Aluminum for Airplane Door Frames: Why Are A7050 and A7475 Priority?

Why is the door frame a special place on the fuselage?

The fuselage of the aircraft is a continuous structural system consisting of skin, frame and reinforcements that participate in transmission. When creating a large opening to accommodate a passenger door, crew door or cargo door, the initial transmission path is interrupted and must be routed around the door cutout area. This makes the door frame area a special structural location. 

The load no longer travels along a continuous path but must be redistributed through: 

• frame above
• bottom frame
• two sides of the door
• door corners
• fastener
• fitting
• connection area between door frame and fuselage structure.

The corners of the door cutout are especially notable because this is where the geometry changes markedly and stress concentrations can easily form. 

The FAA is still recording cases of cracks appearing at the door cutout corner, in which the cause is determined to be related to high operating stress combined with stress concentration in this area. Therefore, material selection for door frames cannot be based solely on which grade has the highest tensile strength. 

What requirements does aluminum for aircraft door frames need to meet?

The door frame must both maintain the shape of the cutout area and participate in transmission and work through many operating cycles. Therefore, we always look at materials under many combined criteria instead of evaluating one mechanical parameter separately.

Important requirements include:

In addition to the above criteria, detailed shape, workpiece thickness, temper and material orientation also need to be considered together. An aluminum grade with very high mechanical properties in thin form may not perform exactly the same when transferred to a thick plate or part that is loaded in many directions.

Why is A7050 suitable for aircraft door frames?

A7050 belongs to the high strength 7xxx aluminum alloy group and was developed for aerospace structures that need a balance between strength, fracture toughness and resistance to stress corrosion cracking.

In the aerospace materials application data we use, A7050 is in the alloy group that can be used for window/door frames along with A7475, A7040 and A7140. This is also the mark that Oristar is adding to the group of materials serving the aerospace industry.

Strengths of A7050 with door frame

The A7050 is notable for its combination of:

• high intensity;
• good toughness;
• Resistant to cracking due to stress corrosion at appropriate temper;
• Ability to apply to plates with large thickness;
• suitable for load-bearing structures;
• Ability to machine parts with complex geometry.

With door frames, the thick section characteristics are especially important. Door frame is not like fuselage skin, which mainly uses thin sheets. Many areas of the frame require larger cross-sections to redistribute loads around the opening, accommodate fasteners and create connection areas with surrounding structures.

Therefore, the material needs to maintain balance in mechanical properties throughout the entire section. The strength of A7050 with door frames does not lie in a single strength value but in the ability to balance strength, toughness and SCC resistance in a load-bearing structure with a relatively large thickness.

What's remarkable about A7050-T7451?

The new A7050 brand name only indicates the alloy composition. When applied to aerospace structures, temper is an important part that determines the actual mechanical properties of the material.A7050-T7451 is a remarkable state of the art aerospace plate because it strikes a balance between strength, toughness and resistance to stress corrosion cracking.With structures like door frames, frames or bulkheads, this balance has greater significance than just maximizing strength.

The mechanical properties of thick plates can vary according to:

• position in thickness;
• rolling direction;
• heat treatment state;
• grain orientation;
• cooling rate during heat treatment.

This is especially noticeable with large load-bearing parts. Therefore, A7050-T7451 a few tens of millimeters thick and A7050-T7451 at a very large thickness should not be considered two completely identical cases just because the alloy and temper names are the same.

How is the A7475 different from the A7050?

A7475 also belongs to the high strength 7xxx aluminum alloy group but is noted for its fracture toughness and damage tolerance. In aviation structures, these are two especially important factors in areas at risk of fatigue crack or stress concentration formation. According to the application data we are using, A7475 is also in the alloy group that can be selected for window/door frames according to each specific specification.

It can be viewed in general as follows:

This table is not intended to determine whether the A7050 or A7475 is “better”. In a specific door frame, the choice also depends on:

• design load;
• thickness;
• embryo form;
• temper; temper;
• material direction;
• processing plan;
• approved specification.

Therefore, do not replace A7050 with A7475 or vice versa without technical approval.

Why are airplane door corners prone to fatigue crack?

