Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
Table 10.1 Comparison of control variables for automatic plasma and MMA
welding
Control parameters SMAW Control parameters PLASMA
Electrode type Current, pulse parameters, current rise/decay
Electrode diameter times, electrode polarity, welding speed,
Arc current electrode geometry, shielding gas type,
Direct or alternating current shielding nozzle size, shielding gas flow
Electrode polarity stabilisation (gas lens), shielding gas flow rate,
Electrode manipulation electrode protrusion. Electrode set back, nozzle
geometry, orifice diameter, plasma gas type,
plasma gas flow, pilot arc current.
The traditional manual control techniques will be outlined in this chapter
and the influence of developments in processes and equipment will be discussed.
Substantial progress has also been made in both the determination of control
parameters, monitoring techniques and automatic process control, and these
advances will be described in detail.
10.2 Manual control techniques
Traditionally, welding processes have been controlled by establishing
satisfactory operating envelopes for a particular application, often by trial
and error, recording the most satisfactory parameters and using these in
production. In some cases, it has been left to the welder to interpret inadequate
drawings and establish conditions which satisfy the design requirements; for
example to produce a fillet weld of a given size.
When improved control is required a welding procedure is established.
This is a formal record of the parameters that have been found to produce the
required result and it is used to specify the steps necessary to achieve repeatable
weld quality. Procedure control has become the accepted approach when
high-quality joints are being produced.
10.2.1 Formal welding procedure control
Formal welding procedure control entails:
∑ establishing satisfactory operating parameters (procedure development);
∑ gaining acceptance of the proposed procedure (procedure qualification);
∑ following the accepted procedure in practice (procedure management).
Procedure development
Welding procedure development involves: selection of the most suitable
welding process; the determination of a suitable combination of welding