Simulation and experiment for crack arrest in remanufacturing ...

Int J Adv Manuf Technol DOI 10.1007/s00170-013-5293-2

ORIGINAL ARTICLE

Simulation and experiment for crack arrest in remanufacturing Jing Yu · Hong Chao Zhang · Dewei Deng · Asif Iqbal · Sheng Zhi Hao

Received: 20 March 2013 / Accepted: 4 September 2013 © Springer-Verlag London 2013

Abstract Remanufacturing is the only way for sustainable development of mechanical equipment manufacturing. For remanufacturing blanks containing cracks, the primary task is the prevention of crack propagation to ensure effectiveness of the manufacturing processes to follow. When pulsed current passes through a specimen, due to the existence of crack, the temperature around the crack tips rises sharply and may even climb above the fusion point of the material, which causes the crack tip to become blunt. In this work, with compressor rotor blade material FV520B as a specimen, the distributions of current density, temperature field, and stress field are calculated at the instant of discharge based on the thermo-electro-structure coupled theory. The crack arrest experiment is performed on high pulsed current discharge device of type HCPD-I. By making comparisons of morphology, microstructure, and size of fusion zone and heat-affected zone (HAZ) around the crack tip before and after energizing, the relationships between the sizes of fusion zone and the HAZ and the discharge energy and the current path are derived. The obvious partition and refined

J. Yu () · H. C. Zhang · A. Iqbal Institute of Sustainable Design and Manufacture, Dalian University of Technology, Dalian, China e-mail: [email protected] D. Deng School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, China S. Z. Hao Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Ministry of Education, Dalian University of Technology, Dalian 116023, China

grains around the crack tip are prominent because of violent temperature change. The experimental and simulation results are found in fine agreement. The high current pulsed discharge can be used effectively to prevent a crack to further expand and show substantial potentials for application in remanufacturing domain. Keywords Remanufacturing · Crack arrest · Numerical simulation · Microstructure · High pulsed current

1 Introduction Remanufacturing is the general name for a series of technical measures and engineering activities of repairing and modifying obsolete equipment and products, guided by life cycle theory, with a goal of improving performance of retired equipment, and governed by the criteria of high quality, high efficiency, energy saving, material saving, and environmental protection. It targets to repair and alter the waste equipment or product with the means of advanced technology and industrialization [1]. Remanufacturing engineering is an important technical means to realize the development mode of recycle economy, which can constantly improve the technology of mechanical and electrical products, reduce cost of the remaining life, extend product life, save material, reduce pollution, and create more profits [2]. The remanufacturing process of retired products includes disassembly, sorting, cleaning, machining, assembly, running in, painting, etc., as shown in Fig. 1 [3]. To remanufacturing blanks with remanufacturability (named after remanufacturing, it possesses the residual life which can complete a whole service cycle), advanced surface technology [4, 5] is applied to recover the surface size and make the remanufactured parts superior to the original ones, or advanced

Int J Adv Manuf Technol

Fig. 1 A general flow chart of a remanufacturing process

manufacturing technique [6, 7] is adopted to process the parts to satisfy the assembly requirements. In the past, the remanufacturing blanks with cracks were considered as irreparable. The core parts of large mechanical equipment (such as engineering machinery, ship, airplane, and largescale compressor) are costly, highly value-added, complexly processed, and possess high technological requirements. During processing of these parts with cracks, it is vital to arrest propagation of the cracks so as to guarantee effectiveness of the repair processes to achieve the purpose of prolonging the life of products.

2 Literature review In most of the engineering components, the failure is caused by fatigue cracks. These cracks initiate at the stress concentration points. The time of crack propagation spans through a major portion of the fatigue life; therefore, it is vital to arrest the crack. A number of investigations have been performed to prolong fatigue life of defective machine components. Increase in the radius of curvature of crack tip and reduction of cracktip stress concentrations have been performed by stop-hole drilling method [8]. Bolts, rivets, pins, and other interference fasteners were inserted into the stop holes [9]. Moreover, introduction of dimples [10] or auxiliary holes [11] in the vicinity of a crack tip can introduce residual compressive stress and effectively arrest crack growth. Reduction in

