Arc welding of galvanized steel sheet

Zinc layer on galvanized steel sheet brings some difficulties during welding. The main problems are: increased sensitivity of welding for cracks and pores, zinc evaporation and dust, oxide slag inclusion, melting and damage of zinc coating. Among them, welding crack, air holes and slag inclusions are the main problems.

Weldability

(1) Cracks
During welding process, molten zinc floats on surface of molten pool or root of weld. As melting point of zinc is far lower than iron, iron in molten pool crystallizes first, and liquid zinc infiltrates along grain boundary of steel, resulting in weakening of inter-granular bonds. Moreover, intermetallic brittle compounds Fe3Zn10 and FeZn10 are easily formed between zinc and iron, that further reduces plasticity of weld metal. Therefore, it is easy to crack  grain bonds under action of welding residual stress.
1) The factors that affect sensitivity of cracks
① thickness of zinc coating, zinc coating of galvanized steel is thinner and less sensitive to cracks, while zinc coating of hot-dip galvanized steel is thicker and more sensitive to cracks.
② The greater thickness of workpiece, the greater welding restraint stress and the greater crack sensitivity.
③ The larger gap is, the greater crack sensitivity.
④ Welding method. Crack sensitivity of manual arc welding is smaller than during CO2 gas shielded welding.
2) Methods to prevent cracks.
① Before welding, V-shaped, Y-shaped or X-shaped groove shall be made on welded galvanized sheet. Zinc coating near the groove shall be removed by oxyacetylene or sand blasting, and gap shall not be too large, generally about 1.5mm.
② Select welding material with low Si content. Welding wire with low Si content should be used during gas shielded welding, titanium type and titanium calcium type welding rod should be used during manual welding.

(2) Air holes
Under action of arc heat, zinc layer near the groove produces oxidation (forming ZnO) and evaporation, that volatilizes white smoke and vapor, so it is easy to cause pores during the weld. The higher current during welding is, the more serious zinc evaporation caused and the more sensitive porosity is. During welding with Ti type and TiCa type electrodes, it is not easy to cause pores with medium current range. However, when cellulose type and low hydrogen type electrodes are used for welding, porosity is easy to occur under low current and high current. In addition, welding rod angle should be controlled in the range of 30 ° to 70 ° as far as possible.

(3) Evaporation of zinc and dust
When galvanized steel sheet is welded by arc welding, zinc layer near molten pool is oxidized to ZnO and evaporates under action of arc heat, forming a large amount of dust. The main component of this kind of dust is ZnO, which has a great stimulating effect on workers' respiratory organs. Therefore, good ventilation measures must be taken during welding. Under the same welding specification, amount of smoke and dust produced by using titanium oxide electrode is lower, while amount of smoke and dust produced by low hydrogen electrode is larger.

(4) Oxide inclusion
When welding current is low, ZnO formed during heating process is not easy to escape, that easily causes ZnO slag inclusion. Melting point of ZnO is 1800 ℃. Large-scale ZnO slag inclusion has very negative effect on weld plasticity. When titanium oxide electrode is used, distribution of ZnO is fine and uniform, that has insignificant effect on plasticity and tensile strength. However, when cellulose type or hydrogen type electrode is used, ZnO content in weld is higher, and performance of weld is poor.
Welding technology of galvanized steel
Galvanized steel can be welded by manual arc welding, MIG welding, argon arc welding, resistance welding and other methods.
(1) Manual arc welding
1) Preparation before welding
In order to reduce welding dust and prevent welding cracks and blowholes, zinc layer near the groove should be removed in addition to proper groove preparation before welding. Removal method can be flame drying or sand blasting. Groove clearance should be controlled within 1.5-2mm as far as possible. When workpiece thickness is large, it can be reduced to 2.5-3mm.
2) Selection of welding rod
Selection principle of welding rod is that mechanical properties of weld metal should be close to base metal, and silicon content of deposited metal should be below 0.2%.
Joint strength obtained by using ilmenite type electrode, titanium oxide type electrode, cellulose type electrode, titanium calcium type electrode and low hydrogen type electrode can help to reach satisfactory index. However, slag inclusion and porosity are easy to occur on welding seam with low hydrogen electrode and cellulose electrode, so they are not generally used.
J421 / J422 or J423 electrode should be preferred for low carbon galvanized steel sheet. E5001, E5003 and other types of welding rods can be used for galvanized steel sheet with strength grade above 500MPa. E6013, E5503 and E5513 electrodes should be used for galvanized steel sheet with strength above 600MPa.
During welding, short arc shall be used as far as possible, arc swing not allowed  to prevent expansion of zinc coating melting area, that ensures corrosion resistance of workpiece and reduce amount of dust.
(2) MIG welding
CO2 gas shielded welding or Ar + CO2, Ar + O2 mixed gas shielded welding can be both used for welding. Shielding gas method has obvious effect on content of Zn during welding. When pure CO2 or Co2 + O2 is used, content of Zn in weld is higher, when Ar + CO2 or Ar + O2 is used, content of Zn in the weld is lower. With increase of welding current, Zn content in the weld decreases slightly.
When gas shielded arc welding is used to weld galvanized steel, welding fume is much larger than during manual arc welding, so special attention should be paid to exhaust fumes. Main factors affecting amount and composition of soot are current and protective gas. The higher current or content of CO2 or O2 in shielding gas is, the larger welding fume is, content of ZnO in fume is also increasing. Maximum content of ZnO can reach 70%.
Under the same welding specification, penetration of galvanized steel is greater than of non-galvanized steel. T-joint, lap joint and downward vertical welding are more sensitive to welding porosity, the higher welding speed is, the easier porosity appears; for galvanized alloy steel influence of welding speed is particularly obvious. During multi pass welding porosity sensitivity of subsequent pass is higher than of previous pass.
Composition of shielding gas has no great influence on mechanical properties of joints, so pure CO2 is generally used for welding. Welding process parameters of I-joint, lap joint and T-joint are given in tables 1-3.

Table 1 Specification parameters for CO2 welding of galvanized steel I-shaped butt joint

Table 2 Specifications for CO2 welding of galvanized steel sheet lap joints

Table 3 Specification parameters for CO2 welding of galvanized steel T-joint (corner joint)