Effect of Friction Welding Parameters on Mechanical Properties of Cast Iron Joints

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[Quarterly Journal of Japan Welding Society, Vol. 12, No. 3, pp (1994) ] Effect of Friction Welding Parameters on Mechanical Properties of Cast Iron Joints By Takeshi SHINODA, Katsuei HOSHINO
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[Quarterly Journal of Japan Welding Society, Vol. 12, No. 3, pp (1994) ] Effect of Friction Welding Parameters on Mechanical Properties of Cast Iron Joints By Takeshi SHINODA, Katsuei HOSHINO and Ryouichi YAMASHITA Abstract The feasibility of using friction welding was used to join similar kinds of cast irons was studied. This study was carried out to examine influence of welding parameters on tensile properties of friction welded joints by similar materials of spheroidal graphite iron castings and gray iron castings. It was found that cast iron, which is difficult to join by fusion welding, can be joined by friction welding without resorting to special measures such as precheating or postheating. Under proper welding conditions, the friction welds are defect free. This paper describes process parameters during friction welding which governing mechanical properties. Materials were selected two types of cast irons, spheroidal graphite iron castings and gray iron castings with flaky graphites which are called ductile cast iron and gray cast iron hereafter in this paper. The friction welding conditions for similar kinds of cast irons in the same diameter can be defined either by the minimum heat input rate or by the friction upset speed. The tensile strength of the joint increases with decreasing heat input rate or upset speed and it is possible to obtain joint strengths equal to that of the base metals. Key Words : Friction welding, Gray cast iron, Ductile cast iron, Tensile strength, Heat input, Friction upset speed 1. Introduction It is well known fact that cast iron is one of the most difficult meterials to join by fusion welding. Welding is usually carried out by gas or arc welding techniques, although electroslag welding making use of the phenomenon of transformation superplasticity has also been attempted in some laboratory scale experimental cases . In all methods of welding cast iron, however, it is necessary to employ extremely high preheating temperatures to prevent cracking. Arc welding of cast iron requires the use of high ductility filler materials such as nickel electrodes and in addition, peening between passes is also performed. In spite of such precautionary measures, however, it is considered difficult to prevent cracking completely because of the low ductility characteristics of the cast iron itself. Other defects such as blowholes are also difficult to eliminate in cast iron under fusion welds. Friction welding has-been successfully applied for welding low ductility materials which are difficult to join by fusion welding techniques'). This is because of the fact that the pattern of heat flow is simple and one dimensional direction in friction welding and also because high compressive residual stresses are present at the surface of these joints. In particular, the friction welding can be successfully applied to joining a much broader spectrum of dissimilar materials than fn.-inn welrlina Friction welding of cast irons was attempted in the present work based on the conviction that, as described above, the process itself is focused on sound principles and should therefore be successfully applicable to cast irons. Gray cast iron with flaky graphite and ductile cast iron with spheroidal graphite were used as the materials for friction welding of similar material joints and the relation between the welding conditions and the mechanical properties were investigated. It can be considered that, in principle, friction welding possesses characteristics that are suitable for joining cast irons. However, there are negative interpretations such as the view of the American Welding Society which holds that friction welding of cast iron is not possible since * Received : 26 May 1993, ** Member, Faculty of Engineering, Nagoya University *** Member, Daido Institute of Technology n ÚŠw ï _ W æ 12Šª(1994) æ3 329 the graphite acts as a lubricant and prevents the generation of sufficient heat from joining. As is evident from the above, although it cannot be said that sufficient work has been carried out, the few studies mentioned indicate that it is possible to produce dissimilar material weld joints of steel and cast iron exhibiting fracture characteristics of the cast iron base metal using friction welding. On the other hand, it appears that studies regarding friction welding of similar materials of cast iron have not been made up to now. 3. Experimental details, results and discussion 3.1 Materials and experimental details 25 mm diameter x 140 mm long specimens of gray cast iron (FC200) with flaky graphite and ductile cast iron (FCD450) with spheroidal graphite whose chemical compositions are shown in Table 1 were employed. Similar cast irons