Coatings have been widely used to improve the surface properties of materials. Especially the protective metallic coatings containing Zn serve as a barrier and a galvanic protection to steel applied in the automotive, building, and other industries.
However, the Zn has some economical problems such as too high demand exceeding the supply and wide fluctuation of price, and the Zn coatings provide insufficient corrosion resistance under severe atmospheric conditions or corrosive aqueous solutions. Additionally, during high temperature processes such as welding process, the Zn coating is likely to melt due to its low melting temperature of 420˚C. The liquid Zn phase tends to penetrate into the copper electrode and the steel in particular advanced high-strength steels (AHSSs), which leads to the degradation of welding electrode tips and the brittle fractures. These problems limit the practical application for automotive industries of Zn coated AHSSs. In order to overcome these problems and to increase the performance of Zn coated steel, the Zn-Mg coatings have been studied extensively. According to previous studies, the Zn-Mg coatings exhibit excellent corrosion resistance compared to that of pure Zn coatings in corrosive environment.
In this thesis, to optimize the deposition condition of the Zn-Mg coating using physical vapor deposition (PVD) process, the Zn-Mg coatings with various Mg contents and deposition temperatures were synthesized on steel substrates. The microstructure, chemical composition, and crystal phase of the synthesized Zn-Mg coatings were investigated, and the effects of those characteristics of coating on the corrosion behavior and adhesion strength of the coated steels were evaluated. Subsequently, the welding properties of the Zn-Mg coated TRIP steels with the optimized Zn-Mg coating were evaluated.
Firstly, the Zn-Mg coating with various Mg contents were synthesized using sputtering process. The Zn-Mg coatings with low Mg content below approximately 10wt.% exhibited the relatively porous columnar microstructure, while the featureless microstructure was developed with increasing Mg content further. The corrosion resistance of the Zn-Mg coatings increased with increasing Mg content in the coatings, which could be attributed to the transition from porous microstructure to dense and compact microstructure. The adhesion strength between the Zn-Mg coating and steel substrates decreased with increasing Mg content due to the amorphous structure of Zn-Mg coating contained high Mg content which led to the brittle fracture of coatings during deformation of the Zn-Mg coated steels.
Secondly, the effect of the deposition temperature on the properties of sputtered Zn-Mg coatings were investigated. The deposition temperature also had a strong effect on the microstructure as well as the coating properties. At the low temperature below 50℃, the microstructure of the Zn-Mg coatings with approximately 13wt.% Mg was the featureless amorphous structure, while the relatively porous crystalline Zn-Mg coatings were synthesized above 100℃. With this microstructural transition of the Zn-Mg coatings, the adhesion strength of Zn-Mg coatings with high Mg content above 10wt.% could be improved. The welding properties of Zn-Mg coated TRIP steel with 13wt.% Mg coating were investigated using the spot-welding test. The Zn-Mg coated TRIP steels showed satisfactory spot-welding properties, including large nugget diameters of approximately 5.25 mm, high tensile load of 17.5 kN, and pull-out tensile failure modes.
Based on the property evaluation results of sputtered Zn-Mg coatings, the Zn-Mg coatings with high Mg content above 10wt.% were studied as an alternative to industrial Zn coating. In order to increase the deposition rate of Zn-Mg coating, the electro-magnetic heating deposition (EMHD) process was developed by the combination of evaporation deposition process and induction heating process. For the purpose of utilizing induction heating in the evaporation process, electro-magnetic simulations and the atmospheric evaporation test with various induction coil designs were conducted to predict the energy efficiency of the induction coils. The induction coil with 4-windings showed the highest energy efficiency among the 3-, 4-, and 5-windings induction coils. The energy efficiency with the 4-windings induction coil was measured to be 43% in the atmospheric evaporation test, which could be utilized sufficiently for the EMHD process.
The Zn-Mg coatings with various Mg contents of approximately from 10 to 28wt.% Mg were successfully synthesized using EMHD process with high deposition speed. As the amount of Mg in the coatings increased above 10wt.%, the columnar microstructure changed to amorphous microstructure. The existent phases in the Zn-Mg coatings were Zn-Mg intermetallic phases such as MgZn2 and Mg2Zn11. As the Mg content in the Zn-Mg coatings increased, the corrosion resistance increased up to 22wt.% Mg. However, the Zn-28wt.%Mg coating showed the reduced corrosion resistance since the passivation layer was not formed during corrosion test.
After the spot-welding process of the Zn-Mg coated TRIP steel synthesized the EMHD process, the nugget diameters of the Zn-Mg coated TRIP steels were in the range from 9.5 to 10.2 mm which exceeded the minimum nugget diameter (4√t), and the tensile loads of Zn-Mg coated TRIP steels were higher than the industrial standard (KS B 0850, 15.8 kN). These results indicated satisfactory spot-welding properties of the Zn-Mg coated TRIP steels. Therefore, it is expected that Zn-Mg coatings could be used as protective coatings for advanced high-strength steel for automotive applications.