Laser welding of modern automotive high strength steels

Transcription

1 Laser welding of modern automotive high strength steels Hardy Mohrbacher Niobium Products Company GmbH, Steinstrasse 28, D Düsseldorf, Germany Abstract: The paper demonstrates the correlation between the chemical composition of steel grades and the hardness as well as the formability of laser welds generated with modern automotive high strength steel grades. At the very low heat input of industrial laser welding and the resulting extremely high cooling rate, the weld hardness is fully controlled by the carbon content of the base materials and not by the carbon equivalent. Correspondingly, the ductility of the weld seam decreases with increasing carbon content as will be demonstrated by Erichsen cup testing on laser welded samples. Based on these results it is possible to derive guidelines in terms of influencing the alloy design, the welding process as well as the material selection for a given application. In particular cases, the benefits that can be obtained with niobium microalloyed materials will be highlighted. Key words: Laser welding, high-strength steel, formability, niobium microalloying 1 Introduction Over the recent years, traditional automotive welding methods like resistance spot welding and MAG welding have been substituted to a significant degree by laser welding. Today, car body structures often involve several tens of meters laser weld seam in the assembling process. The advantage of laser welding is the lower heat input as compared to MAG welding and the continuous seam as compared to spot welding. The low heat input limits the volume of material with altered properties to a minimum while the continuous seam allows a much better force flow and thus leads to a lower specific stressing of the material. Typical laser welding configurations are lap, hem, and edge joints. The butt joint is applied in the manufacturing of so called tailored products such as tailor welded blanks (TWB) and tailored tubes [1]. For these semi-products the forming operation is performed only after joining, thus adding the demand for a high formability to the HAZ generated by laser welding. While laser welding was mainly performed on mild steel grades several years ago, the recent broad scale introduction of high strength and ultra high strength steels in modern car bodies implicates additional concerns with regard to the extremely high cooling speed of laser welding. Therefore, the present research analyses the hardness as well as the ductility of laser welds generated with those materials to allow a reliable prediction of properties based on the chemical composition of the streel. Guidelines are derived in terms of influencing the alloy design, the welding process as well as the material selection for a given application. 2 Hardness of laser weld seams Laser welding of steel sheets makes use of the so-called keyhole welding process. In this process, the laser radiation is focused to a small spot (< 0.5 mm diameter) of extremely high energy intensity (> 25x10 9 W/m²) forming a vapor channel commonly known as keyhole. The welding process relies then on a continuous displacement of the keyhole to form an autogenous weld. The heat-affected zone has thereby a high depth-to-width ratio rendering only a very small volume material with altered properties. Depending on the welding conditions the heat input ranges between 40 and 100 J/mm. The cooling speed is inversely proportional to the heat input, but is generally very high. Values in the order of several hundred K/s can occur in the fusion zone. When estimating the tendency of a steel grade to form hard structures in the HAZ, usually the carbon equivalent is considered. The two most common formulae of the carbon equivalent are: [ IIW] Mn Cr + Mo + V Cu + Ni CE = C applicable for steel grades having more than 0.18% C

2 and Si Mn + Cu + Cr Ni Mo V [ Pcm] = C B CE applicable for steel grades having less than 0.16% C. The higher the carbon equivalent, the lower is the critical cooling speed to form martensite. However, the hardness of martensite solely depends on the absolute carbon content [2]. Microalloyed high strength steel for automotive applications typically has a carbon content of less than 0.10%. However, traditional C-Mn high strength steels as well as modern ultra high strength multiphase steels can considerably exceed this limit. Furthermore these steels can also contain increased amounts of silicon and chromium as well as high levels of manganese. The correlation between the peak hardness and the carbon equivalent was checked for a variety of automotive high strength steels with the chemical composition given in Table 1. These steels were all laser welded with a heat input of around 60 J/mm which corresponds to a standard processing condition for the manufacturing of tailor welded blanks [1]. After welding micro Vickers hardness scans were taken along the weld cross section in the sheet center. The material structure of these welds in the fusion zone is fully martensitic. Table 1 Chemical composition of automotive high strength steels wt.% Steel grade C Mn Si P Cr Nb Ti others H220Y H260P H340LA DP600 CR DP800 CR Mo : 0.02 CP1000 HR B: Mo: 0.20 DP1000 CR Mo : 0.02 TRIP800 CR TRIP800 HR B: TRIP1000 CR It is found that the IIW definition of the CE does not give a reliable correlation with the hardness in contrast to the P cm definition. The best correlation however was found between the absolute carbon content and the weld hardness (Figure 1). This correlation between hardness and the carbon content, also holds for HSLA and ELC steels containing less than 0.1% carbon. Only the hardness of ultra low carbon steel deviates significantly from the linear relationship. Hardness (HV0.2) Hardness data laser weld Fit of data points Hardness data lit. [ref. 2] Ms temperature C-steel HV = 1156(C) R² = 0,99 0,00 0,10 0,20 0,30 0,40 0,50 Carbon content (wt.%) Fig.1 Dependency of laser weld hardness and martensite start temperature on carbon content The present hardness-carbon relationship based on data from laser welding was checked against data published in literature [2]. The latter data were generated by quenching of 2 mm thick platelets of unalloyed C-steel in ice water with 10% NaCl after reheating into the austenite range. The agreement of those data with the present ones obtained after laser welding is excellent. Also shown is the measured martensite start temperature of the unalloyed C-steel [2], which is decreasing with increasing carbon content. This can explain the deviating behavior of the hardness data for the ULC steels. Since the martensite start temperature of this material is close to 500 C, the formed martensite is being auto-tempered upon further down-cooling of the sample. Ultra fast cooling tests ( t 8/5 = s) on ULC steel (0.004 %C) using a Gleeble apparatus resulted in martensite having a hardness of around HV [3]. This value is fully consistent with the extrapolation of the current linear fit. 3 Formability of laser weld seams The ductility of a laser weld seam can be Ms temperature ( C)