crack-tip stress intensity was also introduced by electroplating [12], bonding composite patch [13], and welding [14] for the purpose of retarding or arresting the crack propagation. The theoretical analyses and experiments have proved that these methods can reduce stress concentration factor and extend the fatigue life of the parts. Yet, new fatigue cracks can initiate at the edge of the stop hole; thus, the effectiveness of crack arrest depends on the machining precision and size of the stop hole. If the diameter of the stop hole is too large, the strength of the parts is reduced; if it is too small, the machining process faces difficulty, especially drilling on highly hard and brittle materials. In addition, drilling stop hole is only suitable for surface cracks; it is not applicable to internal defects or embedded cracks. In accordance with the thermo-magneto-elasticity theory and pulsed current technology, the Joule heating effect of a generated electromagnetic field can be used to arrest crack propagation. This method has gradually become a very promising nonequilibrium processing technology for practical solutions. When pulsed current flows through a conductive metal containing a crack, the current will detour and concentrate at the crack tip [15]. The local temperature increases due to Joule heat release that initiates crack tip melting, a round molten hole is formed, and the curvature radius of the crack tip is increased. In this way, mechanical stress concentration can be reduced or even eliminated, and large phase transition and thermal compressive stresses could be generated around the crack tip. This phenomenon causes prevention


of crack growth and at the same time increases potential energy of formation of the main crack, and thus, the purpose of arresting the crack propagation is achieved effectively. The effects of electromagnetic fields produced by current pulses on the kinetics of crack growth were examined in the work of Golovin et al. [16]. The authors found out that when the current pulses were passed at appropriate time, the crack growth was prevented. The papers [17, 18] reported that the temperature field and the stress field of the specimens containing two- and three-dimensional cracks were derived by the methods of integral transformation and complex variable functions. The pulsed current discharge experiments and mechanical tests on embedded crack and welded joints were reported in the work of Fu et al. [19, 20]. In the papers [21, 22], the phase change zone and microstructure near the crack tip were analyzed after performing discharge experiments. It was found that the grain size was refined, and toughness, hardness, and corrosion resistance were increased. These increases were good enough for retarding crack growth and improving the resistance to stress corrosion. A plate containing a crack, under high electric current, was analyzed based on thermo-electro-structure coupled theory. The temperature-dependent characteristic parameters [23] and phase change [24] were also considered in the work. According to Zhou et al. [25], a small crack can be healed under electropulsing without getting it melted. A huge thermal compressive stress is caused by high-rate heating as the effective atoms fill the crack. According to Cai and Yuan [26, 27], the stress generated by the electromagnetic effect and heat effect that in turn were caused by the electric current were calculated in a numerical analysis procedure. The result showed that the thermal stress was beneficial for controlling the damage propagation. Following the work of Qin and Su [28], an analytical expression for modeling healing driving force as a function of passing electric density, crack geometric shape, and sample size was derived from a thermodynamic point of view. The presented theoretical framework was in accordance with the earlier experimental results, and the critical current value for crack healing was also provided. As compared to the currently used methods, the obvious advantages of arresting crack through the heating effect of electromagnetic field are as follows: 1.

Local electric current and heat concentration at the crack tip are used to arrest the crack propagation. The nondefected portion of the remanufacturing blank remains unchanged. Moreover, it is not necessary to detect the location, size, or shape of the crack. This technology makes use of a simple operation, i.e., the curvature radius of the crack tip is increased causing elimination of stress concentration around the crack tip, thus retarding the crack growth.

2.

From the mechanics perspective, thermal compressive stress and phase transition stress are caused in the process of rapid heating and cooling. These phenomena are seen as a benefit to arrest the crack. The important factors affecting crack arrest depend on the following two aspects: (a) microstructure of the fusion zone and heat-affected zone (HAZ) and (b) compressive stresses in the vicinity of the crack tip.

The material FV520B, used in the manufacture of compressor rotor blade, is put under investigation in this paper. Based on thermo-electro-structure coupled field theory, the current density, temperature field, and stress field around the crack tip are calculated with the help of a finite element analysis software. The isotherm around the crack tip is used to estimate the size of fusion zone. The crack arrest experiments are performed on a high pulsed current discharge device of type HCPD-I. The macro/microstructures, before and after discharge, are observed. The sizes of the fusion zone and HAZ under various discharge energies and the crack lengths are quantified. Afterwards, comparisons between experimental and simulated results are presented.

