6351 is a heat-treatable 6xxx-series (aluminum-magnesium-silicon) wrought alloy developed as a higher-strength structural extrusion grade than the ubiquitous 6063. Where 6063 is optimized primarily for surface finish and extrudability in architectural sections, 6351 carries meaningfully more silicon and adds manganese to the mix, giving it noticeably better mechanical properties once solution treated and aged — without giving up the good extrudability and weldability that make the 6xxx family so practical to fabricate.

That balance of higher strength and continued good workability made 6351 a long-standing choice for structural extrusions, pressure piping, load-bearing frames, and general engineering sections where 6063 wouldn't have enough strength margin but a higher-alloy grade would cost more to extrude and finish. In much of the world it has since been substantially replaced by EN AW-6082, a similarly-positioned but more tightly standardized modern alloy, though 6351 remains in service and in specification on plenty of existing equipment and legacy drawings.

6351's extra silicon and manganese compared with lower-strength 6xxx grades like 6063 push its machinability slightly in the harder direction, but the difference is modest — this is still a magnesium-silicon aluminum alloy, and it still shears easily and conducts heat away from the cutting edge far faster than steel or titanium. Cutting forces stay low and surface speeds stay high relative to any ferrous material, so the same high-speed, light-force approach used across the 6xxx family still applies.

The manganese addition, which mainly serves to refine grain structure and improve strength, has a secondary effect on chip formation: it tends to make chips slightly tougher and less prone to gumming than a very soft, low-alloy aluminum, which can actually help chip breaking in some operations. As with other 6xxx alloys, the main quality risk is built-up edge at low cutting speeds or with dull tooling — sharp, positive-rake, polished tooling run at proper speed avoids the smeared finish and dimensional drift that a hesitant approach tends to cause.

Because 6351 is most often machined from extruded structural shapes, watch for wall-thickness variation and residual extrusion stress in thin-walled sections, which can cause parts to move slightly during or after machining if stock removal isn't balanced across the section.