were joined by friction welding process. A continuous drive friction welding machine with a maximum thrust of 6 tons was used for welding without preheating, using friction and forge pressures in the range MPa. The spindle rotating speed was kept constant at 2360 rpm (39.3s-1) and the welding was performed under the specified friction upset distance. Upset and brake timing were kept simultaneous for all experiments. During the experiments, a laser displacement sensor was used to measure the friction upset speed and upset distances. The main spindle rotation, and the forge pressure were also monitored. In addition, a radiative type thermometer was used to measure the temperature variation at the joint interface during welding. These were recorded using multi-pen recorder to analyze welding phenomena. The welds were evaluated by tensile tests for joint strength and by microstructural observations, without any post weld heat treatment. The shape and dimensions of the tensile test specimens specified by JIS Z2201 and are shown in Fig. 1. The weld interface is located at the center of the specimen and the length of the parallel portinn is 20 mm 3.2 Results and discussion Range of experimental conditions and results of welding The same range of friction welding conditions were used for both FC200 and FCD450. Figure 2 is an example showing the range of friction welding conditions for FC200. The upper left area in the figure is the region where friction welding is practically used, where friction pressure, P1 is less than forge pressure, P2. The two horizontal dotted lines indicate the critical friction pressure conditions under which the HAZ widths are 15 mm and 20 mm respectively. It can be seen that the HAZ width is independent of the friction pressure P1 and is influenced only by the forge pressure P2. This is due to the fact that the region softened by heat is expelled as flash roll from the joint interface by the forged pressure P2. Figure 3 is an example of the data recorded during welding. For displacement controlled friction welding, it was found that in the case of ductile cast iron (FCD450) the specified displacement of 3.75 mm was attained slightly later when compared to gray cast iron (FC200). Except for this, no other significant differences were observed in the welding conditions of the two types of cast iron. It can be observed from the chart that Table 1 Chemical compositions of materials. Fig. 1 Dimensions of tensile test specimen. Fig. 2 Operation range of friction welding of FC200 cast iron. 330研 究 論文 TakeshiSHINODAet al : Effectof FrictionWeldingParameterson MechanicalPropertiesof Cast Iron joints Fig. 3 Recording chart of friction welding of FCD450 cast iron P1 : 50.9 MPa, P2: 86.8 MPa. Fig. 4 Effect of friction pressure on friction upset speed (FC200 and FCD450). the temperature at the joint interface rises only during application of the friction pressure P1 and reaches a maximum value of about 1200 K. The temperature decreases rapidly during the forging period of the weld cycle. This suggested that heat generation appeared only at the period of friction stage. Figure 4 shows the relation between friction pressure and upset speed and it can be seen that FCD450 exhibits a higher upset speed compared to FC200. It is believed that this difference is due to the differences in the deforming characteristics and other material constants at high temperature of the two cast irons . Cracks which appeared to be hot cracks formed Fig. 5 Appearances welded joints. of flash roll of friction in the upsetting stage were observed on the surface of the flash roll of some of the welded specimens. However, no macroscopic defects such as cracks or blowholes were present in the weld interface and heat affected zone. Under lower friction pressures, a liquid phase was present at the joint interface and in certain cases liquid droplets were observed. Although the conditions under which these droplets form could not be clarified, it seems that in general, longer friction times promote their formation in case of lower friction pressures. Also, they were observed to occur more easily in FCD45O rather than in FC200. An example of hot 溶 接 学 会 論 文 集 cracks in the flash roll of friction welded FC200 is shown in Fig. 5A and an example of droplet formation in a FCD450 weld is illustrated in Fig. 5B. Compared to FC200 welds, the tendency for crack occurrence in the flash roll is much less in the FCD450 welds. This is due to facts that the region softened by heat is expelled as flash roll from the joint interface by forge pressure P2, and the rather longer friction heat time makes wider parallel HAZ region to interface. As dimensions of HAZ is enough wide to the total upset distance with lower friction pressure, HAZ width depends on forge pressure. This seems particular case for cast iron. The relation between the friction upset speed and the tensile strength of the welds is shown in figures 6 and 7. The friction upset speed was defined as the average displacement per unit time (L1/t1 in Fig. 3). In the case of gray cast iron welds (Fig. 6), the joint strength increases as the friction upset speed is decreased and is approximately equal to the tensile strength of the base metal at upset speeds of to mm/s. The Fig. 6 Effect of friction upset speed on tensile strength of cast iron (FC200). Fig. 7 Effect of friction upset speed on tensile strength of cast iron (FCI)450). 