the end of the 20th century. The double-acting membrane cylinder pressurization system with programmable current and power curve, good millisecond response speed, computer controlled electric resistance welder with precise control and monitoring of process parameters, proven by many countries, their products can fully meet the requirements of various countries. Quality and product safety requirements as defined by military standards such as MIL-W-6858D.

1 BRITISH FEDERAL Product Features

BRITISH FEDERAL is a specialized company of resistance welding machines integrating research, development, production and sales. Mainly for the world aerospace, weapons, automotive, barrel packaging and other industrial fields, providing advanced three-phase secondary rectification and intermediate frequency inverter two major types of DC resistance welding machine.

In response to the continuous improvement of the technical performance, structure, materials, quality standards and management systems of modern aerospace products, we continue to innovate and provide the world's advanced resistance welding machines, including spot welders, to users in the aerospace industry. Longitudinal seam welding machine, circumferential seam welding machine, or vertical and horizontal seam welding machine, according to user requirements, can also provide spot welding, vertical and horizontal seam welding three-in-one replacement of convenient resistance welding machine.

They have many common characteristics, and their advanced nature mainly includes:

・ The rigidity of the fuselage is good. The welding head is designed with low inertia and vertically transported as a light alloy structure with a preloaded linear spherical rolling rail. The slider is operated by the double-acting membrane cylinders of the upper and lower chambers. The membrane cylinder is directly mounted above the rail assembly, which can provide 7~12mm welding working stroke, and 0~150mm auxiliary with stepless adjustment by manual or air motor. stroke.

• WS4000A series control system with 32-bit microprocessor computer control for full process monitoring, it can store 128 sets or multiple times of welding specifications. Can control spot welding, continuous spot welding, seam welding, rolling spot welding and other modes of operation. There is no deviation from the time parameter setting, and secondary closed-loop constant current control and monitoring can be performed for each segment of the welding pulse (1-16 segments).

Constant current control is a necessary condition to ensure aerospace products, such as network voltage fluctuations, workpiece thickness changes, material and surface state changes, etc., will affect the welding terminal voltage, but requires a constant welding current, at this time the sensor works.

Usually, the constant current control is monitored by the last 1/2 cycle, that is, every half wave is sampled several times, the signal is restored by integration, and the CPU is input through A/D conversion to obtain the effective value of the half wave current, and The set current rms value is compared, and according to the difference between the two, the next 1/2 cycle is automatically corrected to maintain a constant current. If the primary voltage fluctuates within ±15%, the secondary constant current accuracy of ±2% of the full range can be achieved by the secondary closed-loop constant current control.

・ Developed the WS98-4000 programming software that can be upgraded at the right time. The welding parameters are fully flexible.

・ Configurable industrial Pentium PC and 10.4" color touch screen display, showing the actual pressure KN value per half cycle (accurate to X.XXKN) and the actual current KA value (accurate to X.XXKA), and simultaneously display the pressure and current curves There is a 6GB memory hard drive with RS232 print interface and Ethernet interface.

・ It can store up to 100,000 solder joint current and pressure data in the hard disk for inquiring at any time, and the quality is highly traceable.

・ The WS4000A control system has a strong self-diagnosis function that can provide a large amount of data, such as input and output conditions, microprocessor status, voltage, frequency, pressure, and so on.

・ A pneumatic pressurization system with extremely fast response and programmable speed is configured. The welding and forging pressure values ​​and their curve shapes can be arbitrarily programmed according to the process requirements. The pressurization system has a high-precision pressure load sensor that has been calibrated by the metrology department. After A/D conversion, the actual pressure value is displayed in real time every half cycle. The pneumatic system is configured with two programmed, MS speed proportional auto-regulation valves. When welding high-resistance or crack-prone alloys, the pre-pressure and forging pressure can be provided in time for 20ms.

・ For seam welding or rolling spot welding, the welding speed adopts the encoder as the detecting component, precise servo drive closed-loop control, and can be arbitrarily programmed according to the process requirements.

・ The transformer, thyristor and silicon rectifier components of the main power circuit are equipped with a water-cooled constant temperature self-protection system.

The above characteristics of the equipment are sufficient to prove that all process parameters (such as time, current, pressure, welding speed, etc.) are in the process of precision control and monitoring during the welding process, which can fully meet the quality requirements of modern aircraft resistance welding.

