Magnesium's HCP Structure Makes It Light and Hard to Form

Magnesium’s defining property, the lowest density of any commonly used structural metal, and its defining limitation, comparatively poor room-temperature formability, trace back to the same crystal structure.

Why HCP limits formability, specifically

A metal deforms plastically by slipping along specific crystal planes, covered in different contexts on copper’s and aluminium’s mechanics pages. FCC metals like aluminium and copper have many such slip systems available in many directions, which is why they can be rolled, drawn, and stamped easily at room temperature. HCP metals like magnesium have, at room temperature, only one easily activated slip plane (the basal plane), a real, specific crystallographic restriction, not a processing failure, which is why magnesium sheet is comparatively difficult to form cold and is more often cast or formed at elevated temperature, where additional slip systems activate.

Why density still wins the argument for many applications

Despite the forming limitation, magnesium’s density advantage over aluminium and steel, covered on this material’s mechanics page, is large enough that die-cast magnesium parts, which sidestep the room-temperature forming problem entirely by casting the part to near-final shape, remain genuinely competitive for weight-critical components.

Sources: slip system counts and HCP formability limitations draw on standard physical metallurgy references; lattice parameters are standard crystallographic values.