第12巻(1999)第3号 331 two dark circles in the figure in the vicinity of mm/s indicate the tensile strength of the welds when P1= P2. This is an unusual welding condition which is not employed normally in friction welding. As shown in Fig. 7, the relation between tensile strength and friction upset speed in the case of ductile cast iron welds (FCD450) is similar to that of gray cast iron welds (FC200), and fracture occurs in the base metal at upset speeds in the vicinity of mm/s in both cases. The fracture surface of an FCD450 weld where base metal fracture occurred is compared to one where fracture occurred in the weld metal (joint interface) is shown in Fig. 8. In the case of weld metal fracture (Fig. 8A), an unwelded region was found to exist near the center of the joint. In the case of base metal fracture specimens (Fig. 8B), no such unwelded interface regions are observable. In these specimens, the hardness of the weld metal is higher than that of the base metal and as a result, fracture was observed to occur near the HAZ. In all cases, however, the fracture was in the brittle mode and the elongation at fracture was only about 2%, which is lower than that of the base metal of ductile cast irons Definition of minimum heat input rate of friction welded materials The concept of welding heat input as used in arc welding cannot be used for friction welding. Usually, the amount of heat generated during friction welding is determined from the friction coefficient of the joining interfaces and the friction torque '. Since these two factors will vary during welding and will depend on roughness of the plating surfaces, their measurement and quantification are quite difficult. Even if quantification is made possible, it would difficult use the quantities as guidelines for obtaining sound welds in actual production in the factory. In particular, the torque will vary during Fig. 8 Fracture appearances specimens of friction (FCD450). of tensile test welded joints 332 Œ _ Takeshi SHINODA et al: Effect of Friction Welding Parameters on Mechanical Properties of Cast Iron Joints the welding process and even if it is measured during welding, the values of initial torque, stationary torque etc. are difficult to define. In this study, by considering the welding process macroscopically, the authors attempted to obtain a simple indicator of heat input which can be utilized even at the factory welding site. In formulating the parameter for heat input rate, indefinite variables such as surface condition, high temperature strength etc. were not considered, and factors that can be easily observed or measured at the welding site during actual production were thought. Referring to Fig. 3, the total heat is generated by the displacement that takes place during friction stage and the length of the base metal is reduced by the final amount of displacement. It was assumed that the ejected flash volume contains the quantity of heat generated in the joint above a certain specified temperature range. Thus, the value obtained by dividing the displacement by the friction time can be taken as the rate of flow of heat during welding and can therefore be defined as the minimum heat input rate necessary for friction welding. It does not contain the heat in HAZ regions, which means it will give a rough estimation of heat input to friction welding. Figure 9 shows the model for defining the minimum heat input rate. The upper drawing shows the state before welding while the lower one shows the state after friction welding is completed. If we assume that the volume of flash roll, Vf (mm3) expelled by application of pressures PI and P2 is equal to the amount of material shrinkage, then Vf is equivalent to the volume of the hatched portion shown in the upper drawing. Fig. 9 Definition of friction welding and simplified heat input rate. Thus, Vf=ƒÎr2ƒÂ(1) where r (mm) is the radius of the specimen and 8 the total displacement. As shown in Fig. 3 the temperature of the flash roll was measured using an infrared radiation temperature instrument and it was found that the peak temperature Tf remained almost constant at 1200 K for both types of cast irons used even if the welding conditions were altered. Thus the distance moved by the friction boundary surface at the temperature Tf during welding, i.e. the heat quantity Qf of the ejected flash roll is given by the following equation, Qf(Joules) =VfcpTf (2) where c is the specific heat (0.394X 10-4 J/g/K) and p the density of materials (7.1 g/mm3). The minimum heat input rate Jmin per unit time and area which is the estimated lower limit of the heat input rate viewed externally, can be obtained by dividing Qf by the friction time tl and the cross sectional area S of the specimen as