The two grades

Grade AGrade B

Property	Aluminium 6061-T6	Stainless Steel 17-4 PH (630)
Grade	Aluminium 6061-T6	Stainless Steel 17-4 PH (630)
Family	aluminium	stainless-steel
Condition as supplied	T6 - solution heat treated and artificially aged at the mill	Condition A (annealed) or aged to H900 / H1025 / H1075 / H1150
Density	2.70 g/cm³	7.80 g/cm³
Density difference	Aluminium 6061-T6 is 188.9% lighter
Tensile	310 MPa	1,310 MPa (H900)
Tensile difference	Stainless Steel 17-4 PH (630) is 323% stronger on the published figures
Hardness	95 HB	32 HRC (annealed) · 44 HRC (H900)
Machinability (shop scale 1–10)	9/10	4/10
Heat treatment	There is no heat treatment after machining. The mill supplies T6 and nothing you do on a machine will raise it; welding drops the heat-affected zone back to O condition and it cannot be recovered without a full re-solution and age, which distorts the part.	Precipitation hardening, and it works properly in the shop: machine in the annealed condition, then age to the required condition. H900 gives the highest strength at 44 HRC, H1150 the highest toughness at around 30 HRC. Distortion is modest but real, so critical dimensions are finished after ageing.
Typical finish	Ra 1.6 µm	Ra 1.6 µm annealed
Best finish	Ra 0.4 µm	Ra 0.4 µm
Cutting speed	500–900 m/min (carbide)	120–200 m/min annealed (carbide) · 60–90 m/min at 44 HRC (ceramic or CBN)
Feed	0.15–0.40 mm/rev turning · 0.10–0.25 mm/tooth milling	0.10–0.25 mm/rev turning · 0.06–0.15 mm/tooth milling
Tooling	Uncoated or ZrN carbide, high positive rake, polished flutes	Coated carbide annealed; CBN or ceramic inserts for the aged condition
Coolant	Flood for deep pockets and tapping; air blast or MQL is enough for open milling	Flood, and never dry on the aged material
Lead time as published	5–10 working days	10–15 working days

What the difference means

Aluminium 6061-T6 rates 5 points higher on the shop machinability scale. On a 1–10 gap of that size the difference shows up as cycle time and tool wear more than as surface finish - and cycle time is what the quote is made of.
The density gap is over 20%, which is large enough to change the part, not just its weight: rotating and reciprocating parts see it as inertia, and a lighter part can sometimes pay for a more expensive material through reduced downstream cost.
These are different families (aluminium and stainless-steel), so the corrosion and temperature behaviour differ by more than any single number above shows. Compare the two grade pages for the service conditions before deciding.
Aluminium 6061-T6 is typically finished to Ra 1.6 µm and Stainless Steel 17-4 PH (630) to Ra 1.6 µm annealed. If the drawing calls a finish that is not achievable on the chosen material without an extra operation, that extra operation is a separate quote line.

Reading this comparison

Machinability is our own 1–10 shop ranking, not a published index - it ranks how the materials behave on our machines, and it is the number to use when comparing cycle time rather than when quoting a datasheet.
Cost is deliberately absent: this tool holds no price list, and a material price from a page that cannot refresh is worse than no price at all. The comparison that matters is how many machining hours each grade takes.

Both columns are read from the published record for each grade, including the shop’s own 1–10 machinability ranking. It compares properties, not costs, and it is not a recommendation.

What you provide

Two grades from the published list

Material Comparison