A summary of the main characteristics observed for the processes analysed in this thesis are present in Table 2.28.
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Table 2.28 – Summary of the processes analysed.
Process Technical Information Results Obtained
RapidArc intended for operation at low voltages;
claims that conventional pulse welding uses 23-26 V, while rapid arc welding can work at 16-19V, allowing the use of high welding speeds;
The increase of welding speed is associated with a reduction in the cycle time, allowing lower spatter emissions and heat input when compared with conventional MIG/MAG welding.
Works at high voltages (compared with other processes);
High welding speeds can be achieved;
High arc stability allowing less spatter emissions;
Arc energy is generally higher than other processes at low WFS and similar to other Pulse Spray processes at high WFS.
STT Elimination of lack of fusion;
reduction in heat input;
generates less spatter and fumes;
background current varies from 50 to 100A, which maintains the arc and heats the parent material;
A reduction of current occurs during short-circuiting, followed by a pinch current, which promotes the detachment of the molten metal from the electrode to the weld pool.
Lack of fusion was generally observed in welding of narrow groove in this project;
Heat input is generally low;
Arc instability associated with short-circuiting splashing is identified;
At the upper limit of WFS (8.26m/min) arc instability phenomena are more significant.
CMT droplet transfer is based on a mechanical oscillation of the wire;
possibility of simultaneous dip transfer and pulse arc welding, with heat input lower than conventional MIG/MAG welding;
The special motion system for controlling the wire speed is incorporated into the waveform control and provides control of the molten metal detachment and arc length;
when the arc plasma is developed the filler wire moves to the weld pool until the wire touches the weld pool and short-circuiting takes place;
The current becomes lower and the electrode is retracted enhancing the droplet detachment.
Droplet transfer is promoted by a mechanical oscillation of the wire;
Self-adjusting of CTWD by changing WFS;
High process stability is developed by the power source characteristics and precise current control mechanism;
Low heat inputs are controlled by the dynamics of the power source and waveform design;
At the upper limit of WFS (8m/min) arc instability phenomena were identified.
CMT-P Technical information was not found. The mechanism of metal transfer that controls this process is controlled pulse spray;
Periodic short-circuiting ends the waveform cycle and short-circuiting frequency decreases with WFS;
The increase of arc energy / heat input level is due to the increase of cycle time;
the remaining arc current waveform does not change;
At the upper limit of WFS (10m/min) arc instability phenomena were identified.
FastROOT modified short arc process able to weld root pass and thin materials without spatter;
allowing positional welding with required penetration beads at higher welding speeds and productivity than TIG welding;
the power source is able to monitor the short circuit and control the timing of droplet transfer;
The accurate control of the waveform in respect to arc current and time is able to satisfy the spatter free condition;
Importance of using a root gap (between 3 and 5 mm) and torch oscillation during narrow groove root pass welding of pipes.
Spatter is often observed using FastROOT, in consequence of arc instability
phenomena;
Positional welding was not tested in this work;
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2.6. CONCLUSIONS
The new processes described in this thesis exhibit a quite a diverse range of waveform control strategies, implemented in software, to achieve their objective of controlled metal transfer with potential benefits to a range of applications;
The complex interrelationships between set and measured waveform parameters have been described in detail, and their relationship to metal transfer has been clarified;
The Burn-Off ratio, i.e. the relationship between melting rate and arc current, is similar for all waveforms characterized;
The arc voltage waveform generally changes in consequence of external setting parameters, e.g. CTWD and shielding gas composition. This leads to significant variations of the arc energy and consequently changes in the way that metal is transferred;
In general, arc energy and arc current play the major role in the melting phenomena, as the depth of penetration and dilution area generally increase with energy and current;
The increase o arc energy is favourable to the reduction of convexity bead shapes, generally observed when short-circuiting waveforms were applied;
Shielding gas composition can play a significant role in process performance and arc stability and resulting bead shape characteristics, as observed from the results obtained for 2.5%CO2 97.5%Ar and 1.5%CO2 54%He 44.5%Ar;
UI diagrams (or cross-plots) have to shown to provide useful information on process performance, and in particular in providing a qualitative assessment of arc stability;
This study has indicated that very complex relationships exist between the parameters that can be set for the different process, and the resulting electrical and bead shape characteristics. This is further complicated by the fact that each process has settable parameters such as ALC where the relationship between the parameter and its effect on the process is not necessarily a clear physical relationship.
However, the very detailed data presented in this Chapter should be of significant help in evaluating process performance for potential applications.