Forging vs. casting

Thousands of aircraft components are critical to consistent, safe operation. Couplings, locking brackets, valves, sling mounts, hubs, and special fasteners are can’t-fail parts. To make them, manufacturers need the best materials with the best processes.

Although casting has been a choice for affordable, quick, and complex parts, forging is a superior process when forming parts that can’t fail. Forging provides numerous benefits for service life and performance in parts subject to extreme stresses and adverse environments.

Metal forming basics

Casting includes heating metal beyond its melting point, pouring it into a mold, sealing it, and letting it cool and set in place. Forging heats metal to less than its melting temperature into a state where it is pliable and soft and can deform easily, but it doesn’t begin to flow. Then, large presses, using extreme pressures, force the metal to form around a special plate called a die in a process called die forging. There are other forging processes, but die forging is the primary method for forming complex shapes in aerospace parts.

Research at the Mechanical, Industrial, and Manufacturing Engineering departments at the University of Toledo1 found forged parts have:

• 26% higher tensile strength – parts can suffer greater pulling stresses; suitable for mount, sling hanging
• 37% higher fatigue strength – greater length of service for forged parts
• 44% greater yield strength – parts can suffer greater stresses before deforming
• Greater deformation area before failure – part will deform more before failing completely

Forging benefits

How and why does the forging process create better and longer-lasting parts, and why are forged aerospace parts superior?

Porosity – Cast parts have higher porosity, the presence of empty spaces within a part, which can appear as small bubbles or tiny jagged voids. Because forging starts with extruded material (bar or shape) and further forms the basic raw material shape into near-net finish part geometry, pores have fewer opportunities to form. Porosity is formed through two main methods: trapped gases and shrinkage voids. During a casting, gases may become trapped if the seal is not completely perfect. These gases will form small, smooth bubbles inside the part, usually close to the surface. Shrinkage porosity occurs when the part cools unevenly, and as certain parts solidify and condense, other portions of the internal part structure have less material to fill the space. Thus, the structure will show small, jagged voids throughout.

Porosity is an unavoidable by-product of casting, although certain methods can be used to minimize it. In forging, the metal isn’t melted, eliminating the opportunity for gas porosity, thereby making forged aerospace parts resistant to failure modes and defects porosity can cause.

Interlocked grain structure – Forged parts have a stronger interlocked grain structure. All metals form a crystalline structure when they cool, but that structure can be deformed quite drastically, which causes the grains to bind and interlock in a more complex pattern. The complex structures produced through deformation energy allows simple aluminum parts to approach the powerful properties of complex alloys such as Inconel. This improved metallurgy translates to greater performance, longer-lasting pieces, and greater fatigue properties. In addition, these properties can be achieved with significantly cheaper base materials. The accessibility of cheaper materials also bypasses the necessity of researching exact alloy compositions and complex testing procedures.

Post-forming processes – Are more effective and simpler with forgings. Forged, near-net shape parts are less expensive to machine because the forgings are made with tighter tolerances. It’s common to reduce machine cycle time by 50% compared to a billet process. Forged material tends to be less abrasive than cast surfaces and will also extend tool life. Forged parts will also have a smoother surface, which makes coatings and paints bind more evenly and cleanly to the part’s surface. Castings often must be smoothed before a coating can be properly applied, which increases time and cost.

Summary

Forging creates a reliable, uniform, and stronger part, with faster turnaround at an affordable price. Full-service forging shops offer design and manufacturing expertise to create forged aerospace parts to exact specifications. A one-stop-forge-shop can perform in-house heat treatment and has a great network of sub-contractor shops for other post-forging processes. A top-tier forging shop will have everything needed to deliver aerospace parts recognized for their quality and precision craftsmanship.

Anchor Harvey

About the author: Kerry Kubatzke is the lead sales manager of aluminum forging company Anchor Harvey, Freeport, Illinois. He can be reached at 888.367.4464 or kkubatzke@anchorharvey.com.

1. “Fatigue Performance Comparison and Life Prediction of Forged Steel and Ductile Cast Iron Crankshafts”