3 Numerical simulation 3.1 Problem statement The material investigated is high-strength martensitic stainless steel FV520B. In this paper, the main purpose is to study on the effect of heat concentration and detour at the crack tip. The heat is produced by electromagnetic field, which, in turn, is produced by pulsed current. In this regard, the slit can be substituted for a crack. The dimension of the specimen is shown in Fig. 2. A unilateral slit is prepared by wire-electrode cutting in the center of the longer side that penetrates throughout the thickness. The crack length is a, and the root radius is 0.1 mm The electric energy is applied on both the shorter sides. Poisson’s ratio of the

Fig. 2 Dimension of specimen in millimeter


material is 0.295, and density is 7,780 kg m3 , while the temperature-dependent properties are presented in Table 1. 3.2 Theories of analysis – Based on thermo-electro-structure coupled method, the temperature and stress fields in the vicinity of the crack tip are calculated when pulsed current is passed through the specimen. In the thermal analysis, nonlinear transient temperature field is worked out. In the stress analysis, the stress field is obtained. –

Electric current analysis [29]: E = −∇φ, J =

1 E, ∇J = 0 ρ

where U is displacement, T is temperature, V is electric potential, F is force, Q is heat flow rate, I is electric current, M is structural mass, C is structural damping, Ctu is thermoelastic damping, Ct is specific heat, K is structural stiffness, Kut is thermoelastic stiffness, Kt is thermal conductivity, and Kv is electric conductivity.

(1)

–

where E is electric field, J is electric current density, φ is electric potential, and ρ is resistivity. Thermal analysis [30, 31]:

–

∂T , q˙ = ρ|J|2 (2) ∂t where q¨ is heat flux, k is thermal conductivity, T is temperature, q˙ is heat generation of Joule heat, β is mass density, Cp is specific heat, and t is time. Thermoelastic analysis [32]:

α is linear expansivity, ν is Poisson’s ratio, I l is stressinvariant, Il = σxx + σyy + σzz , T is temperaturedifferent, T = T − T0 ,T0 is reference temperature, and δij is the Kronecker delta. Coupled field analysis [32]: ⎡ ⎤⎡ ⎤ ⎡ ⎤⎡ ⎤ ¨ ˙ U U M00 C 0 0 ¨ ⎦ + ⎣ Ctu Ct 0 ⎦ ⎣ T ˙⎦ ⎣ 0 0 0⎦⎣ T ¨ ˙ 0 00 0 0 0 V V ⎡ ⎤ ⎡ ⎤ ⎡ ⎤ K Kut 0 U F + ⎣ 0 Kt 0 ⎦ ⎣ T ⎦ = ⎣ Q ⎦ (6) V I 0 0 Kv

q¨ = −k∇T , k∇T + q˙ = βCp

σj i,j + Xi = β u¨i 1 εij = (ui,j + uj,i ) 2 1 εij = (1 + ν)σij − (νIl − Eα T )δij , E i, j = x, y, z

(3) (4)

(5)

where σij is stress, εij is strain, Xi is body force, ui is displacement, u¨ i is acceleration, E is Young’s modulus,

3.3 Numerical simulation results and discussions The specimen of dimension a = 20 mm, used in this section, is plotted in Fig. 2. Five kilojoules of electric energy is applied on both the shorter sides. The distribution of the electric current density, temperature field, and stress field at the crack tip are calculated. Figure 3 shows that due to the existence of the crack, the current cannot pass through the crack surfaces, and thus, the electric current detours and concentrates around the crack tip. This process results in the generation of large amount of Joule heat, and consequentially, the temperature becomes higher than the fusion point of the material. Figure 4 shows the details. From the numerical simulation, the average diameter of the circle-like

Table 1 Temperature-dependent material properties Temperature (◦ C)

Young’s modules (G Pa)

Coefficient of thermal expansion (1/◦ C)

Thermal conductivity (W/m·◦ C)

Specific heat (J/kg·◦ C)

Resistivity ( ·m)

20 100 200 300 400 500 600 700 800 900 1,000 1,200

191 185 181 168 127 54 7.8 6.9 – – – –

10.8 × 10−6 10.53 × 10−6 10.16 × 10−6 11.36 × 10−6 12.24 × 10−6 12.96 × 10−6 13.50 × 10−6 14.58 × 10−6 13.05 × 10−6 – – –

44.06 42.51 37.42 33.87 30.97 29.17 28.86 27.64 23.73 20.26 17.25 17.25

460 474 511 563 621 676 758 1005.4 1005.4 1,005.4 1298.7 1298.7

0.1244 × 10−6 0.1648 × 10−6 0.2876 × 10−6 0.3957 × 10−6 0.5034 × 10−6 0.6744 × 10−6 0.8178 × 10−6 1.1009 × 10−6 1.1175 × 10−6 1.1820 × 10−6 1.2540 × 10−6 1.3760 × 10−6

