Specifying In-Process Weld Rework
The internal scrap rate in a metalcaster’s shop is a major concern for casting producers and customers alike. An excessive scrap rate can hurt a foundry’s competitive position and result in missed schedules and higher prices for finished parts. Every year, the Defense Logistics Agency (DLA) procures numerous cast metal parts used in military applications. Aligning with the DLA Strategic Plan 2018-2026 Focus Area IV: Drive Efficiencies and Innovation in Our Business, pragmatic approval of in-process weld rework, as one of many normal casting finishing processes, has the potential to drive down the internal scrap rate and the resulting high casting costs and lead times.
There is a misconception that in-process weld rework is generally harmful to the structural integrity of a cast part. However, ample cyclic life and fracture toughness evidence indicates that, when done according to governing specifications, in-process weld rework to castings that have been welded, blended, and heat treated results in parts that will be dimensionally, physically, chemically, metallurgically, and structurally compliant with drawing requirements.
Why Weld Rework
Castings are susceptible to having small surface imperfections such as inclusion pits, gas microporosity, cold laps, misrun edges, or dents, that are cosmetically unappealing or non-compliant with customer specifications. In-process weld rework of castings, also referred to as cosmetic weld repair, is a routine casting finishing activity used to mend such casting surface imperfections across almost all alloy families and molding processes.
In-process weld rework is also performed on surface and near-surface discontinuities. These discontinuities are typically harmful to the structural performance of a casting. They include hot tears, cracks from heat treating, and near-surface voids revealed by radiography or ultrasound. For commercial castings, the extent of discontinuities is classified typically by radiographic standards.
Most commonly, in-process weld rework is not noted in specifications. A casting that has been welded, blended, heat treated and has passed all drawing specified inspections generally will be dimensionally, physically, chemically, metallurgically and structurally compliant to drawing requirements. Specifications to limit or document in-process weld rework or surface imperfections are rarely noted for commercial castings. Similarly, military or aerospace castings not classified for service severity typically have no specifications limiting or documenting in-process weld rework.
Sometimes, AMS 2175 (classification and inspection of castings) is specified. For military and aerospace castings specified to AMS 2175, the severity of cast component service is classified as Class 1-4, and surface and/or internal integrity is specified as Grade A-D. Classes require different levels of sampling for nondestructive tests to verify compliance with the specified integrity grade. Significantly, grades directly correlate to the cyclic life of a classified casting’s highly stressed surfaces. AMS 2175 covers almost all casting processes and the full range of casting alloys, so it is also adopted as a commercial standard for safety critical castings, SAE 2175 being identical.
Importantly, AMS 2175 is silent regarding in-process weld rework, only specifying the extent of imperfection in Grades A-D as “graded” in radiographic, magnetic particle, die penetrant inspection and/or visual inspections. This is inspection after all casting finishing processes are complete, including final heat treatment. Those finishing processes include in-process weld rework, if applicable. Welded or not, passing the specified grade indicates the allowable transformed stress for which the casting was designed will enable the intended cyclic life. Conversely, poor in-process weld rework will result in surface and/or subsurface defects that would not pass testing for the specified integrity grade.
Sometimes, in-process weld rework is limited or must be documented for the purposes of assuring surface and internal integrity correlating directly to the design intent for cyclic life, but with AMS 2175, this is an unnecessary “belt-plus-suspenders” precaution. An example is AMS-A-21180 (high strength aluminum alloy castings) that allows the call-out of no-weld zones or in-process weld rework only upon written permission of the purchaser. Maps may be required showing location, size, and depth of welds to be approved by the cognizant design authority in advance for each casting to be welded. Applying AMS-A-21180 implies the cognizant design authority doesn’t have faith the nondestructive testing specified in AMS 2175 for aluminum structural castings will assure safe performance in Class 1 or 2 severe service. That should be a rare requirement, and if required, should be applied only to casting surfaces that are stressed critically.
Sometimes, no-weld specifications or in-process weld rework limitations have been specified incorrectly for legacy defense system castings on the original drawing or in the technical data package (TDP). The consequence of misapplied in-process weld rework specifications can be lack of metalcasting supplier team responses to solicitations, excessive costs and/or excessive delivery delays.
Mechanical properties of welded surfaces, especially cyclic life and fracture toughness, have been well-documented to be the same as the cast parent material that has been heat treated. In-process weld rework that meets AMS 2175 surface and internal integrity grade for the class of service is just another casting finishing process prior to final heat treatment and final net-shaping processes.
To meet the specified grade for surface and internal integrity, the metalcaster will have to use qualified welders, approved weld procedures, and approved weld filler chemistries. If the metalcaster doesn’t have those in-process weld rework manufacturing capabilities, then ASM 2175’s nondestructive testing requirements will reject each casting for flawed welds or the whole lot of castings based on a flawed weld sample. So, specifying no-weld zones or in-process weld rework only upon written permission of the purchaser should be a rare, super-safe requirement on only the most severe service applications.
An effective specification strategy for in-process weld rework only upon written permission of the purchaser is to define, in advance, zones on specific critical casting surfaces for which there are limitations for in-process weld rework.
For example, specify “in zone A, in-process weld rework is permitted per ‘X’ square centimeters: ‘N’ welds of ‘L’ maximum length no closer than ‘S’ spacing.” Then allow the metalcaster to certify compliance with the specification when the finished castings are shipped. A higher level of compliance can be required with both the certification and a map of the weld sizes and positions in the defined zones. This compliance strategy can save many weeks of lead-time by eliminating requests to weld specific castings and the waiting time for approval.
Another effective strategy to simplify and expedite finishing of castings with minor surface imperfections is to allow blending of such imperfections of a maximum depth with light grinding as a practical alternative to in-process weld rework. A common maximum depth that is helpful is 1.5 mm. In addition to a note allowing blending of such shallow imperfections, a note can be made to exempt the specific blend area from dimensional compliance.
Misapplication of in-process weld rework limitations can be superseded with a drawing revision, TDP revision, or a purchase order requirement to enable faster deliveries of quality castings at lower costs.
Click here to see this story as it appears in the September/October 2018 issue of MCDP.