Hitoshi Matsui

Reduction of blowholes in high-speed arc welding of hot-dip galvanised steel sheets

Summary This paper describes an investigation of the blowhole formation mechanism and the substances causing blowholes during high-speed arc welding of galvannealed steel sheets extensively used in the chassis components of motor vehicles. Based on these results, it clarifies the effect of the shielding gas composition on suppression of zinc vaporisation and also examines suitable preventive measures. The results obtained may be summarised as follows: Provided the organic substances adhering to the base material amount to around 1 mg/cm2, they are not the main cause of blowhole formation. Even after thorough degreasing in such a way that the amount of organic substances present is equivalent to less than 1 in 500 parts of the zinc coating weight, numerous blowholes are formed to much the same extent as those found in the base material before degreasing. The gas collected through the blowhole zone being broken apart reveals its main components to be hydrogen and hydrocarbons, including methane. Hydrogen is...

Reduction of blowholes by vibration of the molten pool in arc welding of galvanised carbon steel sheet

Summary This paper examines measures to reduce the number of blowholes formed through vaporised zinc gas bubbles infiltrating into the molten pool through the weld gap in pulsed arc welding of galvanised carbon steel sheet with consumable electrodes by the hot metal being induced to flow along the molten pool base. Longitudinal vibration of the base metal by 1 mm during the welding process in such a way as to produce back and forth flow of the hot metal along the molten pool base leads to a significant reduction in the number of blowholes. The optimum vibration frequency giving the minimum number of blowholes is 30 Hz. This frequency is assumed to correspond to the natural vibration frequency depending on the size and physical properties of the molten pool, etc. To exert the same effect as described above by welding current control, the paper also describes an investigation of the molten pool being vibrated longitudinally by a repetitive current waveform sequence of a high peak/short time pulsed waveform ...

Detection and amplification of the molten pool natural oscillation in consumable electrode arc welding

In the pulsed MAG welding with highly precise periodic drop transfer, welding current was analysed by fast Fourier transform (FFT). By this analysis, peculiar frequency of the molten pool was detected by resonance phenomena with the base plate vibration and it was confirmed that peculiar frequency falls according to growth of the molten pool. Furthermore, natural frequency was detected at the natural welding state that is without vibration of the base plate and confirmed by the resonance method with the base plate vibration. To amplify natural vibration of the molten pool, drop transfer pulse was modulated consisting of regular pulse peak current and higher pulse peak current. The higher pulse peak current makes a change in the arc pressure and the molten pool is pushed down in the period of higher pulse. Because short circuit is apt to occur at the time that the molten pool is at the top of the front side, so it is effective for amplifying the molten pool oscillation that higher pulse phase begins at this time instantaneously or somewhat later. According to this, higher peak was gained at peculiar frequency by FFT analysis. It means that amplifying of the molten pool oscillation was successful. Appropriate period of higher pulse for maintaining stable oscillation is from about 1/4 to 1/2 of the oscillation cycle.

Reduction of spatter in high-speed pulsed MAG welding

Summary Undercut and/or humping are generated when the arc length in high-speed pulsed MAG welding is too high, and spatter is generated if it is too low. Refinement of the droplets from the wire and simultaneous maintenance of one droplet transfer per pulse avoid short-circuiting in a short arc and enable spatter and highspeed/high-efficiency welding to be rendered compatible with each other. The requirements are to maintain a stable droplet shape during one droplet transfer per pulse, to maximise the welding speed limit without spatter generation, and to develop a technology for arc length shortening at the limit of short-circuiting by wire droplet refinement as objectives basically centred on pulsed MAG welding. This paper describes an investigation of the factors controlling droplet transfer through a division being made into the driving force imparted to the droplets and the deformability of the weld metal receiving this force as well as a method of droplet refinement based on resolution of these asp...

