Aluminum is a metal marvel with global engineering significance. Due to the alloy’s favorable properties, aluminum is employed in countless commercial, industrial and military products. For a myriad of applications, some type of joining process is utilized to join aluminum parts together: mechanical, adhesive, soldering, brazing, fusion welding or solid state. This article centers on fusion arc welding, gas shielding processes: GTAW/ variable polarity (VP), VP plasma arc welding (VPPAW) and GMAW.
The secret to success in the fusion arc welding of aluminum lies in the development of a weld execution plan (WEP) prior to production. Aluminum welding requires a comprehensive understanding of the respective alloy. This is due to weldability, manufacturability and engineering variances between each specific alloy. To mitigate risks and liabilities, specified alloys must be completely understood to ensure the weld meets code criteria, design specification service and performance requirements as well as contract warranty and service guarantees.
A WEP reflects a seriousness of purpose because it represents an objective and proactive approach by taking into account prerequisites that must be addressed and resolved prior to the commencement of production. Success as measured to attain code-compliant, high-quality welds in a safe, productive and profitable manner is contingent upon the accuracy and competency of a WEP.
Aluminum production prerequisites requiring careful consideration include:
● Procedure qualification record (PQR)/welding procedure specification (WPS) qualifications
● Work area/shop air cleanliness
● Laser metrology
● Material storage and handling
● Welding equipment configuration
● Welding processes
● Humidity/hydrogen controls
● Base metal/weld filler metal alloys
● Non-destructive testing (NDT) requirements
● Weld joint design
● Weld filler metal
● Process gases/purity
● Design dimensional tolerances
● Welder qualifications
● Materials procurement/certified material test report (CMTR)
● Weld joint cleaning method/timeline
● Equipment calibration
● Personnel development and training
A vital component of a WEP is to understand the base metal and weld filler metal alloys. Aluminum is a highly reactive metal for it readily combines with oxygen to form an oxide on the surface. The nominal melting temperature of aluminum is 1,200 degrees F and the nominal melting temperature of an aluminum oxide is 3,700 degrees F. Thus, unless oxides are removed as close as possible to the actual welding, they inhibit weld pool wetting and fusion characteristics and may form non-metallic inclusion defects in the fusion zone.
Also, aluminum is highly prone to hydrogen porosity during welding. The hydrogen gas solubility of a molten aluminum weld pool is significantly higher in comparison to solid aluminum. Unless precautionary measures are in place during welding, hydrogen gas dissolves quite readily within the weld pool, producing gas pores upon solidification. In other words, as the weld pool begins to solidify, weld pool solubility for hydrogen gas decreases where the gas tries to escape or becomes entrapped (see Figure 1).
The weld pool
The transient nature of an aluminum weld pool is unforgiving relative to coping with any adverse weld process parameter or weld joint condition(s). To be precise, when weld filler metal is being deposited, if the weld pool is contaminated, there is poor joint fit-up and insufficient preheat. The weld pool reacts accordingly in the formation of weld bead defects such as overlap, cracking, porosity, incomplete fusion and undercut. Arc time is of the essence relative to the time available for a welder to manipulate and control molten weld pool behavior.
Weld pool geometry and surface morphography of the weld bead is a function of weld process parameters and arc manipulation and control, such as travel speed, amperage and voltage, as well as weld joint attributes such as fit-up, cleanliness and design.
Additional criteria includes: process gases, gas flow rates, pulse time durations and frequency, and tungsten electrode setback. This relates directly to the weld pool solidification process which is dependent on the weld filler metal chemical composition and process parameters.
All of which contribute to the dynamics of the weld pool melt such that any detrimental or unfavorable aspect of the welding process and the weld joint that is present or introduced during welding prevents the creation of an acceptable weld. From the practitioner’s viewpoint, the tempo of welding and the rate at which the weld pool solidifies transpires very quickly, such that there is no compensating for inadequate planning and preparation once welding begins, which subsequently affects weld integrity and performance (see Figure 2).
To mitigate aluminum welding and fabrication risks, a WEP outlines production prerequisites as determined by diligent planning and preparation efforts. The following are examples of the extent and attention to detail that these planning efforts might include.
1. The enforcement of environmental, health and safety and personal protection equipment policies and practices. Reference pertinent codes and standards, including ANSI Z49.1, Safety in Welding, Cutting and Allied Processes. The promotion of a cultural attitude focused on safety, injury prevention and precautionary awareness is emphasized.
2. A quality management manual that delineates policies and procedures pertaining to: QA/QC oversight; preventative maintenance and equipment calibration; process improvements; in-process inspections; documentation control; and material handling and storage. Moreover, provisions for training requirements for personnel performing activities affecting weld quality. Quality does not come not from inspections alone, but from production process improvements.
