| In recent years, the quantity of automobile production and sales increased year by year.That resulted in more and more serious energy crisis, higher and higher oil prices andtransport costs increasing rapidly. Research on the relationship between the car weight andthe amount of consumed fuel by automotive experts showed that vehicle weight was animportant factor in the decision of the amount of consumed fuel. If the total weight of apassenger vehicle is reduced by 10%, its consumed fuel will fall about 7%. Therefore,reducing the own weight of an automobile is one of the most basic ways to save energy andimprove fuel economy. Galvanized high-strength steels not only have the advantages suchas relatively low yield ratio, better processing formability, higher tensile strength, bettermatching properties between strength and plasticity and so forth, but also have excellentanti-corrosive performance. Galvanized steel sheets, as a new material, have been widelyapplied to automobile body manufacturing. Resistance spot welding is the main connectedmethod in body welding. But the weldability of galvanized high-strength steels isdeteriorated significantly compared to ordinary cold-rolled steels because of the presence ofcoatings and the difference in compositions.Welding currents (8.5KA~12KA), welding time (12cyc~32cyc) and electrodepressures (2.5KN~4.5KN) are changed to research the weldability of galvanized steel sheetswith trademark of CR590T/340Y DP in this thesis. The geometric sizes, outsidemorphologies and microstructures of the resistance-spot-welded joints with everycombination of welding parameters were observed and measured. And their mechanicalproperties, such as tensile-shear loads and microhardness distribution were tested forsummarizing the effect of welding current, welding time and electrode pressure on weldingquality. The following conclusions are concluded based on the experiment results andanalysis:1) When welding currents range from 10.0 to 10.5KA, welding time ranges from 20 to24cyc and electrode pressure is 4.0KN, satisfactory mechanical properties were achievedfor resistance spot-welded joints of galvanized steel CR590/340Y DP. If welding currents are lower such as less than 9.5 KA or welding time is shorter such as less than 20cyc, thenugget diameter will be smaller and the mechanical properties of joints are poorer. Whereashigher welding currents (more than 11.0 KA) and longer welding time (more than 28cyc)will result in the welded joints with deep indentation and splash, and internal shrinkage willbe generated as well. And if the electrode pressure is lower (2.5 ~ 3.5 KN), the mechanicalproperties of the joints will be insufficient and a large amounts of splash will be producedwhereas the indentation rate will be more than 20% if the electrode pressure is too high (4.5KN), which results in poor surface quality.2) The surface of spot-welded joint is divided into three regions from inside to outside.NO.1 ring region in center, whose shape is similar to that of the electrode end, is the areawhere electrode cap and galvanized steel directly contact while welding. The depth ofindentation is increased by the increase of the welding current, welding time and especiallythe pressure of the electrode. Like an arch convex ring, No.2 ring region is located in theoutside of NO.1 ring region. It looks like a slope from the low inner ring to high externalring formed by metal solidification of molten galvanized layer along the edge of theelectrode cap by extruding from the pressure of electrodes. It is formed base metal along thesurface of the electrode cap during continuous deformation in the electrode pressure andheat resistance under the action of the arc. The edge of nugget is located inside No.2 ringregion, whose diameter can closely substitute the bonding one in the absence of splash. TheNo.3 ring region is heat affected zone.3) Welding joint is composed of base metal, heat affected zone (HAZ) and the nuggetzone. The microstructures of the base metal are two phase mixture in which martensite likeisland distributes in the ferrite matrix uniformly. Heat affected zone could be divided intouneven microstructure zone, fine-grain zone and overheated zone by different levels ofheating. Both fine ferrite and martensite after recrystallization and coarse ferrite which grewup by heating consist in the uneven microstructure zone. And Fine-grain zone is made upwith ferrite and martensite after recrystallization which are uniform and fine. Overheatedzone is composed of coarse martensite, mainly equiaxial crystals. Nugget zone is mainlycomposited by coarse lath martensite, whose morphology is columnar crystal.4) Tensile-shear forces of spot-welded joints are related and normally proportional tonugget diameters. But they will decrease when the spot-welded joints have internalshrinkage voids, pores, cracks and other defects. Interfacial fracture, button fracture andtearing along the base metal are three fracture types after tensile-shear test for resistancespot welded joints. Interfacial fracture is caused by smaller welding heat input or shrinkage voids, splash and other defects which make the bond strength decrease. The spot-weldedjoints with button fracture have better bonding properties. At first cracks initiated inheat-affected zone by tensile shear loads because the microstructures and properties areinhomogeneous there. Then cracks propagated along the heat affected zone to form the finalbutton-type fracture. The mechanism of tearing along base metal is similar to the buttonfracture. However, the direction of crack propagation is changed as the direction of thetensile-shear change after cracking in the heat affected zone.5) The tensile strengths of nuggets were improved significantly than those of the basemetal. The tensile strength of the base metal sample with the small size is 675MPa, but theminimum tensile strength of the nugget metal can reach 730MPa under the same condition.This is because the microstructures of the base metal are martensite distributing in theferrite matrix, while the nugget zone is mainly composed by coarse but harder and strongerlath martensite. Under the Circumstances of different welding parameters, the process ofheated and cooling during the welding is different, the resulting form and content ofmartensite is also different, and the same is true of tensile strength which can reach up to amaximum of 1000MPa.6) Splash, pores and shrinkage voids, cracks and other defects generated duringresistance spot welding have a great impact on welding quality. Rational processingparameters should be developed in the actual production in order to get better spot-weldedjoints.
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