Aluminum structural castings: following aircraft's lead.

Inside This Story:

* An understanding of the relationship between the aircraft and metalcasting industries has opened up the opportunity for cast components to play a more important role in aircraft structures.

* Detailed within are the unique requirements of the aircraft industry and how every metalcaster can use this information to expand their customer base.

Cast components have been used on aircrafts for many years. For the most part, they have been relegated to secondary structures, such as doors, handles, inlet rings and other non-structural applications. But recent advancement in the performance of cast aluminum alloys (such as D357) have allowed cast parts to be used as primary, load-bearing structures.

This leads to the question: how have metalcastings been successfully incorporated into aircraft structures?

The answer is that there is now a better understanding of the relationship between the technological requirements of the aircraft industry and those of the metalcasting industry. This is a lesson all metalcasters can learn from--regardless of customer base.

With this understanding, the opportunity exists to successfully consolidate fabricated and welded aircraft components into cast components. But only by understanding the industry and its unique requirements can metalcasters be successful in this realm. This article looks at some of those unique requirements of the aircraft industry and how metalcasters can use the knowledge to their advantage. By understanding similar requirements for their customer's industries, all metalcaster can benefit from speaking their customer's language.

Airframe Structures

Integral to the success of casting use in the aircraft industry is the understanding of aircraft structures, technology and some key design requirement.

With aircraft, first and foremost is the weight requirement for the airframe. An overweight airframe results in decreased passenger capacity, reduced range and reduced weapons payload and could ultimately lead to the cancellation of aircraft programs. Airframe structure weight can be influenced by a number of factors, including magnitude and source of loading of the structure, material properties, assembly methods and the consequences of failure of a particular portion of the airframe. Following is a look at each of these factors and how they relate to airframe structure design.

Structural Loads--Structural loads can be caused by flight conditions, such as lift generated by the wings, gust and turbulence, maneuvering and weapons release. They also can be induced by ground events, such as take-off and landing, side loads due to crosswind landings, taxi and catapulted take-off and arrested landings for naval aircraft. In general, flight loads result in fuselage down-bending and torsion (twist) and wing up bending and torsion. Ground loads generally result in the reverse, with the fuselage being bent up and the wings bending down. In certain circumstances, such as naval aircraft, these ground loads can be very severe and can result in a significant decrease in airframe life.

Consequences of Failure--Consequences of failure determine numerous requirements of an airframe and represent the uniqueness of the aircraft structure market. Chief among these is the economic impact because of airframe failure. Failure of an airframe component means a significant loss of revenue for an airline of the loss of a fighter jet during a conflict. In addition, failure during flight brings about the potential for loss of the aircraft and its passengers and crew.

Static and Fatigue Design--These requirements fall into two major divisions--static and fatigue loading conditions. Static design cases fall into two major categories--one-time failure modes and ultimate cases.

Failure cases include actuator jamming, hard and side landings, inlet hammer shock and crash conditions for seats and consoles. Ultimate conditions include maximum wing upbending, fuselage down-bending due to maximum G-forces, fuselage and wing torsion due to flight maneuvers and engine thrust and reverse thrust cases. For all of these conditions, individual parts and assemblies are allowed to plastically deform (develop permanent set), crack and become unstable so long as they do not rail catastrophically.

Fatigue failures generally do not directly result in catastrophic failures, but they can change the way an airframe functions. As a structural part develops fatigue induced cracks, it no longer resists forces in the manner in which it was designed, which results in a redistribution of loads and stresses and can then result in catastrophic failures in other portions of the airframe.

The ability of the structure to attain the required lifetime depends on the stress level in the structure, any designed stress concentrations (holes, steps, notches, etc.) and the mechanical properties of the materials chosen. Most fundamental to this is to maintain the minimum weight possible in the structure to achieve the aircraft's required range, passenger and payload requirements.