Int J Adv Manuf Technol Fig. 3 Electric current density vectors in ampere per square meter: a global view and b local view

fusion zone is about 0.724 mm. Figure 5 shows that at the energizing moment, the annular compressive stress field is formed in the vicinity of the crack tip, which proves good for stopping the crack growth. The heat effect of electromagnetic field to arrest the crack propagation affects only the area of the crack tip, so the other parts of the remanufacturing blank remain unaffected. Therefore, this method is especially suitable for large-sized remanufacturing blanks.

4 Experimental analysis The crack arrest experiments were performed under ambient conditions on a self-made high pulsed current discharge device of type HCPD-I, as shown in Fig. 6a. It is composed of an electric governor, a high-tension transformer, a rectifying silicon stack, a current limiting resistor, a high-tension switch, capacitors, and load. The basic parameters are as follows: 1.

There are ten capacitors (MWF-10), and the energy storage capacity of each is 1 kJ;

2. 3. 4.

The charging voltage is 4–10 kV; The peak current of discharge is 250 kA; The discharge periodic time is 40–100 μs.

The pulsed current was applied onto the specimen through two copper electrodes, as shown in Fig. 6b. In the discharge process, local fusion and eruption occurred at the crack tip because of Joule heating accompanied by sparks and intense sound. As a result, a small hole was formed, and the crack tip became blunt. With increase of energy input and crack length, the current and heat concentration ahead of the crack tip is larger, and the melting process got more intense. The geometry of the specimens used is shown in Fig. 2. The samples are divided into two groups: the first one was not subjected to discharge process and was employed as reference, while the second one was treated under discharge energies of 3, 4, and 5 kJ. Optical microscopy was employed to measure diameters of the fusion zone and the HAZ. Afterwards, the specimens were made into metallographic samples to examine the morphology and microstructure around the crack tip after discharge by an optical microscope as well as a scanning electron microscope.


Fig. 4 Temperature contour in degrees Celsius: a global view and b local view

5 Results and discussion 5.1 Morphology and microstructure around the crack tip Figure 7 shows the morphology and microstructure in the vicinity of the crack tip after discharge. As shown in Fig. 7a, after performing the crack arrest, the shape of the crack tip becomes obtuse and smooth. There are no serrated convexes or concaves; neither are there any secondary cracks. As the current cannot flow through the crack, the current has to

Fig. 5 Von mises stress contour in Pascal

detour around the crack tip, which causes the current density to increase excessively because of Joule heating. This results in local melting at the crack tip. The partition around the crack tip is distinct. As shown in Fig. 7b, the microstructure of area A is very fine. It is actually the edge of the molten hole covered by fusion metal (white-bright layer). Figure 7c shows area B located outside the white-bright layer. Color change is visible, which means that the local temperature is higher in this region. It does not reach the fusion point but does reach the phase change temperature. Area B is actually the HAZ. The rapid heating increases the nucleation rate. After discharge, the growth of nucleated grains is restrained by the surrounding cool matrix. This, consequentially, results in the formation of fine cryptocrystalline martensite, decentralization of the deformation, and reduction in stress concentration. It also suppresses the grain-boundary cracks and crack propagation so as to prolong the fatigue life [33]. The original microstructure in area C remains unchanged during discharge as shown in Fig. 7d. The reason is that the effect of Joule heat and detour is weak over there, and thus, the temperature of matrix stays low. It is the characteristic that the nondefected part of component remains unchanged. The boundaries around areas A, B, and C are clearly detectable, and they show a huge temperature gradient around the crack tip that would cause diversion of the crack propagation. This is also good for improving the fatigue life [34]. At the instant of pulsed current passing through the specimen, the temperature around the crack tip increases sharply, and it initiates melting. On the other hand, the matrix remains almost at the room temperature. The expansion of the fusion zone and the HAZ with high temperature is restricted by the surrounding matrix which is at low temperature. Therefore, a huge compressive stress, which is a superposition of phase transition stress and thermal stress, is formed around the crack tip. This is also beneficial for suppressing generation of new micro-cracks and preventing crack growth. 5.2 The relationship between the discharge energy and the changes occurring at the crack tip The input energies 3, 4, and 5 kJ are, respectively, fed to the three specimens. The three specimens are treated by the respective discharge energy, and the diameters of the molten holes and HAZ are measured. The attained data are worked upon to obtain the average value. The relationship between discharge energy and sizes of molten zone and HAZ is explained in this section. Table 2 shows the morphology and the microstructure at the crack tip observed under a metallurgical microscope and measurements of the diameters of the fusion zone and the HAZ produced by current burning. When discharge energy is 3 kJ, no melting occurs at the macroscopic level, and only a snuff colored HAZ circle

