ABSTRACT

Gas metal arc welding (GMAW) is currently attracting much interest on account of significant developments in process control over the past few years. These developments are largely associated with benefits gained from the application of modern solid state power devices to welding power supplies. In this paper pulsed current GMAW is considered with emphasis on interactions between pulse parameters, parameter selection, fusion characteristics and process control.

KEYWORDS

Pulsed current gas metal arc welding

metal transfer

power sources

process control

INTRODUCTION

GMAW is arguably the most versatile of all welding processes being capable of joining in any position a wide range of materials, using manual or mechanised techniques on thin sheet or sections hundreds of millimeters thick. Process productivity is potentially high since filler metal is continuously deposited, with little slag, at high deposition rates (associated with efficient wire melting) and suitable for use in narrow gap weld preparations. Good toughness with very low deposited hydrogen levels are achievable meeting the requirements of many demanding applications. Furthermore GMAW allows close control of plate dilution and finds applications besides welding in cladding and brazing.

GMAW has however, yet to achieve the potential outlined above. Historically, two process weaknesses (metal transfer and fusion characteristics) and a number of equipment related short comings have limited the application of GMAW.

Metal may be transferred in a variety of modes. At very low currents short circuiting (dip transfer) is required and not all materials are weldable in this mode. The explosive nature of such transfers gives rise to spatter and intermittent arcing produces a susceptability to lack of fusion defects. At higher currents transfer becomes globular and non projected. Further increases in current result in a spray of small droplets, typically of wire diameter projected across the arc gap.

Synergy is a control technique used in pulsed current MIG welding (Ref 1) where mean current is determined by wire feed speed such that stable wire melting and drop transfer occur. The outcome of this technique is simplified process operation with nominally one knob control. A wide range of methods exist for achieving the above characteristics but only two basic approaches are considered here (see Ref 8). One technique consists of driving the power supply in response to a wire feed speed control signal. This might for instance be used to increase pulse frequency proportionally to wire feed speed demand. Metal transfer can then be controlled by predetermined unit pulses of current (of specified Ip and Tp) while frequency control simply changes the time spacing between pulses with the effect of altering mean current. With this control scheme droplets of uniform size are detached at every mean current (i.e. W/F in constant) and mean current increases approximately in proportion to wire feed rate when low background currents are employed. Process control is then achieved directly from wire feed rate. For this type of control no arc length self adjustment exists i.e. when the torch is withdrawn from the work arc length increases with fixed wire extension.

Arc length self adjustment may also be achieved by incorporating voltage control. Here a voltage error signal is generated (difference between reference voltage and measured voltage) which in effect modifies the wire feed to pulse frequency ratio to achieve the desired arc voltage. Features of conventional self adjusting GMAW are thereby regained. A second so called synergic technique relies entirely on voltage control to produce frequency modulation without any link between power supply and wire feed unit. Having set the required voltage and wire feed rate, the spacing between pre-determined pulses is modulated in self regulating manner i.e. arc current is self regulating. Process control is again of the one knob type and when a welding torch is withdrawn from the work both pulse frequency and mean current then fall in a self regulated manner so as to maintain a given arc voltage. By reducing process control to nominally one knob a range of further possibilities are presented. For instance thermal pulsing and backface control of full penetration. In the first case low frequency modulation of wire feed speed is used to produce overlapping weld beads which can have beneficial effects on fusion. The required changes in current changes are then automatically accomodated by the synergic type control technique. With backface penetration control (as practiced in TIG welding) a radiation signal from the underbead may be used to modulate top face heat input thereby controlling penetration (although this technique has yet to be developed for GMAW).

All of the above techniques rely on steplessly variable control of the current waveform (especially pulse frequency). This may be achieved electronically with solid state devices (transistors and a range of thyristors) which are used in power circuits as current switches or variable resistors. Series regulator circuits use devices as variable resistors and have inherently high response with low ripple. However, at low arc voltages most of the process power is dissipated across power devices and these circuits, although essentially simple, are very inefficient with high cooling requirements. Switching circuits have much lower losses and air cooling is often appropriate. Devices are then either on or off and switched at a frequency characteristic of the circuit design/device capability (typically of order 20KHz). These circuits generate current ripple and have a slower response than achievable with series regulators. One praticularly energy efficient and physically small class of switching circuit are inverters of which there are a number of basic types. With these switching is on the primary side of the transformer and transformers can then be made much smaller than for secondary switched circuits.

The first condition is concerned with drop detachment and reflects the observation that background parameters usually have little influence on the detachment event. It is often observed (see Fig 1) that a peak detachment can occur at high peak currents (Ip) of short duration (Tp) or lower peak currents of longer duration such that (1) is approximately obeyed where D, a detachment parameter, is constant and influenced by wire composition, wire diameter and shield gas type. Typical values for D are given in Table 1. In many situations preferred combinations of Ip and Tp exist and those identified by the writer are also shown in Table 1 where it is interesting to note that preferred peak currents are typically 1.5 times the so-called spray transition current (Is).

The second condition is a simple statement of droplet volume (ϕ) when one drop per pulse is detached where W is wire feed rate, F pulse frequency and A is wire cross-sectional area.

In many situations W = K. where is mean current and K is a constant. Equation 2 then becomes:-

¯F=ϕK.A (3)

Equations 1 and 3 allow pulse parameters to be specified provided K and D are known. The method is then:-

In many commercial equipments, rules of this type are incorporated as control circuitry and the required parameter adjustments are automatically made thereby resulting in considerable simplifications in process operation (see process control).

For many materials spray transfer occurs at currents too high to allow managable weld pool control in any position other than downhand welding. The principal method of overcoming transfer limitations outlined above was developed over two decades ago...