EB Industry’s electron beam welding service focuses on welding to industry specifications. We often need to advise customers and help them understand the differences between standards so that they select the appropriate choice for the assembly they are designing. The three standards we most often apply are:
- AMS 2681 — Welding, Electron-Beam
- AMS 2680 — Electron-Beam Welding for Fatigue Critical Applications
- AWS D17.1 — Specification for Fusion Welding for Aerospace Applications
This article provides an overview of the differences between these standards with respect to the issues engineers must consider when designing assemblies for EBW (Electron Beam Welding).
All of these standards were originally created specifically for aerospace and defense applications, and situations where exceptional weld quality is an absolute necessity. The standards apply to any industry where weld failure is not an option, inclusive of automotive, medical, and oil/gas/energy industries in addition to aeronautics.
The Basic Differences
AMS 2680 and AMS 2681 are Aerospace Material Specifications (AMS) published by SAE Aerospace, which is part of SAE International Group (https://www.sae.org/). According to Wikipedia:
SAE International, initially established as the Society of Automotive Engineers, is a U.S.-based, globally active professional association and standards developing organization for engineering professionals in various industries. Principal emphasis is placed on transport industries such as automotive, aerospace, and commercial vehicles.
The D17.1 standard is published by the American Welding Society, which is a U.S.-based non-profit organization focused on the advancement of welding across all industries. AWS D17.1 was specifically created to consolidate and clarify the requirements of two military standards: MIL-STD-1595A and MIL-STD-2219. The AWS D17.1 specification applies to all fusion welding processes including electron beam welding (EBW), laser welding (LBW), and gas tungsten arc welding (GTAW) to name a few. AMS 2680 and AMS 2681 only apply to EBW.
In general, D17.1 has significantly more defined requirements for the design, processing, and testing of welded assemblies than the AMS specifications. AMS 2681 and AMS 2680 are very similar but AMS 2680 targets fatigue critical applications. According to the AMS 2680 standard:
These procedures are used typically for high quality, electron-beam welding of aerospace components, the failure of which could cause loss of the aerospace vehicle or one of its major components, loss of control, or significant injury to occupants of a manned aerospace vehicle…
AMS 2680 focuses on stress concentration features that could crack or fail under varying load fatigue. All welds are full penetration and procedures are designed to minimize porosity. The requirements have very tight limits.
Each of the three specifications stands on its own merrit and they do not conflict. Although it is not best practice, EB has seen drawings that define different specs for joints on the same assembly (e.g., joint 1 requires D17.1 specification and joint 2 requires AMS 2681). In general, D17.1 covers design and process requirements in detail, while AMS 2681 and AMS 2680 specifications are more focused on specific electron beam welding requirements and parameters.
AWS D17.1 provides extensive guidelines relative to the design of weld joints. The document has specific requirements relative to Groove Welds and Fillet Welds, and the documentation includes a full appendix describing the characteristics of these types of welds under three classifications: A, B, and C. These classifications refer to different requirement levels and acceptance criteria, class A being the most restrictive.
AMS 2680 and 2681 include very little information regarding the physical design of the welded joint. Both documents require that drawings specify parent materials, filler materials (if applicable) and cleaning methods. The documents also have specific requirements relative to join fit, edge preparation, weld start and run-off tabs. Of the two, AMS 2680 has tighter restrictions on the allowable gap between mating parts. For example, AMS 2681 will allow a maximum gap not to exceed 0.010” on a 0.5” thick part, AMS 2680 only allows 0.001” gap.
AMS 2680 requires that all welds must be full-penetration which is specified to avoid stress concentration that is inherent with a partial penetration weld. The specification does include a stipulation that a partial penetration weld can be called out by the engineering authority responsible for the design. However in our experience at EB Industries, we have never seen an AMS 2680 weld call out that was not full penetration. The full-penetration requirement has a significant influence on the design and fabrication of the part and needs consideration early in the design. For example, an exiting electron beam can cause spatter and weld BB’s on the inside of the part. This may cause problems in the operation of the assembly. The problem can be eliminated by adding a backing ring inside the part, but this may cause other problems. Design for full penetration is not a trivial task and is one of the more significant factors to evaluate when specifying AMS 2680.
Both AMS specifications reference the MIL-STD-1595 standard with respect to welder and procedure qualification. As D17.1 was created to replace 1595, D17.1 should be the overriding standard in areas where it overlaps 1595. D17.1 has very clear requirements for welding procedure specification (WPS), procedure qualification records (PQR), and welder/welding operator certifications. The D17.1 specification is highly detailed including, lists, tables, tolerances and sample forms that enhance compliance. The requirements are fairly broad in that they address other welding processes besides EB and laser welding.