Door cutout creates a large change in geometry on the fuselage. When the load must be transmitted around this opening, the door corners become locations that require very careful control of stress distribution.

In addition to the shape of the cutout, there are many factors in this area that make the problem more complicated:

• fastener holes;
• fitting;
• linkage;
• change in thickness;
• transitions between details;
• repetitive loading due to pressurization;
• body deformation during flight.

Each fastener hole creates small discontinuities in the material. If the geometry, surface or local loading is not well controlled, the area around the hole can become the starting point for fatigue crack.

The FAA recorded a crack in the forward upper corner of the door cutout and determined the cause was directly related to high operating stresses and stress concentration at the door corner.

Here's why with door frames, we shouldn't just care about:

• yield strength;
• tensile strength.

But still have to see:

• fracture toughness;
• fatigue behavior;
• crack growth;
• SCC resistance;
• surface quality;
• grain orientation.

Why is fracture toughness especially important for door frames?

In actual operations, it cannot be assumed that an aerospace structure will always be completely defect-free. Small scratches, fastener holes, surface defects or stress concentrations can all create conditions for cracks to form.

Fracture toughness represents the ability of a material to resist unstable crack growth.

With door frames, this is especially important because the door area has:

• complex geometry;
• multiple connection points;
• load is redirected;
• repeated loading;
• stress concentration area.

Therefore, a material with very high strength but inappropriate toughness is not necessarily a good choice for this structure.

This is also the reason why A7050 and A7475 need to be evaluated according to their overall properties instead of just comparing tensile strength.

Are A7040 and A7140 used for aircraft door frames?

In addition to A7050 and A7475, the aviation industry also uses many other generation 7xxx alloys to optimize each structural problem.

According to the material application data we are using, the window/door frame group may also encounter:

• A7040;
• A7140.

However, these are not marks that should automatically replace A7050 or A7475.

Usage depends on:

• aircraft program;
• specification; specification;
• product form;
• temper; temper;
• thickness;
• design requirements.

Therefore, when receiving RFQs for specialized alloys, we always need to fully check the specification instead of determining the material based only on the alloy name.

Oristar supplies aluminum for aircraft door frames according to specification

At Oristar, with aerospace material requirements, we not only check the name but also compare alloy, temper, product form, specifications, specifications and document requirements.

Our current list of materials for the aerospace industry includes grades such as A2014, A2024, A6061, A7050 and A7075.

For other specialized alloys such as A7475, A7040, A7140 or labels specifically indicated on the drawings, we accept inspection and orders according to:

• specification; specification;
• temper; temper;
• material form;
• specifications;
• quantity;
• required documents.

Quality management system AS9100

Oristar has achieved AS9100 certification - a quality management system for the aviation, space and defense industries.

AS9100 is a certification for a business's quality management system, not a separate certification for an aluminum brand.

Deploying this system helps us to more closely control the specific requirements of the aviation supply chain, especially regarding:

• procedure;
• file;
• quality control;
• traceability;
• Manage customer requests.

Documentation and material retrieval

Depending on the requirements of each order, customers can request documents such as:

• CO;
• CQ;
• Mill Test Certificate/Mill Cert;
• heat/lot information;
• material retrieval records.

With aviation parts, batch traceability is especially important.

Heat/lot codes on materials need to be managed consistently with technical records so that the origin can be determined when needed for comparison.

Prepare workpiece before machining

If customers need pre-CNC blanks, Oristar can support as required:

• cut to size;
• cut according to drawings;
• chuẩn bị lượng dư;
• phay 4–6 mặt;
• kiểm soát độ phẳng;
• kiểm soát độ vuông;
• Prepare the workpiece close to the machining size.

Mục tiêu là giảm các bước xử lý thô tại xưởng của khách hàng và chuẩn bị đầu vào phù hợp hơn cho công đoạn CNC chính xác.

Kết luận

Aluminum for aircraft door frames must solve a special structural problem: door cutout interrupts the transmission path of the fuselage and creates stress concentration areas at the door corners, fastener as well as the connection area. For more detailed information, contact Oristar immediately

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