The 6000-series aluminum alloys, particularly 6061 and 6063, are frequently used in modern construction and manufacturing due to their favorable strength-to-weight ratio and corrosion resistance. Although they belong to the same family and look nearly identical, subtle differences in composition lead to vastly different performance characteristics and applications. Using the wrong alloy can compromise a project’s structural integrity or aesthetic finish, making identification necessary. Understanding the fundamental differences in their physical properties and applying simple, comparative field tests provides a practical way to identify which alloy you have.
Understanding Structural Characteristics
The primary components in both 6061 and 6063 aluminum are magnesium and silicon, forming the basis of the 6xxx alloy series. Aluminum 6061 contains higher percentages of alloying elements, including copper and chromium, which are minimal in 6063. This chemical difference dictates the distinct mechanical responses of each alloy, especially after heat treatment to the common T6 temper. The higher concentration of elements in 6061 allows it to achieve significantly greater hardness and strength, making it the preferred choice for load-bearing structural applications.
In the T6 temper, 6061 aluminum achieves a Brinell hardness of approximately 95, compared to 6063 aluminum’s Brinell hardness of roughly 73. This difference translates directly to a higher yield and ultimate tensile strength for 6061, which is why it is often referred to as “structural aluminum.” Conversely, the composition of 6063, with fewer alloying elements, makes it easier to push through an extrusion die to form complex cross-sectional shapes.
The superior extrudability of 6063 results in a smoother surface finish and allows for tighter tolerances on intricate profiles compared to 6061. Both alloys are weldable, but 6061 retains more strength after welding, though it often requires a specific filler metal to prevent cracking. Aluminum 6063 is generally easier to work with for aesthetic welds due to its cleaner chemistry, despite potentially losing strength if not re-heat-treated afterward.
Appearance and Common Uses
Identifying these alloys based on their application context provides the simplest initial clue, as manufacturers select the material based on required performance and finish. Aluminum 6063 is often called “architectural aluminum” because its low alloying content and excellent surface quality make it ideal for visible, decorative elements. It accepts anodizing exceptionally well, resulting in a clean, uniform, and aesthetically appealing finish. This smooth appearance is typically found on window frames, door trim, non-load-bearing railings, and plumbing pipe.
In contrast, 6061 is chosen when strength is the primary factor, even if it means a rougher initial surface finish. This alloy is frequently used for heavy-duty components that must withstand significant mechanical stress or impact. Common applications include truck beds, trailer frames, boat hulls, scaffolding, and structural members in aerospace and industrial machinery. If the metal is a thick plate, large-diameter bar stock, or a raw-finish structural angle, it is likely the higher-strength 6061 alloy.
Simple Identification Tests
Since appearance and context are not definitive, simple mechanical tests can be used to compare the relative hardness of the two alloys. The difference in Brinell hardness is substantial enough to be detected by a comparative scratch test using a hardened steel point or a sharp file. Because 6061-T6 is significantly harder, applying consistent force to both a known 6063 piece and the unknown piece should result in a noticeably shallower, more difficult scratch on the 6061.
Another practical method involves observing the material’s behavior when a small amount is removed, such as by filing or machining. When 6061 is filed or cut, its higher strength and hardness cause it to produce small, brittle, and well-broken chips. The softer 6063 alloy, with its lower tensile strength, tends to create longer, more continuous shavings or chips that can occasionally gum up the cutting tool.
Both alloys have nearly identical density, approximately 2.7 g/cm³, meaning a weight comparison of two equally sized pieces will not reliably distinguish between them. While chemical spot tests exist, they are generally less reliable and more hazardous for the average user than mechanical tests. The scratch and chip formation tests, combined with application clues, offer the most accessible field identification methods.