Magnesium Machining Guide: The Lightest Structural Metal

Magnesium Machining Guide: The Lightest Structural Metal Magnesium is about a third lighter than aluminum — and it brings a fire risk that separates serious shops from casual ones. Here is when to use it and what to ask. Why magnesium exists in the material list Magnesium is the lightest structural metal at roughly 1.74 g/cm³ — about two-thirds the density of aluminum and a quarter of steel. For aerospace, handheld and portable products where every gram counts, that weight saving is the whole reason to choose it. The cost is real: magnesium is less stiff than aluminum per unit volume, and it must be machined with fire-safe practices. Why engineers pick magnesium: weight, damping and EMI Weight is the headline, but it is not the only reason. Magnesium at roughly 1.74 g/cm³ cuts about a third off an equivalent aluminum part and around 75% off steel, and it does it without the machining penalty of titanium. Beyond density, three properties decide whether magnesium is the right call. Vibration damping Magnesium has a high specific damping capacity — it absorbs vibration and converts it to heat faster than aluminum or steel. For housings around motors, optical benches, camera bodies and hand tools, that means less resonance, lower audible noise and longer fatigue life in the surrounding assembly. If your part rings or buzzes, magnesium often quiets it. EMI and RF shielding Magnesium is electrically conductive, so a machined or cast magnesium housing shields the electronics inside it from electromagnetic and radio-frequency interference the way a conductive enclosure should — without the conductive coating, gasket or foil that a plastic enclosure would need. This is why magnesium shows up in laptops, camera bodies, drone gimbals and handheld instruments. Thermal behavior Magnesium conducts heat well — pure magnesium around 156 W/m·K, with alloy values a little lower — so it spreads heat away from hot components instead of trapping it. Combined with low density, that makes it a candidate for heat sinks and enclosures that must be light and cool. The trade-off to watch is creep: standard AZ alloys soften and creep under sustained load above roughly 120–150°C, so elevated-temperature parts move to WE43 or ZK60. Machinability Magnesium cuts at high speed with low tool wear and a fine finish, so complex, feature-dense parts are cheaper to machine than in steel or titanium. That, plus the low density, is why magnesium wins in products where weight and per-part machining cost both matter. The grades you will be quoted GradeWhat it is for AZ31BThe common wrought alloy — sheet, plate, machined parts AZ91DDie-cast alloy — high-volume housings WE43High-temperature, aerospace-grade The safety question you must ask Magnesium swarf and dust are flammable — a hot chip can ignite and it burns extremely hot. A shop that machines magnesium without the right controls is a fire waiting to happen. Ask your supplier directly: do you run magnesium with proper chip control, fire-rated cutting practice, and Class D extinguishing? A serious shop answers with specifics, not "we do everything." Finishing magnesium The metal corrodes readily, so it needs protection — typically a conversion coating (chromate or non-chromate) or anodizing before any paint or powder coat. This is not optional the way it is with aluminum. See the finish options. Magnesium grades, in more detail AZ31B is the workhorse wrought alloy (about 3% Al, 1% Zn) with good formability and weldability, the default for milled plates and brackets. AZ91D is the die-casting grade (about 9% Al) for thin-wall housings at volume. WE43 is a rare-earth alloy that holds strength above 200°C for turbine and aerospace brackets. For higher load, ZK60 (zinc + zirconium) is the stronger forged or extruded option. Most CNC parts are quoted AZ31B. GradeAlZnBest for AZ31B~3%~1%Wrought plate, weldable frames, brackets AZ91D~9%~1% (Mn 0.1%)Die-cast housings at high volume WE43—rare-earth (Y, Nd, Zr)High-temp aerospace, >200°C ZK60—~5% (Zr)High-strength forged or extruded parts Magnesium grades: properties at a glance The two grades you will actually be quoted for CNC work are AZ31B (wrought) and AZ91D (cast). The others appear when temperature or strength force them in. Typical published values from MatWeb and the ASM Handbook are shown below; treat them as design reference points, not mill-cert guarantees. GradeFormDensity (g/cm³)Tensile (MPa)Yield (MPa)ElongationThermal (W/m·K)Typical use AZ31BWrought plate/bar1.77260–290~200~15%~96Machined plates, brackets, weldable frames AZ91DDie cast1.81230–250150–160~3%~72Thin-wall