Material Properties--Material properties represent an obvious influence on airframe weight. Aircraft are generally fabricated using 2000 and 7000 wrought series aluminum alloys. These alloys have ultimate tensile strengths approaching 75,000 psi. Of the alloys typically cast for aircraft structure, the D357 alloy achieves the highest strength, with an ultimate strength of 50,000 psi in the T6 condition.

Equally important to the static strengths of the alloys used are their fatigue properties. Cast alloys typically have a lower fatigue life due to their lower static strength and their inherent porosity. All of these properties may result in a thicker cast structure that, in addition to being potentially heavier, can affect the way the structure itself behaves.

Component Assembly for Aircraft

When considering the impact of loading sources and types, material properties and consequences of failure on the weight of an airframe, a cast aluminum structural part can have a difficult time replacing a single, highly stressed part made from a wrought 2000 or 7000 alloy. The key to using castings as aircraft structures is to consolidate assemblies of parts. By focusing on assemblies, castings can take advantage of the ability to create lower-cost, more geometrically complex parts comparable to assemblies of wrought parts. To do this, the final contributor to airframe structural weight--aircraft structure assembly methods--must be examined

There are numerous methods of joining parts together--the most common being fastened joints, adhesive bonding and welding. Welding adds an undesirable uncertainty in the fatigue life of an airframe. This has precluded its use as an assembly method except under the most controlled circumstances (usually al significant expense). Adhesive bonding is an accepted joining process for composite assemblies, but suffers when subjected to tension loading. This leaves mechanical fasteners as the only acceptable method of assembling aircraft structure in a lightweight, fatigue dominant environment.

Mechanical fastening generally falls into three categories--shear joints, tension joints, and lug and clevis joints. Lug and clevis joints are generally reserved for highly loaded, removable attachments such as landing gears and control surfaces. They are usually heavy and require a large margin of safety since they are a single point failure source.

Further, gaps and the oversized holes have a significant reduction in the fatigue life of the joint due to the bending of the pin and the peaking effect of the small pin in the large hole. All of these complications are taken into account when designing and sizing shear joints in aircraft structure.

This combination of increased thickness at the joint, overlapping joint surfaces, shims, fasteners and sealant causes an increase in the weight of the assembly and provides the opportunity for castings to compete on a basis of airframe weight.

Casting Competition Economics

After looking at the technological requirements of airframes and the opportunities castings have to satisfy these requirements, the economic advantages available to cast structures in airframes become important. Castings have technological opportunities when used to replace assemblies of structural parts. There are also economic advantages that come about due to the assembly methods used to attain the close tolerances required for mechanically fastened assemblies.

To assemble aircraft structures requires tooling to locate the parts in the proper location, tooling to locate the holes, shims and sealant to fill any gaps and fasteners to hold the assembly together. Each comes with its own cost as either a nonrecurring of a recurring cost. Nonrecurring costs include any permanent or portable tooling to locate parts and the attaching holes. Saving these non-recurring costs can help offset the non-recurring costs associated with a new casting. Capturing these savings requires the casting design to be part of the initial fabrication plan.

Recurring costs are much easier to capture. They include the labor to assemble the tooling, details needed to locate the parts, the labor to locate the parts in the assembly tool and to determine the size of any shims and the labor to locate the holes on the parts. Additionally, significant labor is required to drill the holes, disassemble the parts, deburr the holes. apply sealant to the mating surfaces. reassemble the parts, install the fasteners and clean any excess sealant. This can become a very expensive portion of the cost of the assembly. Certain materials, or combinations of materials (joining titanium and aluminum), require power feed drills to drill precise diameter holes within the tolerance required. The power feed systems take additional time to set up and move from hole to hole.

Further complicating this is the potential for detects during the drilling process. These defects can include mislocated holes, holes with short edge margin, oversized holes and holes that are oblong. Also, there are several types of defects that can occur during the installation of the fasteners. These can include improperly seated fasteners (head gap), improper torque of the nuts and collars and incorrect length of the fasteners (inadequate thread protrusion). All of these defects can add significantly to the cost of assemblies due not only to the actual rework, but the documentation and engineering effort required to approve the rework and the inspection required to verify it was performed correctly.