The most significant difference between the AWS standards is that AMS 2680 has specific requirements defined for qualification samples. Samples are required for weld inspection and penetration verification. These samples can be cut from an actual part, or from three simulated parts that have the same material and joint characteristics. This is important to keep in mind because samples may need to be designed and fabricated if actual pieces are too valuable to destroy during production processing.
Welding Process Control
The D17.1 document has sections addressing fabrication that includes requirements for consumables, equipment, pre-weld cleaning, joint preparation, welding, post-weld processing, inspection, rework and repair. These are fairly high-level requirements defined for fusion welding processes in general. AMS 2680 and 2681 specifications go into much greater detail in these areas for items specifically critical to EB welding, with some specific differences.
The biggest process control difference between 2680 and 2681 standards involves the vacuum level. Electron beam welding must occur in a vacuum because air molecules interfere with the propagation of the electron beam. The vacuum level makes a difference in the quality of the weld: the higher the vacuum, the less amount of contamination and residual gasses will be present when the welding occurs. AMS 2681 requires a vacuum of 1×10-3 Torr and AMS 2680 requires 1×10-4 Torr. The higher vacuum level means that an AMS 2680 weld is more expensive because more time is required to reach the proper vacuum level for welding. Additionally, the parts must be cleaned and handled more carefully.
The other big difference between the two AMS specifications is related to rework. Both specifications allow for subsequent full-penetration, partial-penetration, or cosmetic passes that may be needed to rework a weld. AMS 2680 only allows one rework cycle. This requirement has two ramifications. First, extra time should be invested in development to make sure the resulting weld parameters consistently yield the proper weld. Second, it may make sense to create extra pieces in case a problem arises.
Inspection and Testing Criteria
D17.1 has significantly more requirements that apply to the visual inspection process, specifically referencing the AWS welder inspection qualification specification AWS QC1. Additionally D17.1 requires vision tests, inspector training and certification, which the AMS specs do not. The AMS specifications explicitly require cross-sectioning of the weld. This is an important distinction because weld samples will be destroyed by the sectioning process. Engineers should plan ahead to either procure extra parts that can be sampled (i.e., destroyed) in a production lot, or plan fabrication of representative samples.
Nondestructive Examination (NDE) is required by all three specifications, but to varying levels. AMS 2681 is the least restrictive in that the engineering authority designing the part may specify Radiographic Testing, Ultrasonic Testing, Penetrant Testing, and Magnetic Particle Testing. AMS 2680 absolutely requires Radiographic Testing, along with other tests at the discretion of the engineering authority. The level of NDE required for D17.1 depends on the Class. Class A NDE requirements are similar to AMS 2680, while Class C requirements are similar to 2681.
None of the three standards allows for cracks, overlap (also referred to as spatter or cold lap), incomplete fusion or incomplete penetration. The three differ, however, in regards to criteria for porosity, inclusions, undercut and underfill. AMS 2680 is probably the most restricting, followed by D17.1 Class A. However, there is variability in all three specifications relative to the actual size of the weld and the material.
Where to Go Next
The table below provides a quick reference to some specifics discussed in this article. Refer to the actual specifications for exact details.
|Specification||AMS 2681 Rev B||AMS 2680 Rev C||AWS D17.1:2017|
|Type||EBW, Aerospace but non-fatigue critical||EBW, flight critical||Aerospace fusion welding|
|How it is used at EB Industries||The main EB welding spec we use unless otherwise specified||Used for fatigue critical aerospace parts||Qualification for personnel and processes|
|Governing Body||SAE International||SAE International||American Welders Society|
|Rework||Once||Once||Established by weld schedule|
|Joint Design||Specified on drawing||Complete fusion||Complete fusion|
|Vacuum Level||1×10-3 Torr||1×10-4 Torr||N/A|
|NDT Required||If specified||Radiographic inspection required||Only required for class A|
|Underfill||10% of total material thickness||0% of total material thickness||Class A: .005xT Class B: .015xT Class C: .025xT|
|Undercut||10% of total material thickness||0% of total material thickness||Class A: .005xT Class B: .015xT Class C: .025xT|
EB Industries has been providing engineering advice and guidance with respect to electron beam welding and these specifications since 1965. If you have additional questions, please feel free to contact us. The specifications are mature and exceptionally complete documents. Contact SAE or the American Welding Society to obtain copies.