2 IF inverter DC resistance welding

In recent years, BRITISH FEDERAL has developed a MF-PAK intermediate frequency inverter with a control range of 50-5000A, which can be matched with a resistance welding machine with a maximum power of 500KVA to realize intermediate frequency inverter DC resistance welding. The control system used can be the advanced WS4000A series or the WS700ms type controller.
The principle of the IF inverter DC resistance welding machine system is shown in the left figure below, and the waveform is shown in the right figure below.

Although the price of the main components in the IF inverter DC resistance welding machine is still expensive, it is still rapidly entering the practical application field because it has a system advantage:

・Three-phase load balancing; transformer size is small, input power is low; capacitive load, power factor up to 0.9, energy saving.

・Because the frequency is as high as 1000-1200Hz, the secondary current output capability is strong, and the waveform is straight, and the range of nucleation size stability is expanded. There is almost no splash, and the current in the primary stage of the welding is naturally increasing, and the surface quality of the solder joint is good.

・Because of the high IF frequency, the constant current response speed is fast and the response speed is in μs.

• Extends electrode life because there is no significant peak current and splash.

3 BRITISH FEDERAL equipment stability debugging

According to international practice, before the welding machine is officially put into production, the buyer and the seller will also carry out the final delivery acceptance at the user site. In addition to the necessary retesting of the performance indicators and technical parameters, an important part is to use the test piece or the analog part for the process. test. The quality acceptance criteria used are usually based on the quality requirements of Class A welds in the US Military Regulations MIL-W-6858D "Resistance Spot Welding and Seam Welding". This standard is currently the most advanced military standard in the world, and the inspection items are complete. , advanced and reasonable.

Of course, for those conventionally used piston type pressurized systems, the gas path response speed is slow or the response timing is not accurate, and the low-inert frictionless linear rolling guide is not equipped with a resistance welding machine. Although the control system is modified by a computer, the acceptance according to the standard is not Easy. According to incomplete statistics in the previous three years, China's aerospace industry introduced 31 sets of such equipment, of which 18 units were imported from the Commonwealth Company, accounting for 58%, and it was delivered once, and the qualified rate of production was 100%.

For the spot welder, the most difficult to achieve is the crack strength test of the 100 pairs of crack-sensitive alloys, especially their consistency indication. Table 1 below illustrates some of our debugging results.

4 Optimization of welding process parameters

A resistance welding machine that accurately controls and monitors the welding process and its process parameters provides a major means for product quality reliability and safety of use, but it is still important to properly set and optimize the process parameters. In particular, modern aircraft puts higher demands on the manufacturing process, and a lot of crack-sensitive materials are selected, and the combined thickness difference is large. At the same time, such welding machines are expensive to purchase, and it is impossible for enterprises to configure equipment of various capacities as needed. It is often necessary to use medium power (150-200KVA) to weld martensitic stainless steel with thickness of 0.3-0.5mm and seam welding 3.0. High-strength martensitic low-alloy steel of mm, in these special cases, the parameters are not optimized and it is difficult to meet the standard requirements. During the commissioning process, we have observed an example: 0.3mm 403 martensite stainless spot welding, the electrode pressure increased from 2.50KN to 2.71KN, and the dissolution rate was 60%. Down to 48%, the nucleation diameter is widened from 2.03mm to 2.58mm. It can be seen that the parameters change slightly, resulting in a significant change in weld quality.

5 Our advice

In view of the emerging new situations and high requirements, in enterprises with many resistance welding products, it is recommended to invest a certain amount of capital and manpower to carry out computer optimization design of the resistance welding process, and it is absolutely necessary to establish an expert system and database for the entire industry. Because the CNC resistance welding machine with precise control and monitoring is no longer a state in which the parameters of the welding machines are not common, but can be stored for a long time, except for the special size and shape, the results can be shared. .

Welcome aerospace welding experts to visit us at the 9th Beijing Essen Welding Exhibition Booth: Hall 9 9527-9528. Discuss the quality control of aviation resistance welding, equipment function and process optimization.

About the Author:

Zhang Huajian was responsible for the market representative of BRITISH FEDERAL Beijing Office of the British Federal Company from 1993 to 2004. She is the general manager of Beijing Lianxinbang Machinery Co., Ltd. and continues to work as a technical consulting service for resistance welding equipment;

Zhou Liangyu, male senior engineer, the fifth director of China Welding Association, is currently employed by Beijing Lianxinbang Machinery Co., Ltd. Senior Welding Process Consultant