Improving TIG welding productivity using the hot wire technique

TWI Bulletin, August 1985

Graham Hutt, BSc, CEng, MWeldI, MIM, is Head of the Gas Shielded Processes Section of the Arc Welding Department.

Hot wire TIG welding combines the precision of conventional TIG with the production rates of MIG. This article describes the principles of the process and outlines some critical applications in industry.

Because TIG welding is capable of producing high joint integrity it is a preferred technique for producing critical joints. With specialised applications, which require precise control of fusion and heat input, independent regulation of the arc and filler metal addition facilitates low defect levels in all welding positions. However, a major drawback is the relatively low joint completion rate when adding cold filler wire, even in highly mechanised or automated operation. Consequently, as the material thickness increases, the economic feasibility of TIG welding diminishes rapidly compared with MIG and manual metal arc welding.

One approach to higher productivity is to increase the metal deposition rate by addition of a preheated filler wire to the weldpool- the hot wire technique. In this technique wire can be added much faster than with the conventional 'cold' wire process, thus speeding the welding operation whilst maintaining high joint quality.

Welding by the hot wire method was first developed in the mid-sixties and over the years has been applied to a variety of welding and surfacing operations in which fusion of the parent material is provided by a plasma arc, submerged arc or TIG arc. Despite the apparent versatility and advantages of this technique it has been little used until recent years. The trends towards more stringent mechanical and metallurgical properties and use of specialised materials have renewed interest in use of the TIG hot wire welding technique for welding medium and heavy section, high performance components.

Some advantages claimed are:

- retention of the high joint integrity of TIG welding;

- metal deposition rates comparable with MIG or MMA welding;

- preheating of the wire removes contaminants;

- heat input remains controlled.

To investigate these benefits The Welding Institute has used mechanised and manual TIG hot wire welding equipments ( Fig.1 and 2) in its own research programmes and for Member company sponsored projects related to specific welding applications.

Fig.1. Mechanised TIG hot wire system

Fig.2. Manual TIG hot wire equipment: 

Fig.2a) General view;

Fig.2b) Manual welding torch with integral wire guide tube.

This article describes some fundamental aspects of hot wire TIG welding and indicates applications and material types for which it is suitable.

Fundamentals

The TIG hot wire technique is a variant of conventional TIG welding in which the arc is the heat source for melting the parent material, and a resistively heated wire is fed into the rear of the weldpool at an angle of 30-40°from the vertical axis ( Fig.3). Use of this steep angle trailing feed is essential to minimise sagging of the hot wire and for smooth feeding into the weldpool.

Fig.3. Methods of filler wire addition for mechanised TIG welding: 

a) Hot wire;

b) Cold wire;

The equipment in Fig.1 comprises the basic components of a mechanised system, i.e. the welding head with arc voltage regulation and weaving facilities and separate power supplies with controllers for the TIG arc and filler wire heating.

Commercial systems use differing methods of heating the wire and controlling the balance of heat supplied to the wire as the wire feed rate changes ( Fig.4). Current for heating the wire may be supplied from either a separate AC or DC source ( Fig.4a or 4b), or a single power source is used for both the TIG arc and wire heating currents using a switching arrangement ( Fig.4c). The latter equipment incorporates a combined welding torch/wire feed arrangement for manual operation ( Fig.2).

Fig.4. Wire heating and control methods:

a) Separate AC power supply

b) Separate DC power supply

c) Single power supply with switching arrangement

Figure 5 shows the mechanised welding head, tungsten electrode and filler wire. A separate gas shroud prevents oxidation of the hot wire. It is essential that an arc is not formed between the wire and the weldpool and therefore the resistive heating power source has a low open circuit voltage. In AC operation unwanted arcing rapidly extinguishes should a gap arise between the wire and weldpool.

Fig.5. Hot wire addition to the rear of the weld pool

The presence of a current-carrying wire with its associated magnetic field inevitably influences the nearby TIG arc. However, use of AC for preheating the wire minimises possible interaction with the arc. In contrast, a DC power supply can be chosen to take advantage of this interaction as a means of deflecting the TIG arc column forwards. In this case the polarity of the wire is positive with respect to the workpiece.

Hot wire control

The rate at which the wire is fed must be matched with the current flowing through it so that the filler is almost melting when it enters the weldpool. Because wire heating current can be drawn only when the wire is in electrical contact with the workpiece it is essential that the wire addition is controlled to give a continuous feed into the weldpool. Imbalance of these two parameters results in either pre-melting of the wire at too high a current or stubbing-in at too low a current. One other factor is also important in controlling the wire heating, namely, the wire contact tube to workpiece distance, i.e. the length of the wire which is resistively heated. In practice this distance is normally fixed (15-50mm) leaving wire feed rate and wire heating current as the primary control parameters.

The effect of too high a current becomes apparent when the filler wire makes contact with the weldpool. The wire tip is melted back into a balled end thus breaking the heating circuit and producing a discontinuous deposit of metal.At the correct current setting a smooth operating condition is established ( Fig.5). At the other extreme, i.e. where too little current has been applied for the selected wire feed rate, the wire may be incompletely melted. Fortunately the tolerance band for wire control is relatively broad so that once set correctly only minor trimming adjustments are required during welding. Moreover, a knowledge of the empirical relationship between the wire feed rate and the wire heating current permits one knob wire control if required.

Economics

The main economic advantage of the process is that the deposition rate can be increased over that of cold wire TIG to levels approaching those of MIG welding; typical deposition rates are shown in Fig.6. Hence, TIG hot wire is used for welding thicker section material, where the significantly higher deposition rates compared with those of the TIG cold wire process can be fully exploited without any reduction in weld quality. ^[1] When used with a narrow gap preparation the joint completion rate can be increased to maximise the economic benefits.

