Purging for Improved Weld Profile

Purging with the EXEL SG-1 for Improved Weld Profile

By Dennis Cobb / Director of Engineering

It is conventional wisdom that the semiconductor industry was, and remains, the primary driver for the continuous improvement of orbital autogenous weld profiles and internal cleanliness for process gas delivery tubing. However, in recent years, this focus on smooth, clean, particle-free internal weld requirement has migrated from the semiconductor industry to the biopharmaceutical industry, and on to the food equipment industry. The initial purpose of internal tube purging with inert gas (usually argon) was the prevention of surface oxidation on the internal diameter of the tubing surrounding the weld (Heat Affected Zone). The oxides produced on the surface when oxygen is present results in what the industry refers to as “color”. The ASME BPE Standard specifies the acceptable discoloration allowable (very little to none), and references AWS D18.2 which is a chart with photographs (see Figure 1) illustrating acceptable and unacceptable weld coloration (the applicable SEMI specification permits none). However, the current edition of ASME BPE (2009) and appropriate SEMI Specifications also specify the maximum internal convexity of the weld surface. Since both semiconductor and biopharmaceutical industries strive to continuously improve performance, it is likely that the current maximum convexity requirement of the weld on the ID (limited to 10% of wall thickness) will shrink, as cleanliness requirements become progressively more stringent.

Weld Color Samples

Figure 1 - AWS D18.2

In the interest of exercising control over the weld profile, the ID purge is introduced with a certain level of pressure, and in order to maintain consistency and repeatability that pressure must be consistent throughout the weld, and from weld to weld. The internal pressure at the weld location, typically arrived at empirically, is influenced by the gas flow rate and the size of the exit orifice. In addition, measuring the pressure some distance from the weld location (due to pressured drop) will not provide an accurate method of determining the pressure at the weld zone.  Many methods have been devised to deliver the purge gas to the weld site, but few methods exist to control the internal gas pressure, thus the internal weld profile. The semiconductor industry has the most stringent requirements for internal weld profile, so the industry’s SEMI standards have been in the lead for defining the internal purge gas pressure. SEMI Standard F78 includes a chart with minimum and maximum purge pressures and flow rates for various tube sizes and wall thicknesses. Since the pressures are given as a range, they serve as a useful starting point for the empirical determination of the best pressure for the ideal weld profile.

The combination of flow rate and prepurge time determines how much oxygen is still present, and thus, the amount of weld color caused by oxidation. In order for all color to be eliminated, oxygen levels must (depending on the material) fall below approximately 20 ppm. The prepurge time is an empirically derived value dependent on many factors including tube size, gas flow rate, exit orifice size, number, sizes and locations of dead-legs or other components that can trap atmosphere, and thus take more time to purge. Most users are satisfied with the results obtained with pure argon as a purging medium; although a few organizations like to add a small percentage of hydrogen to produce a reducing environment. However, given the dangers associated with the use of hydrogen, it is hard to recommend it unless a critical need exists.

Purging for suitable weld color is relatively simple. However, gauging the internal pressure such that the weld ID profile is acceptably flat is a more difficult task.  Experimentation is required to establish the ideal pressure for a given tube size, wall thickness and material. Once that is determined, the next problem is to correlate the required pressure at the weld with the measured pressure at the delivery inlet. One way to establish the inlet pressure is to repeat the empirical testing with the inlet pressure as the controlled variable. However, this value will only be correct for the specific assembly tested. If another configuration is to be welded, the entire experiment must be repeated. This experimentation requires several trial pressures, with a weld coupon created for each test condition, until acceptable results are obtained; a time consuming and costly activity. In addition, most standard gas pressure regulators are not nearly accurate enough to deliver the exacting performance required.

One excellent solution to the problem is the EXEL Model SG-1 Pressure Controller by Arc Machines, Inc., as described in US Patent Number US 7,057,137: Orbital Exel SG-1Welding Internal Pressure Control. Since the internal pressure at the weld-site is dependent upon the purge gas delivery pressure, the distance from the entry (measurement) point and the weld, and the gas venting rate, a closed loop pressure controller is required, such that a specific, low value pressure can be maintained. Since the pressures are quite low, on the order of 0.7”- 3.0” H2O (0.11 psi), the pressure transducer must be very accurate, and very repeatable (for tube sizes from ¼” through ¾”). The pressure transducer used in the EXEL SG-1 has a range of 0-4kPA (0-0.58 psi, 16” H2O), and an accuracy of 1.5% or full scale, or 0.06kPa (0.0087 psi, 0.24” H2O).  This level of measurement and accuracy permits the EXEL SG-1 to repeatedly maintain the exact internal pressure required for a perfect weld.

The only remaining hurdle is to obtain the corresponding inlet pressure for the known weld pressure, and fortunately, the EXEL SG-1 also provides the capability of measuring the inlet and weld pressures simultaneously. The unit then utilizes this offset for all welds performed with this same configuration. The required weld pressure is input to the device, and it correctly offsets the pressure for the tubing configuration as tested. The input gas is controlled both for pressure and flow, using a variable orifice (iris) controlled by a microprocessor in a closed loop system. This variable orifice is especially important because as the joint is welded shut, the effective purge gas exit area is reduced, so as the joint is sealed the SG-1 is making real-time adjustments to accurately maintain the required pressure.

By correctly selecting the purge time and internal pressure, the EXEL SG-1 system permits the repetition of nearly flat, colorless internal weld profiles time after time while saving considerable time and money compared to “trial and error” methodologies.

Dennis Cobb has spent more than 25 years both designing and managing the design of semiconductor capital equipment. He has a BSME from California State University at Long Beach, and an MSEE from California State University at Northridge. He is a registered professional California Electrical Engineer.
 

References:

ASME Bioprocessing Equipment Standard (ASME-BPE-2009)

ASME B31.3 Process Piping Code (ASME B31.3 2010)

SEMI F78

USPTO 7,057,137 Orbital Welding Internal Pressure Control

Operation Manual, Model SG1 Pressure Controller, Exel Orbital Products by AMI

Need for Weld Purity Drives Purging Methods, Michael J. Fletcher:  Welding Journal, July 2012