A.) Leak Testing
Most Casting used as housings have leak tightness requirements to prevent unacceptable conditions attributable to leaking internal fluids or even high temperature gases in the design application.
External Leakage Classifications:
With fluids such as oil, fuels or water, there are 3 classifications for leakage: a.) Leaks, b.) Seeps and c.) Weeps.
A.) A leak condition is usually caused by large porosity voids being present in the wall sections of the casting that are opened up from machining cuts. Cracks are another condition known to cause leakage. Generally, the leak condition correlates to a pressurized gas leak test above 15 cc/min when tested with a gas pressure between 1 and 2 bar. In most cases, dry air is used as the gas medium for leak test purposes. For porosity conditions that cause a leak, the casting can usually be salvaged with methacrylate sealants if the leak rate is below 40 cc/min. If the leak rate at room temperature is greater than 40 cc/min, then most castings should be rejected as a “Gross Leaker” and assume that the leak rate is too high for long-term sealing with a methacrylate impregnation sealant.
B.) A Seep condition is usually associated with a leak level that allows a fluid, such as oil, to travel through the case and accumulate in the form of a heavy drip. The amount of accumulation and drippage is generally a function of internal pressure and fluid viscosity. A seep condition will be wet to the touch and appear to be ready to drip onto the floor. Generally the seep condition correlates to a pressurized gas leak test between 4 and 15 cc/min, again with an air pressure 2 t0 4 Bar. Generally, this range of leakage is considered salvageable by impregnation with methacrylate sealants.
C.) A Weep condition is associated with a fluid leak level that will “stain” the affected surface area of the casting. While this level of leakage doesn’t drip or fall on the floor, it could accumulate dirt and create a questionable visual appearance that may lead to expensive repairs.
While aluminum castings are normally leak tight, the best practice is to test every casting after machining. Generally, the occurrence rate for parts not meeting leak tightness specifications is statistically considered “common cause”, meaning that leaking castings occur sporadically during serial production. Therefore, traditional batch and hold procedures are not robust enough to assure 6-sigma levels of certainty. For this reason, cast parts with important leak-tightness requirements are 100% tested after machining.
Leak Tightness Test Criteria:
To accommodate the design requirements for any casting used as housing, the most important knowledge is the correlation of long-term leak performance with a fluid, even hot oil, to a short-term leak test using air or helium gas in place of the more viscous oil. If the direct correlation is unknown between component leak test and a down-stream assembly test with helium, it is recommended that the component leak rate specification should be 2.0 cc/min maximum at 2 bar pressure with dry air.
Background on Sealants:
Castings used as housings should have leak tightness requirements to prevent the weeping, seeping or leaking of fluids, such as oil, as the casting is used with the product in operation. While aluminum castings are normally leak tight, the best practice is to test every casting after machining. Generally, the occurrence rate for parts not meeting leak tightness specifications is statistically considered “common cause”, meaning that leaking castings will occur sporadically during serial production. Therefore, traditional batch and hold procedures are not robust enough to assure 100% verification. For this reason, cast parts with important leak-tightness requirements are 100% tested after machining.
To salvage castings that don’t meet the leak tightness spec, methacrylate sealants are used to impregnate the structure. The methacrylate class of sealants are used because they have good sealing properties and are relatively insensitive to surfaces that may have been exposed to oil. Methacrylate solutions are also chemically stable at the process temperatures and pressures necessary to force the liquid solution into the voids, and can be swiftly polymerized (cured) when the temperature is elevated using hot water in the last stage. Approved sealants comply to MIL-I-17563C (chemical resistance), where the test specimens were prepared per the process requirements outlined in MIL-STD-276A. Approved sealants are listed in material standard MS-9543, governed by FCA Materials Engineering.
Thermo-mechanical Properties of Sealants:
Methacrylate sealants are generally classified as a thermo-set plastic with durometer hardnesses in the range of 65-70 Shore D-scale, if the sealant hardness could be measured in the casting. Therefore, the mechanical properties are only significant in their ability to resist leakage of fluids, retain internal pressures and resist breaking down at elevated operating temperatures. For comparative purposes, the following properties are typical:
- Density: ≈1.2 g/cc (aluminum is ≈2.7 g/cc)
- Coefficient of Thermal Expansion: 150 x 10-6 mm/mm/deg.C (7 times more than aluminum)
- Operating Temperature: -40 to 400 deg.F (-40 to 200 deg.C)
C.) X-Ray Inspection
Background on X-Ray inspection:
Production castings will have internal porosity limits to assess the internal quality, which is also a method to verify that the casting process is operating as expected. There are 3 types of porosity: a.) Blind Porosity, b.) Through Porosity and c.) Fully Enclosed Porosity. For any type of porosity, the fundamental root causes are related to both Shrinkage and Gas Molecules, that come out of solution during solidification. The scope of this discussion is related to type c.) Fully Enclosed Porosity, where Non-Destructive Test (NDT) and measurement methods are accomplished with X-Ray based radiographic technologies. Due to the expense associated with NDT as part of the serial production value stream, the most important features under inspection are related to those areas that are part of a safety-critical parking system and other areas that have a significant impact on customer perception due to leakage.
