Laser welds were made in three dual-phase (DP) alloys with ultimate tensile strengths ranging from 450-980 MPa and varying microstructures to investigate effects of heat input on heat affected zone (HAZ) softening. To compare the total heat transferred into the HAZ of all the welds, heat input was normalized using the Rosenthal Equation. It was found that HAZ softening experienced in a DP steel was a function of both martensite content and heat input. Maximum HAZ softening was proportional to the martensite content, and the heat input controlled the completion of softening. Material softening was normalized by martensite content, which showed that the contribution of martensite to material hardness from the three materials is the same; however the materials had different transformation kinetics.
Understanding effects of welding on strength and formability is critical to support wider application of advanced high strength steels in automotive components. In this study, High Strength Low Alloy (HSLA) and DP980 (Dual Phase, 980MPa) sheet steels were welded with a 4kW diode laser. Mechanical properties of welds and parent metals were assessed by tensile and limiting dome height tests, and related to microhardness distribution across the welds. The formability of HSLA welds was insensitive to the welding process and comparable to that of parent metal. For the DP steel, weld formability was much lower than that of corresponding parent metal, which appeared to be due to the formation of soft zones in the outer region of the Heat affected zone (HAZ) of the welds. It was found that increase of welding speed resulted in a slight increase of formability of the DP steel, associated with a reduction in the microhardness difference between base metal and HAZ soft zones.
Limiting dome height (LDH) tests were used to evaluate the formability of laser butt welded blanks of the dual phase 980 steel in comparison with the base metal. Two different lasers were used: diode and Nd:YAG, giving a wide range of welding thermal cycles. A sharp decrease in LDH was observed in the welded specimens due to the formation of a softened zone in the outer heat affected zone. Softened zone characteristics were correlated with the LDH. Larger softened zones led to a larger reduction in the LDH. The welding orientation relative to the rolling direction or to the punch surface did not influence the formability, as the softened zone dominated the formability behavior. It was observed that in both uniaxial and biaxial strain tests, the fracture occurred in the softened zone of the welded samples consistently slightly farther out from the weld centerline than in the location of the minimum hardness.
TRansformation Induced Plasticity (TRIP) steels are promising materials to achieve a better combination of formability and strength than conventional steels due to their unique microstructural makeup. Though welding is a vital part of auto body manufacturing, the weldability of TRIP steels has some complex and poorly understood features, which has served to retard the growth of its applications in the automotive industry. In this study, autogeneous welds were carried out on Al-alloyed TRIP steel using a 4 kW diode laser. Both fusion zone solidification behavior and subsequent austenite transformation products were investigated with optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. In terms of solidification behavior, fusion zones solidified with high temperature -ferrite as the primary phase. Fusion zone microstructure at room temperature was composed of ferrite with a skeletal morphology characteristic of solidification, and austenite decomposition products almost all having a lath morphology. Skeletal ferrite covered about 30% fusion zone area. Upper bainite laths separated by retained austenite films comprised most of the transformed microstructure, about 65% of the fused area. Lower bainite with carbide particles dispersed in an aligned way, chunk shaped retained austenite, lath martensite and twinned martensite were also occasionally observed. The Al content was considered to be for a dominant influence on fusion zone microstructure evolution.
We have used transmission electron microscopy (TEM) and nanoindentation to characterize the dominant phases present in the weld zone of a diode-laser-welded transformation-induced plasticity (TRIP) steel, examining the unaffected base metal as a baseline. The microstructure of the base metal consists predominantly of ferrite, retained austenite, martensite, and occasional large carbide particles. The dominant microstructure of the weld zone is of differently oriented packets having a bainitic morphology. The weld also contains allotriomorphic ferrite, idiomorphic ferrite, as well some twinned martensite that is surrounded by austenite. The TEM analysis of the bainitic-morphology packets indicates that they consist of a lath ferrite phase separated by an interlath carbon-enriched retained austenite. In most cases, the orientation relationship (OR) between the lath ferrite and the interlath retained austenite can be approximated as Nishiyama-Wasserman (N-W). We used site-specific nanoindentation to further characterize the packets and the allotriomorphic ferrite, confirming through the hardness values the conclusions reached by TEM. While martensite was regularly present in the base metal, it was only sparsely distributed within the weld zone, boding well for the weldÕs mechanical properties.
The QP980-DP980 dissimilar steel joints were fabricated by fiber laser welding. The weld zone (WZ) was fully martensitic structure, and heat-affected zone (HAZ) contained newly-formed martensite and partially tempered martensite (TM) in both steels. The super-critical HAZ of the QP980 side had higher microhardness (~ 549.5 Hv) than that of the WZ due to the finer martensite. A softened zone was present in HAZ of QP980 and DP980, the dropped microhardness of softened zone of the QP980 and DP980 was Δ 21.8 Hv and Δ 40.9 Hv, respectively. Dislocation walls and slip bands were likely formed at the grain boundaries with the increase of strain, leading to the formation of low angle grain boundaries (LAGBs). Dislocation accumulation more easily occurred in the LAGBs than that of the HAGBs, which led to significant dislocation interaction and formation of cracks. The electron back-scattered diffraction (EBSD) results showed the fraction of LAGBs in sub-critical HAZ of DP980 side was the highest under different deformation conditions during tensile testing, resulting in the failure of joints located at the sub-critical HAZ of DP980 side. The QP980-DP980 dissimilar steel joints presented higher elongation (~ 11.21%) and ultimate tensile strength (~ 1011.53 MPa) than that of DP980-DP980 similar steel joints, because during the tensile process of the QP980-DP980 dissimilar steel joint (~ 8.2% and 991.38 MPa), the strain concentration firstly occurred on the excellent QP980 BM. Moreover, Erichsen cupping tests showed that the dissimilar welded joints had the lowest Erichsen value (~ 5.92 mm) and the peak punch force (~ 28.4 kN) due to the presence of large amount of brittle martensite in WZ and inhomogeneous deformation.
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