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Stress Relieving Processes for Aluminum Alloys

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Informed manufacturers understand the benefit of knowing about the full variety of stress relieving processes available for aluminum alloys.  Having a stable, durable aluminum product able to withstand stresses is a requirement no matter what industry is being discussed.  That’s why it’s helpful to have a thorough understanding of the various means for relieving stress, thereby lengthening the life cycle of a product.

One of the most common options for relieving stress comes in the form of heat treatment, and there are a variety of alternatives to choose from.  Other possibilities include vibration stress relief, cryogenic stress relief and more.  Knowing which method is most appropriate for a particular application is essential.

The fact that stress relief can be such a complicated process for those who work with aluminum is one more reason it’s important to partner with a committed and dedicated materials supplier who understands the nuances involved with stress relief of aluminum alloys.

What is stress relief?

In the world of mechanics, stress refers to the measure of internal forces caused by the particles within a material exerting pressure on each other.  The term used to measure this stress is known as strain, which indicates the amount of deformity caused by the internal stress of a material.  It is important to note that even if the stress itself is internal, the pressure being exerted on the material can be external in origin, such as a metal bar supporting the weight of another object.  If that object is so heavy that the amount of stress overwhelms the bar, then internal deformities can occur.

The strain within a material can be caused by a variety of mechanisms.  These can include stress applied by external forces to the bulk of the material (Ex: gravity) or to its surface (Ex: contact forces, external pressure, friction).  A common form of stress includes residual stress, which is often left over after the manufacturing process.  Residual stress can cause the alloy to be more susceptible to deformation, corrosion and metal fatigue. Structural stress determines how well the tensile strength of the material will hold up under applied forces and conditions.  There’s also thermal stress, caused by extreme temperature changes leading to expansion and contraction of the metal.

What are the common heat treatment methods for aluminum?

One of the most common means of providing stress relief in metal is through a variety of heat treatment methods.  There are several ways to heat treat aluminum.  It is common for aluminum alloys to be work hardened early in the production cycle.  Also known as strain hardening, the purposeful plastic deformation of a work piece will alter the crystalline structure within the metal, which can then be reset through annealing.

The aluminum is heated to temperatures between 570°F and 770°F for up to three hours. This will relieve the stress that occurred during the work hardening process and help resolve warping and other issues.  One of the benefits of annealing is it can be used on alloys that are considered non-heat treatable, such as those in the 3xxx, 4xxx and 5xxx series.  If there is an H2 in the alloy number, this indicates it has undergone annealing.

Another form of heat treatment is known as solution heat treatment.  This is when the metal is soaked in a solution at a high temperature, between 825°F to 980°F, and then quenched to cool the material quickly.  This will trap dissolved elements that will subsequently precipitate out and lead to an age hardening effect.  Immediately after quenching, the metal will be easy to work with, but it gradually hardens and becomes more difficult.

Other heat treating options include homogenizing, natural aging and precipitation hardening (which is essentially an artificial aging process).

What is vibration stress relief?

Manufacturers have more options than heat treating to relieve stress in aluminum alloys. One of the popular methods is known as vibratory stress relief.  Abbreviated as VSR, this non-thermal stress relief method is used in metalworking to enhance dimensional stability and mechanical integrity, especially for aluminum that has been cast, forged or welded.

VSR is targeted at residual stress, which remains after whatever induced the stress has been removed.  It happens following inelastic deformation, temperature gradients and structural changes.  For example, during welding, the heat causes localized expansion, and during the cooling process, some regions will contract more than others, resulting in residual stress.

VSR is commonly used with precision components that have extremely tight dimensional or geometric tolerances.  It is also good for use with workpieces that have been heavily loaded, such as with lifting yokes, clamshell buckets, crane bases, ingot processors and rolling mills.

Vibratory stress relief targets the metal’s resonant frequency to maximize the amount of stress relief.  The process involves flexing the aluminum with enough amplitude to combine the induced stresses with the residual stresses.  This results in plastic flow, which is what leads to stress relief.

How can cryogenics help with stress relief of aluminum?

Another type of stress relief involves cryogenics.  This is when an aluminum part is put into a special tank and exposed to liquid nitrogen.  The temperature will cool to as low as -300°F and the metal will remain there for a set amount of time, depending on the type of alloy and the thickness.  It is then gradually warmed to room temperature.

The cryogenic process is an alternative to the more common forms of heat treating.  It removes residual stress and bolsters wear and corrosion resistance.  After aluminum has been treated in this manner, it will show decreased tendency to distortion and be both stronger and more durable.  Other benefits include reduced stress cracking, a lower coefficient of friction and greater impact resistance.  Parts that have been treated in this way will be more open to machining and redressing, and the finished parts will have an increased lifespan.

To learn more about the many options when it comes to stress relieving aluminum alloys, contact a friendly and experienced customer service representative at Clinton Aluminum today.

 

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