3. The qualification of PQRs/WPSs in accordance with a weld code and weld specification. A PQR is performed to develop the weld variables required to ensure the weld meets the design service and performance requirements of the weldment. A PQR is a record of weld variables used during the actual welding of a test coupon. It is emphasized that PQR qualification be based upon the actual production welding requirements, such as plate thickness or pipe diameter.
For critical applications, in addition to PQR testing, the employment of full-size production mock-ups should be welded and then NDT evaluated, mechanically tested and metallurgically analyzed. The repeatability of qualified PQR weld variables is crucial to achieve high-quality production welds on a consistent basis.
A WPS is a written document based on a qualified PQR. The WPS provides guidance by detailing welding variables and specified ranges as well as pertinent instructions to be followed throughout production.
In addition to a WPS, an operating instruction (OI) may be used to support production due to the complexities of certain welding applications. An OI is a comprehensive document employed for critical welding applications that complements the WPS by providing a checklist and additional information to further support welding efforts.
4. A keen awareness of aluminum base and weld filler metal weldability, metallurgy and physical properties. Aluminum is an extensive field of study whereby it is important to understand the ramifications of welding. Having knowledge respective to the specified base metal and weld filler metal alloys is key, such as the effects of overheating the base metal during welding, alloy cracking sensitivities and the coefficient of thermal expansion (CTE).
Alloys with a high CTE such as aluminum present problems during fabrication that must be planned for by using proper weld joint and designs via design for manufacturing and welding principles; decreasing weld sizes and weld joint volumes where possible, such as included angle reduction; forming parts to eliminate weld joints; and reducing the number of components.
Due to aluminum’s CTE, welding parts within a fixture is not suggested. As weld joint restraint increases, the likelihood for weld cracking increases.
5. The development and training of personnel, such as welding supervisors, fitters and inspectors and the qualification of welders. Throughout production, achieving synergy by having qualified personnel working together as a concerted team cannot be overstated. Continuous training is crucial to increase quality, productivity and profitability.
6. Optimizing weld parameters, such as the VP controls on GTAW/VP and VPPAW power supplies. The direct current electrode negative (DCEN) and direct current electrode positive (DCEP) arc waveform ratio can be controlled. With DCEN, current flows from the tungsten electrode to the base metal allowing arc energy to be focused, thus achieving deep penetration and narrow weld beads with no cleaning action. Whereas, with DCEP, current flows from the base metal to the electrode, removing oxides off the material surface via a cathodic cleaning action with little penetration.
VP controls provide the ability to program the arc waveform ratio to optimize weld bead geometry and penetration qualities, which is important for NDT-radiographic testing (RT) applications.
On new generation power supplies, arc waveform controls produce exceptional welding characteristics in combating common weld problems such as poor penetration, high spatter levels and burnthrough. Aluminum welding equipment demands a forensic level of attention to detail.
The principal weld process gases utilized are argon, helium or a mixture of the two. Shielding, purging and plasma gases are controlled to low moisture content. Reference AWS A5.32, Specification for Welding Shielding Gases for process gas information.
7. A thorough understanding of the weldment fabrication contract requirements: What welder certifications are required? Is NDT-RT required? What are the weldment dimensional tolerances? What is the shipment delivery schedule? Contract criteria require working out production specifics and details. A preventative maintenance plan and calibration of measuring and welding equipment are imperative.
Base metal materials and welding consumables require proper storage to prevent contamination and damage. It is a good policy to obtain the base metal and weld filler metal CMTRs to maintain material verification, traceability and usage records.
Atmospheric conditions affect aluminum weld quality. All non-aluminum welding activities must take place in other areas away from aluminum welding work. For NDT-RT quality welds, it is very important to develop and implement production methodologies to eliminate opportunities for weld pool and base metal moisture and hydrocarbon/hydrogen gas contamination.
Be knowledgeable of proper welding practices: preheating to evaporate base metal condensation; the purchasing of low dew point shielding, plasma and purging gases; selecting the correct preweld cleaning method; maintaining good housekeeping of work areas and so on.
8. Fabrication drawings that are accurate and unambiguous, including the drawing notes and ensuring that weld symbols are in accordance with AWS A2.4, Standard Symbols for Welding, Brazing, and Nondestructive Examination. Drawings must convey the required information such that shop personnel can fabricate weldments in a time efficient and effective manner. The ability to provide 3-D views on a drawing greatly assists personnel in understanding component orientations. Likewise, in an effort for shop personnel to view complex weldments, the availability of interactive 3-D models allows for real-time viewing where the model can be rotated and viewed from multiple angles.
Locating engineers with computer workstations to the shop floor area is emphasized. The objective is to improve weld quality, production cost and time efficiencies, and profitability by eliminating repairs due to drawing inaccuracies and misinterpretations.
9. The procurement and utilization of quality materials. Provide sufficient lead time to procure quality materials with CMTRs. Also, using the production welding process, verify material compatibility by conducting weld filler metal/gas combination testing and base metal/weld filler metal combination testing. Do not assume, verify welding materials prior to use via weld testing, CMTRs, etc.