shown in equation (3). The term minimum heat input has been used here because the heat quantity refers only to that of the weld boundary surface, which has been expelled during upsetting stage and does not include the heat contents in the HAZ region. Jmin(J/mm2/s) =Qf/tl/S (3) This value was found to be valid in a separate experiment where S45C steel was friction welded and the heat quantity of the joint was measured by calorimetry and is therefore used here too as a parameter. Although this type of approach has not been used up to now, the authors believe that it can be used as a guideline in actual industrial production too. The authors intend to verify the generality of this approach') in the future and investigate whether it can be expanded to include dissimilar metal welding or welding materials of different diameters. Figures 10 and 11 show the relationship between the minimum heat input rate and the tensile strength of FC200 and FCD450, respectively, in the as-welded state (i.e. no post weld heat treatment). It can be observed that there is a general tendency for the tensile strength to increase as the minimum heat input rate decreases. Although there is scatter, the data for FC200 and FCD450 fall almost within the same band. It should be noted here that for both types of cast iron, fracture occurs in the base metal when the heat input rate is J/mm2/s or less. Since equations 2 and 3 refer to welds of the same material with the same diameter, the vol- Fig. 10 Effect of simplified friction heat input rate on tensile strength (FC200). Fig. 12 Relation between friction upset speed and simplified friction heat input rate (FCD450). Fig. 11 Effect of simplified friction heat input rate on tensile strength (FCD450). ume of material ejected from the joint will be directly proportional to the displacement (ca) and inversely proportional to the friction time. In other words, the minimum heat input rate should be proportional to friction upset speed. The data of friction upset speed and heat input rate for FCD450 have been plotted in Fig. 12 and a near linear relationship can be observed. However, the minimum heat input rate, jmin seems to have larger estimation results. This attributes to the thermal properties of material which are average nominal values at certain high temperatures. This indicates that for friction welding of the similar materials with the same diameter, the friction displacement can be used as a function to define the welding parameters. Figure 13 shows the relationship between minimum heat input rate and HAZ width. The HAZ width in this case was defined as the region which becomes darkened when etched with nithal etchant, and this defined HAZ does not include Fig. 13 Effect of simplified friction heat input rate on width of heat affected zone. expelled hot temperature interface zone during upsetting stage. It was found that the HAZ width near the center of each specimen was almost the same as that near the circumference. It can be observed from the figure that when the heat input rate (heat flow rate) increases, i.e. when the heating and cooling rates increase, the HAZ width tends to decrease. It is also evident that the HAZ widths of FC200 and FCD450 depend only on the heat input rate and not on the materials themselves. The hardness distributions of the welded joints of the two materials are shown in Fig. 14. The hardness is maximum near the weld interface, showing values of 45OHv and 300Hv, respectively, for FCD450 and FC200. The microstructure of Takeshi SHINODA et al: Effect of Friction Welding Parameters on Mechanical Properties of Cast Iron Joints investigated and the following conclusions were obtained. (1) Cast iron, which is difficult to join by fusion welding, can be joined by friction welding without resorting to special measures such as preheat and/or post heat treatment. Under proper welding conditions, the friction welds are defect free. (2) The friction welding conditions for similar materials of cast iron specimens of the same diameter can be defined either by the minimum heat input rate or by the friction upset speed (except for the extraordinary case where P1= P2). The tensile strength of the joint increases with decreasing heat input rate or upset speed and it is possible to obtain joint strengths that of the base metals. equal to (3) Under the same conditions, the friction upset speed for flaky graphite cast iron is lower than that for spheroidal graphite cast iron. Acknowledgments Fig. 14 Hardness distributions of friction weld ed joints of cast irons. The authors wish to express their grateful thanks to Professor Kohno of the Daido Institute of technology for his guidance and encouragement regarding this work. The Asahi Works of Mitsubishi Electric Co. for providing us with the materials used for welding. the weld interface in ductile cast iron consisted of martensite and fine spheroidal graphite. In the case of micro-structure in gray cast iron, spherodization of the flaky graphite was observed in addition to the presenc
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