Magnesium Welding Automation With Robots

Weld­ing mag­ne­sium is per­formed for pri­ma­ry man­u­fac­tur­ing or repair.

Prop­er­ties

Mag­ne­sium alloys with a den­si­ty of about 1.74 g per cubic cen­time­ter (0.063 lb. per cu in.), when in cast form alloyed with Alu­minum, Man­ganese, Rare Earths, Tho­ri­um, Zinc or Zir­co­ni­um, dis­play high strength to weight ratio mak­ing them mate­ri­als of choice when­ev­er weight reduc­tion is impor­tant or when it is imper­a­tive to reduce iner­tial forces (for rapid­ly mov­ing machine parts). Mag­ne­sium is rough­ly 20% the weight of steel and 67% the weight of alu­minum. Mag­ne­sium cast­ings exhib­it remark­able damp­ing capacity.

• Pure Mag­ne­sium melts at 650 degrees Cel­sius (1202 degrees Fahrenheit).
• Con­trac­tion from liq­uid to sol­id is 3.9 to 4.2% and from liq­uid at melt­ing tem­per­a­ture to a sol­id at room tem­per­a­ture is 9.7%.
• Mag­ne­sium is used as an alloy­ing ele­ment in the pro­duc­tion of cer­tain alu­minum alloys.
• In cast iron foundries pro­duc­ing nodu­lar cast iron, mag­ne­sium is used to make the graphite par­ti­cles nodu­lar. It is also used for cathod­ic pro­tec­tion of oth­er met­als from corrosion.

Safe­ty

Safe­ty pre­cau­tions must be under­stood and fol­lowed. Mag­ne­sium oxi­dizes eas­i­ly. If ignit­ed when it is in the form of machined turn­ings or pow­ders, it burns intense­ly. Machin­ing must be per­formed under con­trolled con­di­tions, with extin­guish­ing agents on hand.

Spec­i­fi­ca­tions

• Cast­ing alloys are cov­ered by Spec­i­fi­ca­tions ASTM B80, B94 and B199.
• Wrought alloys by ASTM B90, B107 and B217.
• Filler met­als for Weld­ing mag­ne­sium alloys are spec­i­fied in
• AWS A5.19 Spec­i­fi­ca­tion for Mag­ne­sium Alloy Weld­ing Elec­trodes and Rods
• ASTM B 448 Spec­i­fi­ca­tion for Mag­ne­sium-Alloy Weld­ing Rods and Bare Electrodes
• SAE AMS 4397 Mag­ne­sium Wire, Welding.

Char­ac­ter­is­tics

Weld­ing mag­ne­sium alloys require low­er amounts of heat to melt than oth­er mate­ri­als. How­ev­er, they are sus­cep­ti­ble to dis­tor­tion, due to the high ther­mal con­duc­tiv­i­ty and coef­fi­cient of ther­mal expan­sion. Ade­quate pre­cau­tions must be taken.

Alloy­ing Elements

Because mag­ne­sium is too mechan­i­cal­ly weak to be used as is, it must be alloyed with oth­er ele­ments which con­fer improved prop­er­ties. The Mg-Al-Zn group of alloys, con­tains Alu­minum, Man­ganese and Zinc, which are the most com­mon alloy­ing ele­ments for room tem­per­a­ture appli­ca­tions. Alloy­ing ele­ments Tho­ri­um, Ceri­um and Zir­co­ni­um (with­out Alu­minum) are used for ele­vat­ed tem­per­a­ture, form­ing the Mg-Zn-Zr group.

An increase in alloy con­tent depress­es the melt­ing point, enlarges melt­ing range and increas­es weld crack­ing ten­den­cy. High alloy con­tent needs less heat for melt­ing and lim­its grain growth, show­ing high­er weld­ing mag­ne­sium efficiency.