Fig.6. Deposition rates that can be achieved with the TIG hot wire process compared with conventional TIG cold wire

Applications

The TIG hot wire technique is selected where higher productivity is required without sacrificing weld quality and is applicable to most of the materials welded by the cold wire TIG process. The applications outlined here have been selected to indicate the range of material thicknesses which may be welded and to show specific fabrications where benefit has been gained.

Sheet and plate materials

The examples in Fig.7 illustrate a variety of joint configurations in 3mm mild steel and 6mm austenitic stainless steel welded by manual and mechanised hot wire TIG respectively. The significant features of these welds are the good fusion and weld profile and welding speeds which are higher than would be expected for cold wire TIG operation.

Fig.7. Sheet material welded by manual and mechanised TIG hot wire techniques with conditions (see table below):

These examples point to potential applications for high productivity welding of, for example, stainless steel components requiring a high degree of cleanliness, where low current MIG arc stability becomes a problem, or for welding of materials such as 9%Ni steels which give rise to magnetic arc blow when using consumable electrode processes.

Process pipe and transmission linepipe

Thick section pipework and piping components intended for use in critical areas of chemical or power plant are an application of major interest. Because materials are of medium or heavy section thickness with specific requirements for mechanical or corrosion resistance properties, they are traditionally fabricated by either cold wire TIG welding or a combination of a TIG root weld and MMA filling passes.

TIG hot wire offers the potential for maintaining the desired weld properties and improving productivity for this type of component. Heat resistant, nickel based alloy reformer catalyst tubes 75-125mm OD and 9-25mm wall thickness ( Fig.8) for a high temperature and pressure environment were TIG hot wire welded in conjunction with a narrow joint preparation for maximum benefit. Figure 9 shows welding of an internally clad (stainless steel) pipe to a flange using a combination of austenitic stainless steel and Inconel filler materials. A requirement for rapid completion of pipeline girth welds has led to development of a multi-head TIG hot wire system which utilises four welding heads to weld each quadrant of the pipe vertically downwards ( Fig.10).

Fig.8. TIG hot wire welding of cast reformer catalyst tubes

( Courtesy Scomark Engineering)

Fig.9. Mechanised welding of a 180mm OD x 54mm wall C-steel pipe, internally clad with 2.4mm of austenitic stainless steel, to a stainless steel flange. Runs 1 and 2 - austenitic stainless steel filler wire, remainder - Inconel filler wire

( Courtesy Equipos Nucleares SA)

Fig.10. Four head welding machine for linepipe girth welding
( Courtesy Saipem SpA):
a) General arrangement
b) Welding head

Containment and pressure vessels

A recent survey of developments in gas shielded welding ^[2] has highlighted several applications of the TIG hot wire technique for welding of containment andpressure vessels. For example, Harris ^[3] has described a system with four welding heads for welding stainless steel core barrels 61 mm thickness in the horizontal position. A modified Vpreparation was selected for a two-sided welding technique ( Fig.11). Welding of 50mm thick stainless steel pressure vessels using hot wire TIG in the horizontal and vertical positions has also been reported by Urantani etal, ^[4] in this case a narrow gap preparation (9.0mm gap width) was selected to increase further the joint completion rate, see Table below.

Examples of narrow gap TIG hot wire welding procedures for stainless steel, Ar + He shield

Welding position	Joint preparation, dimensions in mm	Number of passes	Welding parameters
			Welding current (1)		Pulse condition		Welding voltage,	Welding speed, mm/min	Wire heating condition,	Wire feed rate, mm/min
			I P A	I B A	T P sec	T B sec	V		A x V (2)
Horizontal		1	160	120	0.3	0.5	10	90	-	500
		2	280	200	0.5	0.7	12	130		900
		Remainder	220	160	0.4	0.4	10	110	90-120A	800
			380	330	0.6	0.6	12	150	(2-4V)	1800
Vertical		1	200	70	0.3	0.3	11	60	-	500
		2	280	100	0.5	0.6	12	100		900
		Remainder	220	160	0.4	0.4	11	80	90-120A	800
			380	230	0.6	0.6	12	130	(2-4V)	1800
Note (1)		(2) Heating length: 40-60mm

Fig.11. Joint configuration for welding of core barrels with parameters as tabulated. ^[3]

Japanese investigators ^[5] have described vertical-up welding of 9%Ni steel LNG tanks utilising a DC wire heating power source for deflecting the welding arc forward to provide better control for positional welding.

Surfacing

Surfacing requires a high deposition rate linked with controlled fusion and therefore low dilution from the parent plate, low dilution being particularly important in the case of corrosion resistant materials.

The hot wire TIG technique is capable of achieving both of these goals, because it has independent control over fusion and filler wire addition. Applications include deposition of both corrosion resistant and wear resistant surfaces ( Fig.12).

Fig.12. C-steel clad with austenitic stainless steel using a lateral weave pattern

Summary

The hot wire variant of TIG welding is attractive for the higher deposition rates which can be achieved for both welding and surfacing operations. It is applicable to a wide variety of material types including some which aredifficult to weld using other arc processes.

The technique is based essentially on conventional TIG equipment with addition of a facility for electrically heating the wire. Thus the benefit of higher deposition rates is not gained by complication of the equipment. In practice,the operator is usually presented with one additional control for minor trimming adjustments to the power supplied to the wire, so as to maintain a continuous and smooth feed of wire into the weldpool.

Acknowledgements

Thanks are due to D Patten, R Davenport, M Churchley and B J Bartlett for their assistance. This work was financed jointly by Research Members of The Welding Institute and the Materials, Vehicle and Chemical Requirements Board.