The two alloys are often compared because both are heat treatable 6xxx grades with good corrosion resistance, yet their magnesium and silicon levels differ enough to change strength, extrudability and surface quality. Understanding those differences prevents over-specifying strength where appearance matters and under-specifying where load capacity is critical.
Temper designations also matter: T5 denotes cooling from the extrusion press followed by artificial aging without separate solution treatment, while T6 denotes a full solution treatment, quench and artificial age to peak strength.
Composition and Designation
Alloy 6061 typically contains 0.8 to 1.2 percent magnesium and 0.4 to 0.8 percent silicon, together with controlled copper and chromium. Alloy 6063 carries lower magnesium and silicon, roughly 0.45 to 0.9 percent magnesium and 0.2 to 0.6 percent silicon, and is essentially free of copper.
The lower alloy content of 6063 improves extrudability and produces a brighter, smoother surface after anodizing, while the higher content and copper in 6061 raise strength at the cost of extrusion speed. Both are covered for composition by EN 573-3 and for mechanical properties by ASTM B209 for plate and ASTM B221 for extrusions.
Mechanical Property Comparison
In the T6 temper, 6061 plate typically reaches about 310 MPa tensile strength and 275 MPa yield strength. Alloy 6063-T6 is considerably softer, with tensile strength near 240 MPa and yield strength around 205 MPa, while 6063-T5 is lower still at roughly 185 MPa tensile and 145 MPa yield.
Elongation favors 6063, which deforms more before fracture and tolerates bending and forming better than 6061 in the same temper. For a given load, a 6063 section usually needs a larger cross section than a 6061 section to reach the same safety factor.
Extrusion and Surface Finish Behavior
6063 extrudes quickly into complex thin-walled shapes with excellent dimensional control and a smooth as-extruded surface, which makes it the default choice for architectural window frames, door sections, heat sinks and trim. It responds well to clear and color anodizing and produces consistent, defect-free oxide layers.
6061 extrudes more slowly and is usually reserved for thicker, load-bearing profiles. Its surface is slightly less bright after anodizing because of the alloying elements, but it accepts anodizing, powder coating and painting without difficulty.
Welding, Machining and Selection Guidance
Both alloys weld readily with 4043 or 5356 filler, but both lose strength in the heat-affected zone. 6061 machines considerably better and is the preferred grade when threads, bores or precise pockets are required. 6063 machines acceptably but tends to be gummy.
Select 6061 when structural strength governs, such as brackets, machine frames, trailer components and marine hardware. Select 6063 when surface appearance, intricate profile shape and low weight govern, such as window systems, handrails, light fixtures and display structures.
Frequently Asked Questions
Q: Which is stronger, 6061-T6 or 6063-T6?
6061-T6 is stronger, reaching about 310 MPa tensile and 275 MPa yield strength compared with roughly 240 MPa tensile and 205 MPa yield strength for 6063-T6.
Q: What does the T5 temper mean?
T5 indicates that the alloy was cooled from an elevated temperature during extrusion and then artificially aged without a separate solution treatment, giving moderate strength and good dimensional stability.
Q: Why is 6063 preferred for architectural profiles?
Its lower alloy content allows faster extrusion of complex thin walls and produces a smoother surface that anodizes to a bright, uniform finish.
Q: Can 6061 and 6063 be welded together?
Yes, the two alloys can be joined with 4043 or 5356 filler, but the joint strength is governed by the weaker heat-affected zone and should be factored into the design.
Q: Which alloy is better for machining?
6061 is clearly better for machining because its higher alloy content gives cleaner chips, better surface finish and more consistent results in turning and milling operations.