Fundamental study of an arc welding procedure for hot-dip zinc-coated steel sheet

Summary Arc welding of hot-dip zinc-coated steel sheet faces a number of problems, such as the generation of spatter, blowholes, and pitting, through the zinc being vaporised during the welding process. To solve these problems, this paper examines the method of zinc vaporisation being positively promoted both in front of the weldpool and in the joint gap. This exploits the property of easy zinc vaporisation in an ordinary single electrode process which is extensively applicable in practice. The results suggest that, to reduce spattering, it is effective to perform high-speed welding at a relatively high current as well as to avoid short-circuiting through the droplets from the wire being refined using a pulse current waveform with a high peak current and short peak time. Both the welding current and welding speed have little effect on the number of blowholes generated. To reduce blowholes, the general rule is that the application of pulsed MAG welding with a high arc time ratio is more advantageous than s...

Control of wire tip deflection in a consumable electrode type arc welding torch

Summary The fundamental requirements for torch construction have been clarified as follows: to provide the specified curvature for a wire and to be able to control its rotation at the same time, so that the accuracy can be improved when aiming at a target position in a welded joint. In order to provide a set curvature for the wire of 1.2 mm diameter, it is necessary to provide the torch with both a radius of curvature of 90 mm or less and a curve with a bend angle of 75° or larger. When the wrist (namely the secondary, axis) of a robot is twisted, it is difficult to restrict the rotation of the wire inside the torch only by selecting both a simple curvature and a bend angle for the curved torch. The rotation of the wire tip can be restricted by providing a wire path with two curves whose centres of curvature are in opposite positions to each other. By using a wire reforming type torch such as the above, even when the bend direction of the curvature of the wire supplied changes and/or when a twisted wire i...

Study on automatic removal of spatter from an arc welding torch nozzle

Summary A method to automatically remove the spatter which sticks to the torch nozzle during GMA welding using magnetic electrode wire without touching it has been developed. By selecting a suction method with magnetic force which does not require maintenance work on both the torch and the removal equipment used in various non-touching methods, the countermeasures for decreasing the sticking force of spatter, the removal mechanism and a suitable removing force for it have been examined. With a carbon nozzle, the sticking force of spatter is less and is stable for a long time. With a silicon nitride nozzle, the sticking force of spatter is unstable even at the initial stage and there are some cases where it increases to about ten times that of the carbon nozzle. With a copper nozzle, the sticking force of spatter is less at the initial stage, but after use for a long time, it becomes impossible to remove because it has fused with the nozzle and stuck firmly. The impulse and peak value of the suction force ...

Abrasion phenomena of the contact tip in consumable electrode arc welding

ABSTRACT In GMA welding, presently, fluctuation of the tip of welding wire has been able to restrain precisely, and metal transfer also has been able to regulate periodically, that is stable arc-like TIG welding has been realized. Then at this stage, reduction of contact tip abrasion is the most important subject to maintain arc stability for a long time. In this investigation, many factors are evaluated with abrasion amount, and new material of contact tip is recommended. Results obtained are as follows. Using uncoated welding wire increases contact tip abrasion rapidly as to Cu-coated welding wire. On the contrary, other welding conditions concerning contact point have almost no influence. Under the same current, the abrasion of contact tip is reduced accordingly as welding wire feeding rate becomes higher. Therefore, the abrasion is accelerated violently at the time of arc ignition because of heat accumulation at the contact point by wire stopping and high current which is needed for arc ignition. That is the welding wire which is induced to contact point with room temperature has considerable influence to cool the contact point. As a measure to reduce the abrasion at arc ignition, shortening the period of high current is effective. To cool the contact point positively, thermoelectromotive force was applied. Constantan (Cu-455Ni) has low thermoelectromotive force and high melting point than Fe. So, Cu–Ni alloy (Cu-43.3%Ni-0.92%Mn) was selected and examined as the contact tip material. The abrasion of Cu–Ni alloy contact tip becomes less than about two-tenth of the abrasion of Cu–Cr contact tip. As the conclusion, the best mixed measures which are effective to reduce the contact tip abrasion are using Cu-coated welding wire, wire retract type arc ignition and Cu–Ni alloy contact tip.