Part 2: Weld Parameters —WPS Unicorns Don’t Exist
In Part 1, we made the case for eliminating guesswork from weld settings by using tightly controlled Welding Procedure Specifications (WPSs). Now it’s time to dig deeper into how those WPSs should be written — specifically for the Gas Metal Arc Welding (GMAW) process.
It’s common for WPSs to include wide voltage and wire feed speed ranges in the name of “welder flexibility.” But in practice, this often opens the door to unintended transitions between GMAW transfer modes — especially from short‑circuiting into globular or spray transfer. When that happens, the entire premise of the WPS breaks down.
Let’s talk about how to write a WPS that actually controls the process.
Don’t Write One-Size-Fits-All Procedures
The temptation to write a “universal” GMAW WPS that covers everything — from short‑circuiting root passes on 1/8″ material to spray transfer fill passes on 1″ plate — is strong. On the surface, it feels efficient. Why write three procedures when you can jam it all into one?
But in practice, this approach is the fast track to inconsistency and noncompliance.
A single WPS cannot realistically handle short‑circuiting and spray transfer, flat and vertical positions, or root and fill passes — unless you’ve fully qualified each scenario with multiple PQRs and clearly documented every variable range and transfer mode. Even then, it becomes a mess to manage and almost impossible for a welder to follow correctly without confusion.
Yes, qualifying multiple procedures takes time. Yes, it costs money. And no — a good WPS isn't cheap. But it shouldn’t be. Your WPS is the foundation of your weld quality system. It’s not a formality; it’s a technical document that defines exactly how to recreate tested and approved welds with repeatable results.
Trying to write a “unicorn” WPS that does everything with one document often means it does nothing well. You’re left with a procedure that spans too many ranges, allows for transfer mode drift, and fails to deliver predictable, inspectable, code-compliant welds. A unicorn WPS looks great in the office — until it costs you in the field.
A better approach is to break your procedures out by application. Write one WPS for short‑circuiting open root passes. Develop a second for spray transfer fill passes. Create a third for vertical-up welds in structural joints. Each of these can be tuned with the correct voltage, wire feed speed, gas, and technique for the specific job at hand — and each one gives your welders clear, usable parameters without the guesswork.
This isn’t over-engineering. It’s what professional, code-compliant fabrication demands.
Understanding GMAW Transfer Modes
GMAW supports several distinct metal transfer modes: short‑circuiting, globular, spray, and pulsed spray. Each is governed by a unique set of voltage, current, shielding gas, and wire diameter conditions — and each behaves differently in the arc.
Short‑circuiting transfer occurs when the electrode wire touches the weld pool and extinguishes the arc momentarily, usually around 100–200 times per second. This low-heat, low-current mode is ideal for root passes, thinner materials, and out‑of‑position welding due to its small, fast-freezing puddle.
Spray transfer, by contrast, uses a higher current and voltage to project a fine stream of molten metal droplets across the arc column. It produces deep penetration, high deposition rates, and minimal spatter — but is generally limited to flat and horizontal positions due to its fluidity.
Globular transfer lives in between — and not in a good way. It’s marked by large, inconsistent droplets that fall into the joint by gravity rather than arc force. This mode is difficult to control, produces excessive spatter, and rarely appears on qualified procedures for good reason.
What Causes Transfer Mode Transitions?
Transfer mode is a function of voltage, amperage (which is directly influenced by wire feed speed in GMAW), shielding gas composition, and electrode diameter. With all else held constant, small adjustments to voltage or wire feed speed can move the arc from short‑circuiting to globular — or even spray.
For example, using 0.035″ ER70S‑6 wire with 75/25 Argon/CO₂ shielding gas:
At 18.5–20.5 volts and 225–275 IPM, you’re operating within the short‑circuiting transfer range.
Pushing up to 22 volts and 300 IPM puts you at the edge of globular transfer.
Any further, and you may be dipping into spray, depending on your exact setup.
If your WPS allows these settings without clearly identifying the intended transfer mode, then it’s not actually controlling the process — it’s just handing the welder a wide-open range with no context.
Why Transfer Mode Drift Matters
Allowing your WPS to span multiple transfer modes results in uncontrolled heat input, varying puddle behavior, and inconsistent mechanical properties. Short‑circuiting creates a fast-freezing, low-penetration weld — perfect for vertical-up or thin material. Spray transfer, on the other hand, is deeper, hotter, and far more fluid — it can’t be used safely out of position.
If your welders unknowingly shift from one transfer mode to another during production welding, you’ve lost the very thing a WPS is supposed to provide: control. You can’t guarantee the welds in the field match what was tested during procedure qualification, and that’s a major liability — especially on critical or code-governed work.
What AWS Actually Allows in a WPS
When writing a WPS under AWS D1.1 (or any AWS structural code), you are allowed to list ranges for variables like voltage, amperage, wire feed speed, travel speed, and electrode extension. But those ranges aren’t arbitrary — they must be supported by sound testing and qualification data, and they must reflect the behavior of the welding process used during procedure qualification.
The goal of a WPS is to ensure that the welds produced in the field are consistent with the welds that were tested and passed mechanical, visual, and NDT evaluation. AWS provides room for practical ranges, but expects procedure writers to use engineering judgment to keep those ranges tight enough to prevent variability and mode drift. This typically means ±1 to 2 volts, and ±25 to 50 IPM from the qualified values — assuming those values remain within the same transfer mode and result in the same weld profile, fusion, and mechanical properties.
If your welders need to switch to a fundamentally different process setup — such as transitioning from short‑circuiting to spray — you’re no longer adjusting within a range. You’re using a different process, and that demands its own WPS and supporting PQR.
How to Build Transfer Mode-Specific WPSs
Writing a proper WPS starts with intention. Select the transfer mode based on the joint design, material type and thickness, and welding position. If short‑circuiting is appropriate, validate the arc behavior and bead profile within a tight range — ideally within 1–2 volts and 50 IPM of the centerline values.
Perform test welds across this envelope, use macroetch samples and bend tests to confirm penetration and fusion, and then lock in the qualified window. Document your exact wire diameter, contact tip to work distance, shielding gas type and flow rate, and clearly identify the intended transfer mode on the WPS. If your WPS allows for multiple transfer modes, you need either:
Multiple PQRs, each tied to different settings and outcomes, or
Multiple WPSs, each for specific joint conditions and weld positions.
The Bottom Line
Transfer mode drift is a silent killer in welding quality. You can follow a WPS perfectly — but if the ranges allow the arc to cross into a different transfer mode, you’re welding outside the envelope of what was tested and qualified.
Precision wins. Lock down your voltage and wire feed speed. Specify your transfer mode. Write your WPSs to prevent drift — not enable it.
Don’t guess it. WPS it.