3 characterized by measuring the fracture elongation in a tensile test performed along the weld seam. Since the weld seam has a hardened structure it will fracture at lower strain than the base material. More frequently, however, the ductility of a laser weld seam is evaluated by the so-called Erichsen cup test. The indentation depth at splitting is measured for the base material as well as on the welded sample keeping the weld seam along the azimuth of the indenting sphere (Fig. 2). The indentation depth on the welded sample is related to the value of the base material. This ratio, called Erichsen index, is a measure for the formability of the weld seam. The correlation of the Erichsen index with the hardness of the laser weld seam is displayed in Figure 2. As expected, the Erichsen index is being reduced as the hardness of the weld seam increases indicating a corresponding loss of ductility in the weld seam. The data are well fit by a second order polynomial. EI (%) HAZ softening EI = -0.13xHV Hardness (HV) Fig.2 Correlation between the laser weld seam hardness and the Erichsen index When loading a weld seam transversely to the laser weld, the material will typically fail away from the weld seam in the base metal, which has a lower strength then the hardened HAZ. However in a few steels failure can occur in the HAZ. This is the case for some ultra high strength steels having martensite as the majority phase [4]. Here, the heat input by laser welding leads to tempering of the existing martensite resulting in a drop of hardness and strength. A loss of strength and ductility can also occur in the HAZ of ULC steel due to the formation of large grains promoted by the high transformation temperature. 4 Laser welding process adaptations A possible way to improve the ductility of a laser weld seam is to reduce the seam hardness by either increasing the heat input during welding or by performing a post weld heat treatment. If the heat input is increased the cooling speed is being reduced, since more heat has to be dissipated from the weld zone into the base material. Several techniques to increase the heat input exist, e.g., defocusing or double focusing. In the latter technique, the available laser energy is focused to two spots next to each other [1]. Accordingly the intensity of each spot is halved. Due to the lower intensity the welding speed has to be reduced by about 20% compared to single spot focusing. Industrial practice has shown that double focusing enables a more robust process, which is less prone to transmissive energy loss caused by local gap openings along the butt joint. Hardness (HV 0.2 ) Single focus Double focus (HI +20%) Single focus Double focus 1 mm Position (mm) Fig.3 Microstructure and hardness of H340LA steel (see Table 1) after single and double focus laser welding.

