Advancing aviation's material: aluminum metalcasters serving the aerospace industry have been hindered by a limited list of approved casting alloys. A new research project aims to bulk up that list.

AFS Technical Department, Schaumburg, Illinois

More than three million components (excluding rivets and fasteners) are typically assembled into a single airplane. Each casting consolidation of these components offers the potential for significant savings in tooling, inventory, labor, rework, materials, design, testing and manufacturing. However, potential casting consolidations suffer a severe setback because currently, only a limited number of casting alloys have been approved for use by the Federal Aviation Administration (FAA). For a casting alloy to be used in an aviation component, the material properties data must be developed and validated on a component-by-component basis. This often results in a very conservative design approach, using casting safety factors to increase material section thickness.

Cast airframe components provide an opportunity to take full advantage of modern finite element designs with non-conventional shapes and uniform loading, in addition to simplifying a supply chain to remove manufacturing chokepoints. Effective utilization of cast components is estimated to reduce tooling costs alone by 25-35%. Because of this, an initiative is underway to add high priority metalcasting alloys to the Metallic Materials Properties Development and Standardization (MMPDS) handbook, which is the authority on statistically validated materials for the FAA.

The American Foundry Society (AFS) is teaming up with the American Metalcasting Consortium, several metalcasting facilities and airplane manufacturers to begin developing statistical-basis allowables for sand-cast E357 aluminum, which will be qualified, tested and collected following MMPDS procedures and practices. The goal is to remove the perception that castings are not reliable or reptitive. The metalcasting industry, like the polymer composite industry, must prove multiple suppliers can meet delivery with consistent properties. This requires metalcasting facilities to use well-defined alloy specifications with tight process control. The successful application of castings in airframes requires statistically validated mechanical and physical property data associated with value-added structural casting alloys.

From Investment to Sand Cast

The initial project will focus on developing the statistical-basis allowables for sand-cast E357 at six thicknesses from 0.125-2.5 in. Currently, only investment cast E357 is allowed by the MMPDS-02 handbook at five thicknesses (from less than 0.5 in to 2.5 in.). Design engineers need specific, quantitative documentation on the capability of castings as a function of different processes with different section thickness (cooling rates). The correlation among metallurgical features, non-destructive evaluation (NDE) signature and mechanical properties needs to be developed to build a technical basis for acceptance criteria based on performance. The group working on the project has verified that the procedures used to produce statistical-basis allowables are approved by the advisory board of the MMPDS handbook.

The development of statistical-based property for E357 aluminum will open the doors for a broader range of casting applications. Other benefits include:

* The Engineering Support Activities at the U.S. Defense Logistics Agency will be able to make cast alloy conversion/replacement decisions with confidence due to the statistical data on tensile, compressive, shear and bearing properties from the MMPDS handbook.

* Lead times with castings are shorter vs. forging and assemblies from sheet, plate and extruded mill products.

The project also will incorporate the use of computer aided engineering and modeling tools to lend reliability and repeatability in the process. The goal is to be able to predict how casting alloys will act in different designs during solidification with the aim of showing design engineers that the material properties proven to exist in test bars will exist in real life applications.

Weld repair procedures will be a third focus of the study. In-process welding is an economical means to fix localized cosmetic or processing damage to a casting. While individual studies show that proper heat treatment and after-welding inspection does not compromise the structural integrity of cast aluminum, defining the best practice provides a tool that results in a quality casting that can save cost and time, particularly during short production runs. The research team will develop a practice for in-process weld repair of aluminum-silicon hypoeutectic alloys, track the mechanical properties to demonstrate the effectiveness of the practice and provide a statistical basis for acceptance.

Plan of Attack

The 36-month program will start this spring after the final participating metalcasting facilities have been selected. Eight tasks have been outlined for the project:

1. Test Plate Design & Gating Definition

2. Collection & Assimilation of In-Process Welding Practices

3. Tooling Production & Cast 36 Test Plate Sets

4. X-Ray & Grade 36 Test Plate Sets

5. Welding Trials on Test Plates

6. Excise & Serialize Coupons from Test Plate Sets, Test & Analyze Samples for Properties

7. Identify Outliers & Compile Test Data

8. Correlate NDE, Structure & Properties

For a description of the design goals for each task, see Table 1. Each test casting will receive NDEs to assure that the data set provides results for the casting with greater than Grade C quality. After the samples are machined, they will be x-rayed to establish the local quality and allow stronger conclusions to be drawn from the pooled data.

After testing, approximately 10% of the pooled population will be investigated metallurgically to verify the interpretation of the x-ray. Both cooling rate and porosity will provide the basis for weld repair and dynamic properties. Static properties are insensitive to micro-porosity but dramatically affect fatigue and fracture properties. These heats will have sufficient castings to characterize the effect of weld repair on properties.

Once this initial program has been completed, it will be used as the framework to add other materials, such as stainless steel 16-6 and aluminum alloys 201 or 206, to the MMPDS handbook.

For More Information

"Review of Reliable Processes for Aluminum Aerospace Castings," M. Tiryakioglu, J. Campbell and N.R. Green, 1995 AFS Transactions (96-158).

Table 1. Tasks and Deliverables for Cast E357
Aluminum Statistical Properties Study

Task                                Deliverables and/or Milestones

1. Test plate design & gating       Gating for three test plates
   definition                       defined for production
                                    utilizing computer-aided
                                    engineering and analysis.
                                    Modeled results validated with
                                    real-time radiography of the
                                    mold filling.

2. Collection & assimilation of     Recommendations published
   in-process welding practices     internally for in-process
                                    weld repair practice.

3. Tooling production & cast test   Twelve heats with six
   plate sets                       replicate plates cast for
                                    characterization. Half
                                    retained for work on weld
                                    repair.

4. X-ray & grade test plate sets    Each test plate graded to
                                    assure greater than Grade
                                    C achieved prior to testing.
                                    Statistical evaluation of
                                    capability of NDE techniques.

5. Welding trials on test plates    Weld repair trials conducted
                                    using the working recommended
                                    practices for in-process weld
                                    repair. Specimens created for
                                    mechanical property testing.

6. Excise & serialize coupons       Coupons and machine tensile,
   from test plate sets; test       compression, shear and bearing
   & analyze samples for            samples cut. Test data
   properties                       analyzed and results suitable
                                    for statistical analysis
                                    compiled.

7. Identify outliers & compile      Outliers identified both in
   test data                        terms of properties and test
                                    sections with x-ray
                                    indications. Statistical-basis
                                    allowables for six thicknesses
                                    of sand cast E357 developed.

8. Correlate NDE, structure         Metallurgical evaluation
   & properties                     conducted to the sample sub-
                                    sets and outliers to correlate
                                    NDE signature, property and
                                    metallurgical nature of the
                                    indication.