Int J Adv Manuf Technol Fig. 6 The discharge device and process: a discharge device and b discharge process

is observed around the crack tip. However, under the metallurgical microscope, there is a little melting around the crack tip, and the average diameter of the crack tip expands from 0.2 to 0.344 mm. When the discharge energy is 4 or 5kJ, the obtuse crack tip and local fusion phenomenon

Fig. 7 The microstructure around the crack tip after discharge. a Morphology of the crack tip after discharge. b Microstructure of the white-bright layer. c Microstructure of the HAZ. d Microstructure of the matrix

can be observed directly. A circular hole is formed at the crack tip. The fusion metal is rapidly solidified, and a whitebright layer, covering the crack tip, is formed. The snuff colored HAZ surrounds the layer from the outside. The average diameters of the holes are, respectively, 0.537 and

Int J Adv Manuf Technol Table 2 Average diameter of fusion zone, HAZ, morphology, and microstructure around the crack tip

0.702 mm for the discharge energies of 4 and 5 kJ. With increase in discharge energy, the melting process becomes more severe, and diameters of the molten hole and HAZ become larger. The local fusion and eruption show existence of compressive stresses around the crack tip. It is beneficial for retarding the crack propagation. It can, thus, be safely stated that the curvature radius of the crack tip can be controlled by adjustments in the discharge energy. Furthermore, the stress concentration factor can be reduced, and crack growth can be halted. The relational curves between discharge energy and diameters of the fusion zone and the HAZ are obtained by experiments and simulation as shown in Fig. 8. Larger sizes of fusion zone and HAZ are obtained when the discharge energy is larger; thus, they are in positive correlation. The average difference of diameters of fusion zone between measured and calculated results is less than 7 %. 5.3 Crack length versus sizes of fusion zone and HAZ The specimen geometry in this section is the same as that described in Section 3.1, and the crack lengths are

Fig. 8 Sizes of fusion zone and HAZ versus discharge energy

Int J Adv Manuf Technol Table 3 Diameter of fusion zone and HAZ versus current path

3.

4.

sizes of fusion zone and HAZ are proportional to the load energy and inversely proportional to the current path. In practice, the optimal size of fusion zone is obtained by controlling the discharge energy and current path. The agreement between experimental and simulation results is very well. The difference of fusion zone between numerical and experimental results is less than 7 %. The results of simulation can provide reference for the experimental parameter selection. There is no research on the application of heat effect of electromagnetic field to arrest crack propagation on remanufacturing as yet. The research relates basic theory and process equipment, which needs to be further strengthened.

Acknowledgments The authors would like to acknowledge the Ministry of Science and Technology (MOST) of China and the National Basic Research Program (973) on Mechanical Equipment Remanufacturing (grant no. 2011CB013402).

a = 20 mm and a = 25 mm, respectively, for the two specimens. Five kilojoules of energy is fed to the shorter sides. The relationship between the diameter of fusion zone and HAZ versus current path is shown in Table 3. When the load energy and the specimen geometry are same, the crack length is larger, and the sizes of the fusion zone and the HAZ are larger. Due to existence of a crack, the current cannot pass through the crack; hence, for the same specimen size but with different crack lengths, different current paths are generated. In fact, the melting condition depends on the current path. A smaller current path causes higher current and heat concentration, and thus, the melting is more severe.

6 Conclusion This paper presents simulation and experimental analyses for electromagnetic heat effect on crack arrest in remanufacturing. The factors affecting the sizes of fusion zone and HAZ are the discharge energy and current path. Some conclusions are summarized as follows: 1.

2.

Application of electromagnetic heat effect on arrest crack propagation is a kind of high efficiency and simple repair method for a remanufacturing blank. In the repair process, it affects only the area of crack tip, so the other parts of the blank remain unaffected. Therefore, this method is especially suitable for large-sized remanufacturing blanks. The experimental results suggest that after discharge, the crack tip gets blunt, the singularity of crack tip is removed, and the crack initiation is barricaded. The

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