Effect of Friction Time on Flash Shape and Axial Shortening of Linear Friction Welded 45 Steel ⁎ W.-Y

中国科技论文在线 http://www.paper.edu.cn

Materials Letters 62 (2008) 293–296

Effect of friction time on flash shape and axial shortening of linear friction welded 45 steel ⁎ W.-Y. Li a,b, , T.J. Ma a, S.Q. Yang a, Q.Z. Xu a, Y. Zhang a, J.L. Li a, H.L. Liao b

a Shaanxi Key Laboratory of Friction Welding Technologies, Northwestern Polytechnical University, Xi'an 710072, PR China b LERMPS, Université de Technologie de Belfort-Montbéliard, Site de Sévenans, 90010 Belfort Cedex, France Received 23 October 2006; accepted 8 May 2007 Available online 24 May 2007

Abstract

The influence of friction time on the flash (upset metal) shape and axial shortening during linear friction welding of 45 steel under an appropriate welding condition was examined. It was found that a sound weld with the fine structures could be formed as the friction time not less than 3 s. The axial shortening was exponentially increased with increasing the friction time. The periodical ridges presented in the flash were formed through the reciprocating motion and expelling of the plasticized materials. The flash in the friction direction presents an undulating- ribbon structure, while that in the vertical direction looks like a little curly swirl. The curved ridges were caused by the non-uniform extrusion rates of materials in the middle and edge resulting from the non-uniform temperatures. © 2007 Elsevier B.V. All rights reserved.

Keywords: Linear friction welding; 45 Steel; Friction time; Axial shortening

1. Introduction including steel, intermetallic materials, aluminum, nickel and titanium alloys with the greatest emphasis on aircraft engine Friction welding is a solid state process for joining materials alloys. The process has also been demonstrated as an effective together with the help of the frictional heat generated from the way for joining copper to aluminum for electrical conductors [3]. movement of one component relative to another one under a Although available for more than 10 years since the design and force. Rotary friction welding (RFW) is the most popular build of a prototype LFW machine by The Welding Institute method. Friction stir welding (FSW) and linear friction welding (TWI) of UK [3], there are almost no extensive investigations on (LFW) are the relatively new processes aimed at extending the LFW except some pioneer work [4–7]. current applications for RFW to non-axisymmetric components. LFW involves a solid state joining of materials through the FSW offers an attractive alternative to conventional fusion relative reciprocating motion of two components under axial welding processes for joining light metals, especially, aluminum force as shown in Fig. 1(a). This process is observed to have four and its alloys [1]. Recently, FSW has been world-widely studied distinct phases, including the initial phase, the transition phase, because of the excellent properties (particularly ductility) of the equilibrium phase, and the deceleration (or forging) phase, as welds [1,2]. The world-wide industrial acceptance of the schematically shown in Fig. 1(b) [4–6]. At the beginning, the economic benefits and high joint quality produced when using two components are brought in contact under a given axial force. conventional rotary friction welding to produce joints in round During processing, frictional heat and deformation strain are section metallic components led to the development of LFW. generated and results in continued plasticization of the Non-round or complex geometry components, such as aircraft interfacial region between the workpieces and displacement of engine blades to discs (blisks), can be welded using LFW [3]. plastically deformed material toward the weld edges to form a LFW has been used successfully to join a range of materials flash (upset metal). Once sufficient plasticization has occurred, a forging force is applied, to produce a consolidated joint seam ⁎ Corresponding author. Tel.: +33 3 84583160; fax: +33 3 84583286. with the limited thermomechanically affected zone (TMAZ) and E-mail address: [email protected] (W.-Y. Li). heat affected zone (HAZ). If sufficient frictional heat has been

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Fig. 1. Schematic of the linear friction welding process.

