Weld pool feature extraction in double-arc Metal Inert Gas (MIG) welding

Design Team Members: Daniel Fernandes

Supervisor: Professor Jan Huissoon

Background

Arc welding is one of several processes for joining metals. By applying intense heat, metal at the joint is melted. After the metal cools and solidifies, a metallurgical bond is created. MIG welding, (Metal Inert Gas welding) is a form of arc welding uses a continuous spool of metal wire, to perform the welding. The wire feeds out the end of the welding gun, and a welding arc is formed at the gun tip using electric current. During welding (see Figure 1), an inert gas is used to shield the surrounding air from the intense heat of the weld arc.

components of the MIG welding tool with arrows and labels
Figure 1 – Essential components of MIG welding

There are two main parameters that affect the results of MIG Welding. The first is the heat level, which directly determines the required voltage to create the welding arc. The second parameter is the speed at which the wire is fed out of the welding gun. Often one parameter will affect the other, so they need to be adjusted during the welding process.

Project description

infared spectrum image of of weld pool being formed, in black and white
Figure 2 – Infrared spectrum image of weld pool being formed

The University of Waterloo’s Welding Laboratory contains a MIG Welding Robot that possesses two welding tips on the same robotic arm. During MIG Welding, each tip feeds a wire into the welding torch, and melts the wire, forming a hot liquid weld pool. This pool cools to form the weld bead, which consists of solid metal. Most of the visible features of the weld formation are clearer in the infrared spectrum, as seen in Figure 2, which represents one frame of the process. The dark regions surrounding the weld pool are the solid (colder) areas of the metal.

The purpose of this project is to provide numerical and visual measurement of the weld width, length and/or shape based on digital images of the welding process. The information obtained may eventually be used to control the welding process, or to suggest ways of improving welds, although this project will focus mainly on characterizing the features in the digital images.

Design methodology

Welding equipment
The setup must include several components, many of which are already available in the welding laboratory. Specifically, the welding robot will perform the weld by moving its robotic arm according to pre-programmed instructions. In addition, the robotic arm must be fitted with a MIG welding gun, and connected to the wire-feeding spool.

Capture equipment
In order to capture images of the welding process, the robotic arm must also be able to hold the camera that is used to film the welding. The camera must be retrofitted with an infrared filter.

Computer hardware
In order to store the images received from the video camera, a computer must be used in conjunction with a “framegrabber card”, which takes a video input and converts it to a series of still images. These images must be stored on the hard drive of the computer in order to be processed at a later time. They could potentially be processed in real-time.

step by step phases of the project with direction arrows
Figure 3 – project phases

Software
The software’s user-interface is to be written in Visual C++. The mathematical manipulation and feature detecting portion will be performed mainly in MATLAB, and will use various techniques to process the image in two stages: pre-processing and feature extraction.

Pre-processing
The pre-processing stage will make the image clearer and easier to quantify. This may involve some noise removal, in order to ‘clean up’ the picture and avoid false detections. In addition, techniques such as thresholding and edge-detection will be used to reveal important features in the image.

Feature extraction
Using pattern recognition techniques, the locations of features of the weld image will be mathematically computed and stored. Specific measurements would include a measure of the orientation, width and length of the weld relative to the camera’s frame of reference. In addition, it may be possible to fit a curve to the boundary of the weld pool.