The assembly of aircraft structures requites tooling to place the parts in the proper location and locate the holes, shims and sealant to fill any gaps and fasteners to hold the assembly together.

Weight efficient aircraft structural assemblies require numerous holes, which add significant cost to the assembly because of the labor involved, the cost of the fasteners and the cost of reworking the defects. Replacing these assemblies with castings can provide a significant cost savings.

Total Cost

A key difference between design simplification and numerous other cost reduction methods is the effect on product design. Methods of procedural changes and improvements seek to eliminate unnecessary labor and support systems (tools), leaving the engineering design configuration essentially intact. The focus of parts consolidation is the necessary redesign of a product to minimize the volume of detail parts and the tools, labor and rework costs associated with assembly. Because of the necessity for redesign, many design improvements are relegated to emerging programs where such a structure will likely undergo design revision anyway.

At first glance, replacing an assembly of numerous, inexpensive parts with a more complex and inexpensive monolithic component provides only modest returns. However, a more encompassing review of the advantages monolithic designs have over a process can be seen in Fig. 1, which is a comparison between determinant assembly and monolithic approaches to the 767 main landing gear door uplock structure.

[FIGURE 1 OMITTED]

Components of part cost are divided into recurring and non-recurring costs and include the following (at a minimum):

* recurring costs--raw material, manufacturing and inspection labor, rework, scrap, inventory;

* non-recurring cost--design, tooling, structural testing, part qualification, manufacturing planning.

The accumulated cost to produce a part over its typical production life will often mask the significant cost drivers. Part design, while miniscule compared to accumulated product cost, is the single most influential element. Engineering design determines 70-80% of a product's cost. This relationship is important when consolidating parts into more monolithic structures. Such redesign requires engineering ingenuity and an appetite for new approaches to component design that capitalize upon the process advantages inherent with castings.

To succeed in marketing and securing castings for aircraft structural components, metalcasters must possess the knowledge of the engineering of the end-product. Only by evaluating the casting within this framework will the potential of cast components be realized.

Table 1. Comparing Castings and Assemblies

                                 One-Piece Casting

Shims eliminated         25 immediately, 7 more gradually

Impact to inventory         27 part numbers eliminated

Impact to                 65% reduction in internal flow;
process now                 factory shortage eliminated

Impact to                7 drawings, 19 sheets eliminated;
engineering             7 drawing changed, 1 drawing added

Impact to planning    30 detail, 2 assembly plans eliminated;
                              revise 4 assembly plans

Impact to tooling          eliminate 2 FAJs immediately,
                                 2 more gradually

Effective                             1 year
implementation

Concerns                      possible weight growth

Savings/airplane                     baseline

Payback period                     27 airplanes

                                Precision Assembly

Shims eliminated                  most gradually

Impact to inventory                 no change

Impact to                   <20% in assembly flow time
process now

Impact to                     many changes over time
engineering

Impact to planning          all detail, assembly plans
                            must be revised; extensive
                               vendor coordination

Impact to tooling         eliminate 2 FAJs after process
                          control established, eliminate
                                 2 more gradually

Effective                    some improvement evident
implementation                     in 6 months

Concerns                 variation reduction of numerous
                        details involving many suppliers;
                          engineering definition of hole
                      coordinates; possible detail redesigns

Savings/airplane                15-20% of baseline

Payback period                          ?

This article was adapted from an article in the Winter 2004 issue of Engineered Casting Solutions.

For more Information

"Airplanes--The Sky's the Limit for Aluminum Use," Aluminum Now, Spring 2000, p. 8.

"Casting Conversions Fly to the Forefront at Boeing," A.T. Spada, Engineered Casting Solutions, Fall 2000, p. 27-32.

David Heck is a senior specialist engineer for Boeing--Phantom Works, St. Louis, Mo.