Weldability Evaluation and Microstructure Analysis of Resistance Spot Welded High Manganese Steel in Automotive Application

Transcription

1 Weldability Evaluation and Microstructure Analysis of Resistance Spot Welded High Manganese Steel in Automotive Application Dulal Chandra Saha, Chul Young Choi, Sangho Han, Kwang Geun Chin, Ildong Choi and Yeong-Do Park* Dept. of Advanced Material Engineering, Dong-Eui University, Busan , South Korea School of Materials Science and Engineering, Pusan National University, Busan , South Korea, TWIP Steel Research Engineering, POSCO, Gwangyang , South Korea Dept. of Division of Marine Equipment Engineering, Korea Maritime University, Busan , South Korea Corresponding author: Abstract High Manganese steel is one of the promising candidates for automotive application due to its outstanding mechanical performance, impact resistance, and high stain hardening rate. In this paper, resistance spot weldability of high Manganese steels (high-mn Steel) were investigated in order to get high reliability in welded joints of automotive components. Microstructural characterizations, cross tensile test, microhardness tests of spot welded parts were conducted. The effects of weld current on the microstructural characteristics, mechanical properties, and fracture modes were investigated using optical microscopy and scanning electron microscopy (SEM). The hardness in the weld nugget was observed to be lower than that in the base metal. The failure initiation was observed to occur basically at the boundary of the weld nugget. Also welding imperfections of welded parts were investigated. Liquation cracking in Heat Affected Zone (HAZ) and void were found most common welding defects in spot welded parts of high-mn steels. Furthermore, the effects of welding imperfections on weld quality and failure criteria were observed. Keywords Resistance Spot Welding, High Manganese steels, Weldability, Microstructure, Welding defects. 1. Introduction High Mn austenitic steel is one of the promising candidates for Twinning-Induced Plasticity (TWIP) steels because of easy control of the stability and stacking fault energy (SFE) of austenite as a function of Mn content. The TWIP steels exhibit outstanding mechanical - 1 -

2 properties of high tensile strength (1000MPa) and large ductility (65%) due to high strain hardening rate [1]. Spot welding is the primary method of joining automotive structural components. The welding process as well as the resulting microstructure and mechanical properties of high Mn steel joints are not a subject widely studied. In this paper, weldability of high Mn steels has been evaluated. Microstructural changes of welded parts were examined using both optical microscope and Scanning electron microscope (SEM). Microhardness distribution from base metal to weld metal has been measured and welding imperfections were observed. Microstructure of welded parts was examined to find out major welding defects. HAZ liquation cracking, porosity and microporosity, and void were found most common welding defects. The location of liquation cracking and the cracking orientation were investigated. Furthermore, effects of welding defects on weldability were examined by using progressive cross tensile test. 2. Materials and Procedures Materials In this study high manganese (TWIP980) steel of 1.4 mm thickness was used. The used material contains 0.6% C, 18% Mn and 1.5% Al with minor alloying elements. The bulk specimen has been cut and made sample for Cross tensile test [2]. Specimens were clean properly with ethanol to remove dirt and oil before experiment. Welding equipment and parameters Welding was carried out using a medium frequency DC inverter spot welder. The 8.0 mm tip diameter of Cu-Cr DR type electrode were used under constant water cooling rate of 6 l/min. The details experiment conditions are shown in Table 1. Table 1. Welding schedule (ISO (2004)) used in this study. Condition ISO Schedule Schedule [Cycle, 60Hz] Electrode Up No. Weld Cooling Down Hold force [kn] Squeeze slope Pulse Time Time Slope Time Mechanical Test and metallographic evaluation Weldability evaluation and microstructure analysis of fractured welded parts were performed using Cross Tensile Test (CTT). The Vickers microhardness measurement across the weld nugget and heat affected zone (HAZ) and of the base metal was carried out with a load of 200 gm and loading time 10 sec. The microstructure was revealed using color etching of Nital (3%) - 2 -