• Alu­minum is the most effec­tive ingre­di­ent in pro­vid­ing improved results. In per­cent­ages of 2 to 10%, with minor addi­tions of zinc and man­ganese, it increas­es strength and hard­ness, at the expense of less duc­til­i­ty. Mag­ne­sium alloys con­tain­ing more than 1.5% Al are sus­cep­ti­ble to stress cor­ro­sion and must be stress relieved after welding.
• Zinc com­bined with alu­minum helps to over­come harm­ful cor­ro­sive effects of iron and nick­el impu­ri­ties that may be present in mag­ne­sium alloys. The high­er the Zn con­tent (over 1%) the high­er the hot short­ness, caus­ing weld cracking.
• Man­ganese improves yield strength (slight­ly) and salt-water resis­tance of mag­ne­sium alloys. High­er melt­ing point requires high­er heat input to melt. Grain growth adja­cent to the weld reduces strength.
• Tho­ri­um or Ceri­um can be added to improve strength at tem­per­a­tures from 260 to 370 degrees Cel­sius (500 to 700 degrees Fahren­heit). Zir­co­ni­um in small amounts is a grain refin­er that improves weldability.
• Beryl­li­um is some­times added to reduce the ten­den­cy of mag­ne­sium to burn while melt­ing. No adverse effect on weld­ing has been observed. It may be ben­e­fi­cial, in a braz­ing alloy, in reduc­ing the dan­ger of igni­tion dur­ing braz­ing in furnace.
• Cal­ci­um is added in small amounts to reduce oxi­da­tion, but it can increase the risk of weld cracking.

Process­es

Weld­ing mag­ne­sium is gen­er­al­ly per­formed with arc process­es using direct cur­rent with reverse polar­i­ty (elec­trode pos­i­tive). Wrought alloys are usu­al­ly more weld­able than cer­tain cast alloys.

Met­al Trans­fer Modes for Gas Met­al Arc Weld­ing mag­ne­sium (GMAW) or Met­al Inert Gas (MIG)

• Short cir­cuit mode — filler touch­es the work many times per sec­ond and extin­guish­es the arc, the met­al is sup­plied as a sequence of drops.
• Pulsed arc mode — a pow­er sup­ply pro­vides a mod­u­lat­ed cur­rent. The arc is unin­ter­rupt­ed and the met­al is trans­ferred intermediately.
• Spray trans­fer mode — met­al is trans­ferred with a spray of droplets.
• The most used shield­ing gas is gen­er­al­ly argon while mix­tures with heli­um are acceptable.

Gas Tung­sten Arc for Weld­ing mag­ne­sium (GTAW) also known as Tung­sten Inert Gas (TIG)

• Alter­nat­ing cur­rent machines or direct cur­rent reverse polar­i­ty (elec­trode pos­i­tive) pow­er sup­plies, with high fre­quen­cy cur­rent super­im­posed are used.
• For thin sheets both are suit­able, for heav­ier sheets alter­nat­ing cur­rent is pre­ferred as it pro­vides deep­er penetration.
• Direct cur­rent straight polar­i­ty (elec­trode neg­a­tive) is not pre­ferred because it lacks the cathod­ic clean­ing action.

Elec­tron Beam weld­ing mag­ne­sium has been used for repair­ing expen­sive cast­ing on alloys con­tain­ing less than 1% Zinc. The rel­a­tive weld­abil­i­ty of the dif­fer­ent mag­ne­sium alloys is sim­i­lar to that dis­played for the more com­mon arc processes.

The con­di­tions have to be strict­ly mon­i­tored because of the dan­ger of devel­op­ing voids and poros­i­ty due to the low boil­ing point of Mag­ne­sium and the still low­er one of Zinc. A slight­ly defo­cused beam may help to obtain sound welds.

Laser Beam is a pre­ferred method for weld­ing mag­ne­sium because of its low heat input, ele­vat­ed speed and lim­it­ed defor­ma­tion. How­ev­er this method has a ten­den­cy of devel­op­ing porosity.

Resis­tance weld­ing mag­ne­sium for either spots or seams is per­formed on wrought alloys like sheets and extru­sions, essen­tial­ly with equip­ment and con­di­tions sim­i­lar to those used for aluminum.

Repair­ing Cast­ings: One of the most com­mon Weld­ing mag­ne­sium appli­ca­tions is repair­ing cast­ings either as cast or after ser­vice. Prepa­ra­tion is impor­tant and should exclude con­t­a­m­i­na­tion from extra­ne­ous mate­ri­als. Gen­er­ous bevels should be pre­pared to allow for full penetration.

Pre­heat­ing: The need for pre­heat­ing when weld­ing mag­ne­sium is dic­tat­ed by the degree of joint restraint and by met­al thick­ness: for thick walls and a short weld­ing bead, it may not be required. Pre­heat­ing should be per­formed in a fur­nace with a pro­tec­tive atmos­phere for reduc­ing oxi­da­tion. One of the rec­om­mend­ed pro­ce­dures to min­i­mize weld crack­ing is to weld from the cen­ter towards the sides (one half after the oth­er). Ther­mal shocks should be avoided.

For more infor­ma­tion on mag­ne­sium weld­ing, con­tact Robots​.com experts today online or at 877−762−6881.