housings at volume WE43Wrought/cast1.84250–280170–190~7%~51Aerospace, service to ~250°C ZK60Wrought, forged/extruded1.83300–340210–2609–11%~117High-strength structural parts AZ31B is the default for milled parts: it is available in plate and bar, welds well, and holds fine features. AZ91D is a casting grade — you do not machine a billet of it the way you would AZ31B; you specify it when the part will be die-cast at volume and only finish-machined. WE43 and ZK60 carry a price premium and are reserved for temperature or strength. Confirm the exact grade, temper and mill cert on the first quote — a "magnesium part" is not a material specification. Fire risk and how a real shop controls it Magnesium's hazard is the swarf, not the bulk part. Fine chips and dust ignite near 450°C. Control starts at the machine: flood with oil-based coolant or run near-dry, keep chips short with sharp tools, and never let them pile into a hot nest. Chips go to a sealed steel bin, the floor and machine get vacuumed, and the cell keeps a Class D extinguisher — dry powder, never water — within reach. At our Dongguan shop magnesium runs in that controlled cell. Tools and coolant Magnesium machines so freely it grabs a dull tool, so keep cutters sharp and positive-rake. Run a higher spindle speed and lighter feed than for aluminum. A water emulsion reacts with magnesium to release hydrogen and heat, so use straight oil or machine dry with an air blast. We verify every incoming bar with handheld XRF before it reaches the machine, so the alloy matches the mill cert. Bottom line on coolantUse straight oil or dry machining for magnesium. Avoid water-based emulsion coolants — they generate hydrogen and heat at the cut and turn a manageable job into a fire risk. Class D fires: how a safe magnesium cell is run Magnesium fires are metal fires, and they do not behave like a wood or oil fire. A Class D extinguisher uses a dry, non-reactive agent — a sodium-chloride-based powder, a graphite-based powder, or a copper-based mix — that smothers the burning metal and forms a crust. Never aim water at burning magnesium: the water dissociates into hydrogen and oxygen, the hydrogen ignites, and the fire escalates violently. Carbon dioxide and standard ABC dry chemical are also wrong — CO² reacts with burning magnesium and can feed it. What actually ignites The hazard is the swarf, not the billet. Fine chips and dust ignite near 450°C, and a pile of hot chips is exactly that. Grinding dust is the worst case because the particles are fine enough to form a suspended cloud that can flash. The controls are mechanical before they are chemical: short chips, sharp tools, no chip nests, and immediate removal to a sealed steel bin away from the machine. Dry machining and coolant discipline Run magnesium dry with an air blast, or with a straight (non-water) oil for finish passes. Water-based emulsion reacts with the metal to release hydrogen and heat at the cut, which is why serious shops ban it for magnesium. If you see a shop putting magnesium through the same water-mix coolant as aluminum, walk away. Housekeeping and disposal Chips are collected wet or dry in dedicated, labeled steel containers and never mixed with aluminum or steel scrap, which can create an ignition source or a thermite-like hazard. The machine enclosure, chip tray and floor are vacuumed between jobs. A dry-powder Class D extinguisher and a bucket of dry sand sit within arm's reach of every magnesium station, and every operator knows the one rule that matters: smother it, never spray it. Surface finishes that actually protect magnesium Magnesium's oxide layer is thin and porous, so bare parts corrode fast in humid or salt air. The fix is a conversion coating — chromate (DOW 17) or chrome-free — that primes paint or powder. Anodizing (HAE or non-chrome) builds a harder oxide for wear, and it is one of the few finishes that works on both aluminum and magnesium. We anodize, plate, paint and powder-coat in-house or via partners, and salt-spray test every coated part before it ships. Corrosion and the surface treatments that stop it Magnesium is the most anodic of the structural metals, and its natural oxide is thin and porous, so a bare part corrodes fast in humid or salt air — and it corrodes faster still where it touches a more noble metal. Two things follow: the part needs a protective finish, and the assembly needs galvanic isolation. Galvanic corrosion comes first In a bolted joint, magnesium in contact with steel, stainless or aluminum acts as the sacrificial anode and pits where the metals meet. The fix is mechanical: insulating washers, gaskets, or a