3 Weld Size (mm) Peak Load (kn) May 15-18, 2011 / Grand Hilton Hotel / Seoul, Korea for 30 seconds + HCl (10%) for 5 seconds + solution of distilled water (100ml) and Na 2 S 2 O 5 (10gm) for 10 Seconds. 3. Results and Discussion Weldability evaluation The weldable current range were determined using 4 t (where t is the thickness of used steel) condition. The suitable current range is considered between the current at which minimum nugget and button diameter is equal to 4 t to expulsion. A wide welding current range is desirable because it would be possible to control the nugget diameter within a prescribed range even if welding current fluctuates. Fig.1 indicated that the welding current has an obvious effect on spot welded high Mn steels. Comparing to other AHSS steels, high Mn steels showed lower welding current range (from 5.5 ka to 7.5 ka). Rich chemical compositions of base metal are influencing higher resistance during resistance spot welding which trends to lower expulsion current range. Another reason is that due to higher base metal strength fully contact diameter cannot be obtained [3]. Expulsion is occurred at 7.5 ka current, this condition confirmed after analyzing dynamic resistance curve. Nugget diameter were found higher than button diameter because fracture always initiated in heat affected zone (HAZ); so opening area of two ends of nugget are not included in button diameter. It can be seen in weld growth curve that nugget diameter continuously increases with increase of weld current (Fig. 1a) and finally large drop due to expel out of molten metal from fusion zone (star mark in Fig.1). As the weld current increased more heat is generated at the interface of two sheets (as resistance value is higher at interface) which leads to increase nugget size. Weld strength curve (Fig. 1b) also indicating same phenomenon. Higher failure load can be observed with larger nugget size a 4 t Nugget Diameter Button Diameter Welding Current (ka) b Weld Current (ka) Fig. 1. Weldability of high Mn steels; a) Weld growth curve, b) Weld strength curve

4 Microstructure analysis Figure 2 shows microstructure at welded zones of 980MPa cold rolled TWIP steel sheets. An ellipse nugget is formed between two sheets and heat affected zone (HAZ) is formed around the nugget. Microstructure at nugget appears to be fully austenite because this area is rapidly cooled by water-cooled electrodes. Microstructure at HAZ changes form fusion zone structure to base metal structure depending on reached highest temperature. Nuggets are formed at the center of two sheets. As the microstructure is fully austenitic at the end of solidification, it will remain austenitic upon cooling to room temperature and exhibit distinct solidification structure when viewed metallographically. The final solidification structure consists of dendrite morphology in microstructure. The dentritic structure is mainly result of segregation of alloying [4], and low diffusivity of these alloys at elevated temperature. Due to extreme high heat inputs, some grain coarsening could be observed in Fig. 2b. As a result of base metals strengthened by cold working, recrystallization and grain growth can result in significant HAZ softening and a distinct HAZ. The grain size in HAZ is clearly larger than that of the base metal. Since the HAZ is heated to temperatures approaching the solidus temperature of the alloy, many of the alloys that are present in the base metal may dissolve. This can lead to a supersaturation of the austenite matrix during cooling. Precipitation of alloy usually forms along grain boundaries. Need further investigation to indentify elements presents along grain boundaries. Figure 2f shows different colored microstructure in fusion zone. Each color microstructure is one grain which is consisting of different dendritic and interdendritic structures. During investigation of elements segregation in fusion zone, it was found that Mn is segregated in interdendritic areas while Al segregated at dendritic areas. Although along the fusion line the temperature gradient is steepest but the solidification growth rate is lowest. As the weld centerline is approached, the growth rate increases while the temperature gradient decreases. Fig. 2g and h show dendritic morphology in nugget center, also some porosity can be seen in that area. During solidification, alloys which have higher solidification temperature are solidified in dendritic area while alloys with lower solidification temperature are solidified interdendritic region. Dendrites initially grow as primary arms and depending upon the cooling rate, composition and agitation, secondary arms grow outward from the primary arms. Likewise, tertiary arms grow outward from the secondary arms

5 a b c d e f g h Fig. 2. Microstructural changes from the base metal to the centre of the weld nugget via optical microscopic (Magnification: 50X) observations. a) Base Metal, b) Heat Affected Zone (HAZ), c) HAZ liquation cracking, d) HAZ liquation cracking in lower magnification (10X), e) transition zone between fusion zone and HAZ, f) fusion zone, g) and h) Fusion zone at center. Micro hardness test Figure 3 shows the hardness profile of high Mn steels. As base metal of austenitic stainless steel is not transformable, no phase transformation occurs in the HAZ. However, grain structure of this region is affected by welding process. Some grain growth (30~35 μm) was observed adjacent to the weld nugget. Hardness at nuggets shows considerably lower value than that at base steel sheets. Hardness at HAZ also shows higher than that at base steel sheet and this values decrease from base steel sheet side to nugget side. Due to fine grain in base metal (5~7 μm) compare to weld metal, hardness value is much lower in weld metal. Strain hardening effect can be observed in base metal which is mainly due to cold rolling of base metal, but there is no hardening effect in fusion zone. Hardening effect can be attributed by observing hardness values in base metal and weld metal. Another reason is that weld metal containing lower carbon percentage due to the decarburization during resistance spot welding, which made it less hard than base metal. Due to presence of porosity and microporosity in weld center, lower hardness value can be observed around weld center (Fig. 3b)