Material shrinkage is considered a “common cause” mode because it is traceable to the physics of the process and must be addressed by specific engineering actions directed at controlling the location where the final solidifiction occurs.
Porosity from Gas Molecules:
Porosity voids formed by hydrogen gas are typically large and traceable to a “special cause” condition. The most frequent special causes of gas porosity voids are excessive die lubricants and cooling line leaks into the die cavity. As the molten metal contacts water and oil, the hydrogen molecules are dissolved into the aluminum, coming out of solution as bubbles during the solidification phase of the process.
Porosity Effect on Mechanical Properties:
In reality, not all porosity is detrimental to the strength ofa part. If the porosity occurs along a neutral axis plane, the stress level would be quite low and not particularly affected by the porosity. However, since FEA analysis assumes that material properties are homogeneous, attention must be given to those areas in the casting that are highly stressed (>70% of YTS). If the design of any casting region has >70% of YTS, a carefully controlled inspection process should be defined. The inspection frequency should be commensurate with the applied stress design factor. For any surface where the applied stress is >90% of YTS, a 100% visual verification with X-ray is recommended.
The main objective for inspecting castings with X-Ray is to determine if there are internal conditions that adversely affect the part. The inspection technology and method must be able to assess the internal voids in a way that they can be graded to an industry or part-specific standard that is associated with an estimate of the porosity size. Acceptance criteria is generally associated with both the size grade and location. Limits for size grades are established for specific locations in the casting, depending on the sensitivity for leakage and applied stress. By using grade limits and location criteria, the casting operations are generally capable of qualifying parts as OK or NOK, as well as using the information to make effective countermeasures to mitigate conditions that cause rejects.
D.) Casting Surface Inspection and Fettling
Background on Surface Inspection and Fettling:
The as-cast surfaces of the final part design must be governed to a surface profile tolerance that is sufficient to allow the cavity inserts to operate in production for as long as feasible. The most common failure mode of the casting dies is associated with heat checking, which is the formation of a network of small cracks on the die cavity surface. These small thermo-fatigue cracks cracks in the cavity surface result in small raised fins being formed on the casting surface. Under the high pressures associated with the die casting process, the molten aluminum gets force into these small thermo-fatigue cracks in the cavity. When the standing fins on the casting are found, they can generally be abrasively removed by shot blasting or other deburring methods (robotically or manually). When continuous casting cycles aggravate the thermo-fatigue die checks into deeper and longer cracks, sometimes small sections of the die steel will separate. In this case the resulting defects are frequently termed as “break outs”. These heavier break-out defects are more difficult to remove. The removal of these surface defects in the foundry is referred to as “fettling”. Sometimes temporary off-line machining operations are used to remove these types of defects until the die surface can be reconditioned by welding or cavity replacement.
Surface Profile Tolerances:
Surface profile tolerances are specified against a reasonable amount of combined accommodation for variances associated with shrink rate assumptions, cavity machining, parting line flash-over and cavity surface wear. When new dies are constructed, the die builders are frequently given a smaller tolerance band during prove out and PPAP verification. This allows the casting plant to operate within the design tolerance over long-term production.
Maximum Flash Height Tolerances:
Because thermo-fatigue related heat checking will eventually occur in any cavity steel, the condition of standing fins on the casting surface must be managed by the casting plant. While the general notes for surface profile tolerance do accommodate some variation, once the on-set of heat checking occurs in the die cavity steel, it doesn’t take long for the height of a standing fin to exceed the general surface profile note. In addition, conventional CMM measurement systems aren’t conducive to making thorough surveys of complete surface profiles and frequently allow standing fins to pass thru automated mapping routines. Therefore, the most common quality verification tool is a simple GO/NOGO gauging tool to assess whether the standing height exceeds a general note tolerance limit. In some cases, the surface profile tolerance must protect for potential interference with rotating components or elastomeric seals. For these conditions, the specific areas and governing tolerances must be identified on the 2D print. Frequently, these surfaces are machined so that the risks for interference with mating parts or seal leakage are eliminated.
The general surface profile tolerances should be measured by use of “Point Cloud” inspection technologies. These measurement technologies are accurate and repeatable within 40 micron and fully scan the cast surfaces using either laser or optical interferometry methods. For measurement of standing fins or parting line flash, simple hand tools capable of measuring depth are generally accurate and repeatable enough initially quantifying fin or flash conditions.
The most common manufacturing methods to remove or reduce flash or standing heat-check related fins are abrasive sanding or shot blasting. Using abrasive sanding disks or belts, the entire casting surface defect can be removed to the base surface. With shot blasting media, the entire casting surface is impacted by the media, but the standing fins and flash lines are removed more aggressively than the surface they are grounded to. In some conditions, it may be easier to machine the casting defects. In this condition, cycle time is provided for a pre-set machining tool to pass over a surface in case there is a standing fin or flash line that might interfere with a mating part.