Quality assurance in resistance welding
Quality assurance measures for resistance spot , projection and seam welding should ensure that the expected connection quality is achieved through the welding process. The basics of quality assurance are presented in EN ISO 14554 and in DVS data sheet 2915. The EN ISO 14554 describes the required quality assurance measures, the DVS leaflet 2915 their process.
conditions
Quality assurance encompasses all the requirements for the design, the material, the manufacturing process and the personnel in order to achieve the expected connection quality. An essential prerequisite for the quality of the connection is the weldability , which depends on the weldability of the material, the welding reliability of the construction and the weldability of the production.
construction
The construction should be "weldable" and proof of sufficient load-bearing capacity must be provided by calculation and testing. The required overlaps and flange widths must be available. A shunt due to the structural design and the devices and tools must be excluded. Depending on the requirements, the security classes and the execution conditions must be specified. The welding device and the welding tool must guarantee the safe execution of the welding process. The permissible gap widths must be able to be adhered to. The production processes upstream of welding (punching, bending, pressing) must be coordinated with the subsequent spot welding.
material
The materials to be welded must be suitable for this. EN ISO 18278-1 defines the weldability of metallic materials by the ability:
- to make the weld continuously;
- to withstand the operational loads that arise.
Certain physical material properties are particularly important for weldability:
- same or close to each other melting temperature,
- low electrical conductivity,
- low thermal conductivity,
- high deformability.
production
Welding equipment
conditions
In the manufacturing process, the welded connection is created by the action of the parameterized welding device on the workpieces to be connected. In order to achieve the desired quality, the welding equipment must be suitable for the welding process. This applies to the mechanical structure as well as the controls and power units . The devices and welding tools must be set up appropriately and the welding settings appropriately set. These requirements can only be met if the welding equipment has sufficient adjustment reserves (electrode force, follow-up behavior , welding current).
Set up and adjust
If welding parts are being produced for the first time or if production is restarted, the machine and control must be set up. For mass production, the devices or welding tools must be exchanged and their cooling system connected. For large tools, care must be taken to ensure that the current is distributed evenly (e.g. for extended projection welded joints). Different immersion depths of the device or the workpiece change the impedance of the welding machine and thus the welding current. This must be taken into account when setting the parameters. The welding parameters must be set depending on the welding task .
Monitor
During the production time, the entire production system must be monitored to ensure that the target quality values are maintained. In the event of deviations, you must react in good time. The causes of the quality deviation must be found and eliminated, be it within the welding production or upstream production processes.
staff
Quality assurance should be placed in the hands of a responsible supervisor who will advise on the choice of materials and the weldable construction. The selection of welding equipment, devices, tools and electrode materials is one of the tasks of the supervisor. She is also responsible for the proper functioning of the welding and monitoring equipment , for optimal parameter setting of the devices and for compliance with all technological specifications as specified by him in a welding instruction. These tasks can only be carried out with the appropriate qualifications.
Quality features and quality assurance in production
Quality features
Depending on the component requirements, different criteria are defined for the weld seam quality. Compliance with these requirements must be proven. Using equipment for online monitoring is a cost effective practice.
Quality assurance
Levels of quality assurance
According to DVS leaflet 2915-1, quality assurance must be carried out in four stages:
- Setting up the welding equipment with all devices and tools and setting the welding parameters according to the results of welding tests and destructive testing of the welded samples or components.
- Welding tests on the component or on special test pieces typical for the welded connection on the component. Destroying the first components and determining the dimensions of the
Welds and / or the load-bearing behavior according to the specified criteria; Setting up and calibrating online monitoring facilities .
- Release of production.
- Monitoring of the production depending on the prescribed safety class
- Safety class A: random sampling and additional monitoring of the welding conditions by measuring and / or process monitoring devices .
- Safety class B: Random sampling (sample type, component testing, test criteria, boundary conditions, test execution and sampling frequency depend on the product and the welding conditions). Possibly additional monitoring by measuring and / or process monitoring devices.
- Safety class C: spot checks only after changing electrodes.
The results of the monitoring should be documented and statistically evaluated in order to initiate timely measures if the tolerance limits are exceeded.
Quality control loops
Quality assurance and monitoring is often organized in quality control loops within welding production. The picture opposite shows a quality assurance concept for the manufacture of body components with three quality control loops (QC).
- QRK 1:
The first process control loop is close to the process and comprises the technical controllers of the welding equipment including process stabilization through constant current control (KSR) and intelligent controllers . This is to ensure an optimal welding process. Maintenance at the production or assembly workstation ensures trouble-free production. Observation of the welding by the operator and visual assessment of the welding result are an initial quality assessment directly at the production workstation. If malfunctions occur, these are eliminated by the operator or a maintenance engineer. The flow of parts is usually only marginally restricted and only when process disruptions occur. A direct quality statement for individual welded connections can only be made through online process monitoring .
- QRK 2:
Within the second quality control loop, the quality of welded joints is randomly checked at separate test workstations after leaving the production workstation. In the event of quality complaints, the flow of parts is interrupted. Causes of poor quality must be identified and remedied. This can lead to longer interruptions in production, since a larger quantity of parts is already affected and must be checked back by the time the malfunction occurs.
- QRK 3:
In the product control loop, the entire product that has passed through various production / assembly workstations is subjected to an overall check.
All control loops provide information for improving upstream processes.
Individual evidence
- ↑ EN ISO 14554-1 "Resistance welding of metallic materials" Part 1: "Comprehensive quality requirements " ; Part 2: "Elementary quality requirements"
- ↑ a b c d DVS - German Association for Welding and Allied Processes eV: Quality assurance for spot, projection and roller seam welding , DVS Leaflet 2915-1, 1979.
- ↑ Resistance welding. Weldability. Part 1: Assessment of the weldability for resistance spot , roller seam and projection welding of metallic materials , EN ISO 18278-1: 2004
- ↑ a b Haepp, HJ; Hopf, B .: Requirements for a future quality assurance system for laser welding with the remote welding process , Laser Magazin, 23 (2006) H. 4