Development of Requirements for Resistance Spot Welding Dual-Phase (DP600) Steels Part 1 — The Causes of Interfacial Fracture
Weld fracture was investigated in relation to weld parameters and steel sheet characteristics
BY M. MARYA and X. Q. GAYDEN
ABSTRACT. The resistance spot welding of two galvanized DP600 steels with 1.8 and 2.0-mm sheet thickness was investi- gated to enhance our understanding of weld fracture during a well-established au- tomotive quality control test. The effects of several process parameters (including weld current, weld time, and weld force) on zinc expulsion, mechanical properties, defects, and microstructures of resistance spot welds were all studied in relation with the two major types of weld fracture (i.e., interfacial and button pullout). It was es- tablished that the slight differences in chemical composition and galvanized coating between the two selected DP600 steels affected negligibly weld microstruc- tures, and were of no measurable conse- quences to weld fracture. In contrast, the difference of 10% for their sheet thickness was responsible for smaller weld diame- ters, lower weld tensile-shear forces, larger shrinkage voids, and more frequent interfacial fractures in welds from the thicker DP600 steel. For the two steels, oc- currence of weld interfacial fracture was eliminated using long weld times (>20 cy- cles), low currents (<9 kA), and high forces (>900 lb, or 4.0 kN); i.e., process parameters that increased weld diameters while preventing zinc ingestion into the fu- sion zone. The effect of zinc was most prominent in welds that were made ab- normally fast (e.g., ~5 cycles), where both solidification cracking and a change in type of weld fracture were found due to in-
M. MARYA, formerly with G. S. Ansell Depart- ment of Metallurgical and Materials Engineering, Colorado School of Mines, Golden Colo., is now with NanoCoolers, Inc., Austin, Tex. X. Q. GAY- DEN is with Materials and Process Laborator , General Motors Research and Development, War- ren, Mich.
terdendritic low-melting point con- stituents.
The drive for vehicle mass reduction has led to the introduction of new engi- neering materials. The Ultra Light Steel Auto Body (ULSAB) project has shown that car body mass can be reduced by 25% using advanced high-strength steels (AHSS) and innovative processes (Ref. 1). Among the AHSS, the dual-phase (DP) steels have been the subject of particular attention owing to their good combination of high strength and ductility. The term dual-phase steel refers to the predomi- nance of two phases, the body-centered- cubic (bcc) a-ferrite and the relatively harder body-centered-tetragonal (bct) martensite. These two phases are pro- duced by some annealing in the A1–A3 “intercritical” temperature, where austen- ite and ferrite are formed, and a subse- quent rapid cooling where the austenite is eventually transformed into martensite. Since suitable amounts of ferrite and martensite in DP steels may be produced from a combination of heat-treating para- meters, compositions of DP steels may vary significantly between steel makers, as
Automotive Steels Dual-Phase (DP600) Resistance Spot Welding Properties Microstructures Defects
found in Part 2 of this study (Ref. 4) to be published in the near future. To create yield strengths of 350 MPa and ultimate strengths of 600 MPa, the DP600 steels (as commonly referred by the ULSAB Con- sortium.) (Ref. 1) were all found to pos- sess about 15% martensite. Compared to precipitation-strengthened or solid- solution-strengthened HSLA steels, DP600 steels exhibit a slightly lower yield strength, a continuous yield behavior due to enough active slip systems in the ferrite phase, and a more uniform and higher total elongation (over 21%) (Refs. 1, 2). These last two properties explain their rel- atively good formability; a property, which combined with their high strengths, has made DP steels attractive for automotive applications (Refs. 1–3).
The acceptance of DP steels in manu- facturing environments has been ex- tremely gradual though DP steels offer ev- ident advantages, and they have demonstrated a good weldability (Refs. 3, 5–8). However, on automotive factory floors and in testing laboratories, the def- inition for weldability can differ, and so are the tests and criteria used to differen- tiate “good” welds from “bad” welds. In the simplest of all quality-control tests, a handheld chisel is inserted in between spot-welded coupons to force the welds to fracture, and, based upon a visual inspec- tion, determine if the same welds would be appropriate in vehicle applications (Ref. 9). In early chisel tests, spot welds in DP600 steels had been found to under- perform welds in traditional automotive steels; i.e., non-AHSS (Ref. 1) (e.g., inter- facial-free steels, low-carbon steels, rephosphorized steels, mild steels, or HSLA steels). Especially with the thicker DP600 steel gauges, welds were seen to fracture in the same plane as the sheet sur-
172-s NOVEMBER 2005