4 Double focusing was also found to be a very effective means in reducing the seam hardness as shown for a microalloyed HSLA steel in Figure 3. The so obtained hardness reduction is in the order of 130 HV as compared to single focus welding. The micrograph of the weld cross section indicates that the width of the HAZ is slightly increased using double focusing. A similar hardness reduction was observed with mild ELC steel (C: 0.03%). However, when welding ULC IF steel (C: %), the hardness reduction induced by double focusing is very limited. This is because the ULC steel experiences already an auto-tempering effect with single focus welding as was explained earlier. In steels having rather high carbon content the hardness reducing effect of double focusing is less pronounced for a different reason. Such steels have a low martensite start temperature as shown in Figure 1. Therefore auto-tempering by the weld heat itself is nearly ineffective. Erichsen index (% of base material) TRIP 800 (CR) TRIP 800 (HR) TRIP 1000 (CR) as welded 72 h at 20 C 2 s at 600 C Post weld heat treatment Fig.4 Effect of post weld heat treatment on formability of laser weld seams in TRIP steels (compositions see Table 1) Due to their comparably high carbon content, TRIP steels experience considerable hardening by laser welding. The very high hardness of the fusion zone in combination with hydrogen take-up during welding can cause additional embrittlement of the weld seam leading to very limited formability as indicated in Figure 4. The formability is being improved after 72 hours as hydrogen has effused from the weld zone. The hardness however still remains on the initial high level. Therefore post-weld heat treatment techniques have been developed to temper the martensite and to accelerate the hydrogen effusion in the fusion zone. A practicable thermal cycle is 600 C for 2 seconds generated by induction heating [5, 6]. Such a treatment reduces the hardness by up to 200 HV and thus improves the formability to an acceptable level without broadening the HAZ. 5 Metallurgical considerations From a metallurgical point of view, the most effective way of improving the laser weldability is to reduce the carbon content of the steel. The contribution of other elements is negligible as under typical welding conditions for thin sheet the cooling rate is high enough to form a martensitic weld zone in all automotive steel grades except in ultra low carbon based steels. Reducing the carbon content of a steel, however, usually involves a reduction of mechanical strength which can be compensated by other strengthening mechanisms such as grain refinement. Good examples for this concept are microalloyed HSLA steels with ferritic-pearlitic microstructure (Table 2). The carbon content does typically not exceed 0.08%. The hardness of a laser weld remains thus below 380 HV (Fig. 1) and the formability of the laser weld seam attains a high level (Fig. 2). The standard microalloying element in such steels is Nb providing efficient grain refinement via thermo-mechanical rolling. To obtain a yield strength of 500 MPa and above, additional microalloying elements such as Ti and V have to be involved. Table 2 Typical chemical compositions of microalloyed HSLA steels Min. yield wt.% strength C Mn Si Nb Ti other Cold rolled MPa MPa MPa Hot rolled 355 MPa MPa V: MPa (HTP) MPa B: 0.002

5 In hot rolled steels with ferritic-bainitic microstructure a yield strength above 500 MPa can be obtained with an even further reduced carbon content. Microalloying by a combination of Ti and B promotes the formation of bainite. Generally ferritic-bainitic steels are widely used for the manufacturing of wheels or chassis parts. A particular alloying concept is that of low carbon ( %) and high Nb ( %), which is also known as the HTP concept (Table 2) [7]. Such steel is already being used for automotive parts [8]. Due to the low carbon content, the weldability of this steel type is excellent as shown in Fig. 5. Laser welding will still result in a martensitic structure of the fusion zone. The maximum hardness however is limited to values below 350 HV due to the low carbon content. Thus requirements with regard to cold cracking avoidance are unconditionally fulfilled. Other industrial welding techniques applied to this alloying concept result in a fully bainitic structure of the fusion zone giving very favorable properties with regard to weld toughness. improved formability especially when highly localized strains occur during the forming operation. The additional strength increase provided by microstructural refinement allows reducing the carbon content to values that are comparable with those of HSLA steels of similar strength. Conclusions It was shown that predominantly the carbon content determines the mechanical properties of laser weld seams in automotive steel sheet. The development of high strength steels with as low carbon content as possible is thus recommended to obtain an acceptable hardness and formability of the laser weld seam. Microstructural refinement and precipitation hardening induced by niobium microalloying help to achieve this goal. Those steel grades where an elevated carbon level is unavoidable require in-situ or post-weld heat treatment to obtain an acceptable hardness and formability of the laser weld seam. References: [1] Mohrbacher H. International Sheet Metal Review, 2001, 3 (3): 34~38 [2] Rose A. Steel-Strengthening Methods, Zürich, Climax Molybdenum Company: 135~145 [3] Sauvage F, Kaplan D. The Book of Steel. Paris: Lavoisier Publishing, 1996: 823 [4] Sperle J-O. Iron and Steel Today, Yesterday and Tomorrow, Conf. Jernkontoret Vol. 2, Stockholm 1997: Fig.5 Transformation behavior for simulated HAZ (peak temperature 1350 C) of HTP steel (0.03% C, 0.10% Nb, 1.75% Mn) Dual phase steels with a tensile strength of 600 MPa are nowadays routinely used in automotive body construction and have a typical carbon content of around 0.10%. Although this steel is usually not considered to be critical concerning weldability, a reduction of carbon can offer assets in terms of reduced hardness and weld formability. Furthermore, the peritectic range can be avoided improving castability in the steelshop. Microalloying of Nb to such steels has shown a significant microstructural refinement yielding in a strength increase and an 413~426 [5] Verrier P. Arcelor CRDM. Private communication, 2002 [6] Haferkamp H, Ostendorf A, Bunte J, Bormann A, Schülbe H, Meier O. Viertes Industriekolloquium SFB 362, 2004: 121~126 [7] Hulka K, Bordignon P, Gray M. Niobium Technical Report No. 1/04, CBMM, 2004 [8] Haensch W, Klinkenberg C. 2nd Int. Conf. On Thermomechanical Processing of Steels: Verlag Stahl & Eisen, 2004: 115~120