6 Hardness (HV, 200 kgf) May 15-18, 2011 / Grand Hilton Hotel / Seoul, Korea a b BM HAZ WM HAZ BM Distance (mm) Fig. 3. The microhardness distributions in the welded zones (welding current 7.0 ka); a) diagonal direction, and b) transverse direction. Welding imperfections There are some internal discontinuities, which are revealed through metallographic examination by cross-sectioning a welded part. Voids and cracks are most common discontinuities which were found in this experiment. Cracks are originated from the HAZ, due to liquation of grain boundaries and large internal stresses in the HAZ and it could be found to fill up by liquid metal, leaving an outline, rather than an opening crack [5]. a b c d e f g h i Fig. 4. Welding defects observed in SEM analysis (magnification 5000X); a) porosity in fusion zone, b) and c) HAZ liquation crack, d) void at center of fusion zone and Optical Microscopic observation (Magnification 50X) e) HAZ Liquation crack, f) and g) intergranular liquation cracking, h) porosity in fusion zone, and i) void in center of fusion zone

7 Figure 4e illustrates the crack which occurs in HAZ of high Mn steels. The crack in HAZ propagates along grain boundaries and the trace of liquation is observed, indicating that the grain boundaries were weaker and contained a little amount of low melting point liquid films when the crack took place. These features confirm that the crack in HAZ of spot welded high Mn steel belongs to liquation cracking. The causes of liquation cracking are associated with grain boundary segregation aggravated by melting of boundaries near the weld. During spot welding, the grain in HAZ became coarser than that in base metal and melting of grain boundaries occurred, as shown in Fig.4. Since melting nucleates preferentially at high energy crystal defects, such as boundaries and surfaces, there is a gradual increase in melted boundary width up to the weld nugget. Under these conditions, solute atoms of low solubility in the matrix tend to diffuse to the melted boundaries to lower melting point of the liquid. At the same time, the HAZ is forced to upset because its expansion to match the temperature rise is constrained by adjacent base metal at a lower temperature. On being cooled, the low melting point liquid films form at grain boundaries to reduce the boundary strength and the tensile stress begins to develop in HAZ because of earlier upsetting. Liquation cracking in HAZ occurs along grain boundaries when the strength of grain boundaries weakened by low melting point liquid films is below the developed tensile stress. Fig.5. Full view of liquation cracking (Magnification, 3000X) Optical microscopic examination was used to identify crack location and appearance of cracks, and their orientations. Fig. 4f shows the intergranular fracture characteristics of the cracks. Cracks initiate near the fusion line in the HAZ and propagate away from the nugget into the base metal. Cracks tend to follow the grain boundaries, while keeping the overall outward direction. Most of them are wide at their bases and become narrower as they enter the base metal. Wide cracks have tree-like structures, i.e., large trunks (wide opening of cracks at the bottom) formed from fine roots (grain boundaries). Many of them are fully or partially filled [5]

8 Fig. 6. SEM and EDX analysis in HAZ liquation cracking area. To identify the effect of liquation cracking on weld strength, progressive cross tensile test were carried out. The welded parts were fractured progressively with 15%, 30%, 45%, 60%, 75%, 90% and 100% of total strain. Each sample has been cut and observed the failure initiation area. It has been observed, most of the cases, fracture path are not following liquation cracking zone. There is less effect of liquation cracking on weld strength. A B Fig. 7. Progressive cross tensile test results, A) 60%, and B) 75% of total stain. 4. Conclusions - Due to high chemical composition in high Mn steels, expulsion occurred earlier compared to other AHSS. - In high Mn steels weld nugget contains the dendritic structure in the center of the nugget. Features of HAZ are the boundary melting and coarsening. - Due to less carbon percentage, no hardening effects, and coarse grain size in fusion zone hardness value is lower than base metal. - The weld nugget has a high liquation cracking susceptibility. The cracks appear in the nuggets when the welding current is higher than 5.0 ka. The high welding current increases the cracking tendency because of increasing the tensile stress developed during cooling. - Liquation cracking which were occurred during welding process of high Mn steel has little effect on weld strength

9 References [1] J.-E. Jin, and Y.-K. Lee: Mater. Sci. Eng. A Vol. 527 (2009), p [2] K. Hofman, M. Soter, C. Orestte, S. Villaire, and M. Prokator: A COMPARISON OF AC TO INVERTER DC RESISTANCE SPOT WELDING AND THE EFFECTS ON DUAL-PHASE 600. [3] H. Oikawa, T. Sakiyama, T. Ishikawa, G. Murayama, and Y. Takahashi: Resistance Spot Weldability of High Strength Steel (HSS) Sheets for Automobiles (NIPPON STEEL TECHNICAL REPORT No. 95 January 2007). [4] John, C. Lippold, and D. J. Kotecki: Welding Metallurgy and Weldability of Stainless Steel Chapter 6, p [5] H. Zhang, and J. Senkara: Resistance spot welding, Fundamentals and Applications