produced during the previous phase to soften the interface bonded in the direction as shown in Fig. 1(a). The use of 45 steel material, larger wear particles begin to be expelled from the in this study was based on its well established structure and interface, and axial shortening of the workpieces begins as a property data bank for a better understanding of the nature of the result of the expelled upset. Heat is conducted away from the LFW process in the future study. The parent 45 steel presented interface and a plastic zone develops. In the plasticized layer the typical quenched martensite structure as shown in Fig. 2(a). formed at the interface, the local stress system with the assistance The LFW machine (XMH-160) developed in Northwestern of the oscillatory movement extrudes material from the interface Polytechnical University (China) was employed to join 45 steel into the flash. It has been shown that the weld integrity is blocks. To examine the effect of friction time on the axial strongly affected by the rate of flash expelled under the shortening, it was changed but the other welding parameters appropriate initial conditions [5]. were fixed at an appropriate value according to the primary As LFW is an emerging technique, research and develop- study as shown in Table 1. The forging force was not exerted for ment work is required to develop both scientific and practical eliminating its influence on the axial shortening. knowledge of the process. The primary experiment with Ti alloy The polished cross-section was etched by 3% nital. The proved the good performance of LFW joints [8]. The control of macrostructure was observed by a digital camera. The micro- axial shortening will benefit the assembly precision of LFW. All structure was investigated by an optical microscopy (OM). The the factors influencing the heat input, such as frequency, axial shortening was estimated by measuring the heights of amplitude and axial pressure, will affect the axial shortening. specimens before and after welding. Hence, as the first stage of the study, the objective of the present 3. Results and discussion work was to investigate the influence of friction time on the axial shortening during LFW of 45 steel. After LFW for different friction times, it was observed that the weld in specimen 5, i.e. welding for 3 s, presented a well flash. The 2. Experimental procedures microstructure analysis showed that this joint was sound as shown in Fig. 2(b). The structures from the weld center to the parent metal were The quenched 45 steel blocks with a configuration of 10 mm determined as very fine ferrite + pearlite in weld center, ferrite + in width (W), 17 mm in length (L) and 45 mm in height (H) were pearlite in weld edge, tempered sorbite in TMAZ near weld, tempered

Fig. 2. Typical OM micrographs of the cross-sections of (a) parent 45 steel and (b) linear friction welded 45 steel (etched). 中国科技论文在线 http://www.paper.edu.cn

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Table 1 Linear friction welding parameters in this study Specimen 1 2 3 4 5 6 Friction time (s) 0.5 1 1.5 2 3 4 Frequency (Hz) 33 Amplitude (mm) 4 Pressure (MPa) 80.5

troosite in HAZ, tempered martensite in HAZ near parent metal. These results mean that the weld has experienced different thermal histories. For example, the tempered martensite, troosite and sorbite correspond respectively to the temperature ranges of 150–200 °C, 350–500 °C and 500–600 °C [9]. The relatively quick cooling rate also influenced the microstructure of the weld. The systematic analysis of the microstructure evolution and its effect on the properties of weld are ongoing. Fig. 3 shows the effect of friction time on the axial shortening of specimen. It is clearly seen that the axial shortening was exponentially Fig. 5. Schematic of the flash and ridges. increased with increasing the friction time. According to the previous study [8], the average heat input for the used LFW machine is almost proportional to the friction time when the friction force, frequency and stage of LFW the friction heat is not much enough to soften the amplitude are fixed. However, owing to the complex heating and interfacial materials and the interface temperature is relatively low. As softening of the interfacial materials, the relation between the friction the progressing of welding, the increase of contact area significantly time and axial shortening will also be complex. The fit of the results increases the heat generation and in return an obvious flash is formed. yields the following simple formula: And thus the axial shortening increases remarkably with the interface temperature rise and softening of materials following the extruding 1:69 Hs ¼ 1:06t ð1Þ process under the reciprocating motion. Therefore, the friction time is a key parameter in controlling the formation of a perfect joint. An where, H is the axial shorting (mm) and t is the friction time (s). This s appropriate friction time should be determined before welding for the equation can explain well the change of axial shortening with the sake of joint quality. friction time under the welding conditions in this study. At the first Fig. 4 shows the macrographs of the specimens welded for 1.5 s and 3 s. It is clear that the flash length for 3 s is much longer than that for 1.5 s, which is consistent with the larger axial shortening. Through a detailed observation of the flash, it was found many ridges presented in the flash of four sides. Other investigators also observed this phenomenon [3–7]. However, it was observed that the shapes of flash in the repeating friction direction and the vertical direction to friction were different. As schematically shown in Fig. 5, the flash in the friction direction presents an undulating-ribbon structure with the almost uniformly distributed ridges, while the flash in the vertical direction looks like a little curly swirl and its length is much shorter than that in the friction direction. The frequency of ridges was consistent with that of the oscillation. Therefore, it was considered that the periodical ridges could be attributed to the reciprocating motion and the expelling of plasticized materials. On the other hand, the ridges appeared like the arcs. This fact suggests that the extrusion rates of Fig. 3. Effect of friction time on the axial shortening.

Fig. 4. Macrographs of linear friction welded 45 steel specimens with friction times of (a) 1.5 s and (b) 3 s. 中国科技论文在线 http://www.paper.edu.cn

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materials in the middle and edge are different owing to the non-uniform curly swirl and its length is much shorter than that in the temperatures and thus various softened materials in the interface. friction direction. Moreover, the ridges appeared like the arcs, which were caused by the non-uniform extrusion 4. Conclusions rates of materials in the middle and edge resulting from the non-uniform temperatures. Based on the results obtained in this study, the following conclusions could be drawn: References

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