conversion coating plus sealant at every dissimilar-metal interface. Design it in, because no coating fully survives a metal-to-metal joint under load. Conversion coating and anodizing A conversion coating — chromate (DOW 1 or DOW 7) or a chrome-free alternative — passivates the surface and gives paint or powder coat something to key into. It is the standard pre-treatment and is not optional the way it is on aluminum. Anodizing (HAE-type, or non-chrome processes) grows a harder, thicker oxide for wear and corrosion resistance, and it is one of the few finishes that works on both aluminum and magnesium. Building the full stack For parts that will see weather, the stack is conversion coat, then epoxy primer, then topcoat or powder. For hard wear or cosmetic surfaces, anodize or electroless-nickel plate instead. Every coated part should be salt-spray tested to an agreed number of hours — typically 96 to several hundred depending on the environment — before it ships, and the test report should ship with the parts. Tolerances and surface finish you can hold Magnesium is stable and easy to hold to tight numbers. On our 80+ CNC mills and lathes (3-, 4- and 5-axis) we hold machined features to ±0.005 mm and critical dimensions to Cpk ≥ 1.67, the automotive threshold we run on IATF 16949 work. As-machined surfaces land near Ra 1.6 µm; a finishing pass reaches Ra 0.8 µm. Thin walls need support, but the material holds form well. DFM and tolerances for machined magnesium Magnesium is one of the most machinable metals there is: low cutting force, high metal-removal rates, excellent surface finish and long tool life. That machinability is why tolerances come cheap — but a few design rules keep the part cheap and the fire risk low. Design rules Keep wall thickness uniform and above about 0.8–1.0 mm for machined features; thin unsupported walls flex and chatter, and a scrapped magnesium part carries a fire-safety overhead on top of the usual scrap cost. Avoid deep, narrow pockets that trap chips and heat. Use the largest practical internal corner radius to help the tool clear chips. For cast magnesium (AZ91D), add draft and avoid undercuts — the gate and ejector layout are part of the design. Threads and assembly Magnesium is soft, so threads strip under repeated torque. Where a fastener is removed and reinstalled, specify a threaded insert — Helicoil or a solid bushing — rather than tapping the magnesium directly. For one-time assembly, a coarse thread with a longer engagement length is the safer choice. What you can hold On our mills and lathes we hold machined features to ±0.005 mm, and grinding or wire EDM tightens that to ±0.002 mm. Critical dimensions run to Cpk ≥ 1.67 on IATF 16949 work and ≥ 1.33 elsewhere — Tell us which numbers are functional and which are default tolerances on the drawing; it changes the price. Choosing magnesium for lightweighting and cost Pick magnesium when weight is the spec, not a nice-to-have: drone frames, camera and audio bodies, surgical handles, anything worn or carried. Against aluminum it buys about 33% lower mass but costs more per kilo and needs the fire-safe and finishing steps above, so it pays off on low-volume, high-value parts. We take magnesium from a single prototype (MOQ 1) to volume, with lead times of 3 / 7 / 30 days by complexity, and confirm grade, finish and controls on the first quote. What drives the price Magnesium costs more per kilo than aluminum, but its free-cutting nature cuts cycle time, tool wear and energy per part, so the machined part is often cheaper than a harder metal with the same feature count. The real cost adders are the fire-safe cell, the mandatory coating, and grade verification. Quote the finished part, not the raw bar — a mill cert and a salt-spray report are part of what you are buying. Cast or machined: when to switch at volume Prototypes and low volumes are machined from AZ31B plate or bar — fast, no tooling, MOQ 1. As volume climbs, the same housing often moves to AZ91D die casting: you pay a die, but the per-part cost drops and the thin-wall geometry comes free. The crossover is usually in the hundreds to low thousands of parts, and it depends on wall thickness, draft and the finish. A shop that does both can machine the prototype, cast the production run, and finish-machine the cast part to the same drawing — which keeps one supplier accountable for the whole ramp. Magnesium vs aluminum vs titanium Magnesium's real competitors are aluminum and titanium. The choice usually comes down to a single question — is weight or stiffness the binding constraint? PropertyMagnesium (AZ31B)Aluminum (6061-T6)Titanium (Ti-6Al-4V) Density (g/cm³)1.772.704.43 Tensile (MPa)260–290~310~950 Stiffness (GPa)~45~69~114 MachinabilityExcellentExcellentDifficult CorrosionPoor bare; needs coatingGoodExcellent Fire riskYes — Class D controlsLowLow Relative costMidLowHigh Pick magnesium when mass is the constraint and the load is modest: it is lighter than aluminum and far cheaper and easier to machine than titanium. Pick aluminum when stiffness, corrosion or cost dominate and you can afford the extra mass. Pick titanium when strength, stiffness and corrosion at minimum weight all matter and the budget allows it. In practice, many programs migrate: start in machined aluminum, lightweight to magnesium where the grams pay off, and reserve titanium for the few parts that truly need it. When magnesium is the wrong call Skip magnesium if stiffness per unit volume is the spec, if the part lives in a hot, sustained-load environment above AZ alloy limits, if the design requires bare metal in a corrosive environment, or if the program cannot accept the Class D fire-safety overhead on the shop floor. In those cases aluminum or titanium is the honest answer, and a good shop will say so up front. Nex-G magnesium machining capability Nex-G machines magnesium in a dedicated, fire-controlled cell at our Dongguan facility in Hengli. The plant is 6,800 m² with 100+ staff and has run since 2006 (Zhuohang), holding ISO 9001, IATF 16949 and ISO 14001 certification (URS, valid to 2027). Our IATF 16949 scope is manufacturing only — we do not perform design, per clause 8.3 — which keeps the boundary clean: you own the design, we own the process. CapabilityDetail Machine count80+ CNC — Mazak, Brother, TSUGAMI, Sodick Mill / turn tolerance±0.005 mm Grind / wire-EDM tolerance±0.002 mm Process capabilityCpk ≥ 1.67 (IATF), ≥ 1.33 (general); live SPC above 1.67 Material verificationHandheld XRF on every incoming bar CertificationsISO 9001, IATF 16949, ISO 14001 MOQ / lead timeMOQ 1; 3 / 7 / 30 days by complexity Inspection and traceability Every magnesium order carries a mill cert, an XRF alloy confirmation, a first-article inspection report, and the salt-spray result for the finish. Critical dimensions are SPC-tracked to Cpk, so the capability number on the quote is backed by data, not a promise. For a magnesium part, that means one supplier handles XRF material verification, the fire-safe machining cell, the conversion coating or anodizing, and the salt-spray report — so the part arrives finished and documented, not just machined. Send the drawing and we will return a quote that names the grade, the finish, the tolerances and the fire-safe controls. Frequently asked questions Can magnesium be welded? Yes — AZ31B welds well with GTAW; AZ91D castings are harder to weld, usually designed as one piece. Is magnesium weaker than aluminum? Lower stiffness per volume, but its strength-to-weight ratio is excellent, which is why it wins wherever mass is the constraint. Do you machine magnesium in China? Yes — at our Dongguan facility (ISO 9001 / IATF 16949 / ISO 14001), with XRF alloy checks and Class D fire controls on a dedicated cell. Is magnesium safe in a finished part? Yes. Once machined, coated and assembled, a magnesium part is no more of a fire hazard than aluminum — the risk lives in the machining swarf and dust, not the finished component. Magnesium is used in aircraft, laptops and cameras for this reason. What is the difference between AZ31B and AZ91D for my part? AZ31B is wrought plate and bar — the default for machined parts. AZ91D is a die-casting alloy you choose when the part will be cast at volume and only finish-machined. If you are CNC-machining from solid, you almost certainly want AZ31B. Does magnesium need a coating, or can I leave it bare? Bare magnesium corrodes in humid or salt air, so it needs a conversion coating plus paint or powder, or an anodize, before it ships. Bare parts are for dry, indoor, temporary use only. How do I get a consistent quote? Specify the grade (AZ31B unless stated), the temper, the finish with the required salt-spray hours, and mark which tolerances are functional. Magnesium pricing moves with the alloy market, but a complete drawing keeps quotes comparable. What is the lead time for a magnesium prototype? A single prototype ships in about 3 days, small batches in 7, and volume in 30 — assuming the grade and finish are standard. Exotic grades like WE43 add sourcing time. Machining magnesium parts?Send the drawing — we will confirm the grade, the finish, and our fire-safe machining controls.Request a quote Related articlesAluminum Machining Guide: Grades and FinishesPlastics Machining GuideCNC Milling vs Turning