System and Method for Controlled Vibration Stress Relief of Metal ...

System and Method for Controlled Vibration Stress Relief of Metal Parts with Residual Internal Stresses Claudiu NICOLA, Adrian VINTILĂ, Marcel NICOLA, Viorica VOICU, Maria Cristina NIŢU, Marian DUȚĂ Research, Development Division for Electric Equipment and Energy Efficiency National Institute for Research, Development and Testing in Electrical Engineering-ICMET Craiova, România [email protected], [email protected], [email protected] [email protected], [email protected], [email protected] Abstract— The paper presents a new method and a new equipment for the dimensional stabilization of metal parts using controlled mechanical vibration technique. The dimensional stabilization through vibrations consists in using controlled vibrations to reduce the residual stress in metal parts or structures. The mechanical characteristics of metal parts have two basic components: the functional mechanical strength and dimensional stability in time. These two components are interrelated to each other. The method is based on a new algorithm for adaptive vibration, depending on the behavior of the metal parts. The equipment for the stress relief through vibrations is a complex process computer system including a process computer, a frequency converter, an eccentric vibrator, a flexible shaft, an induction motor and an acceleration transducer. The system reduces the level of residual internal stresses in a certain metal part, following its own stress relief algorithm implemented in LabVIEW environment. Keywords—stress-relief, mechanical vibration, application, LabVIEW, dimensional stability.

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I. INTRODUCTION The dimensional stability is conditioned by the unstable residual stress. These unstable residual stresses lead to cracking and dimensional changes in metal parts [2]-[5]. The problem consists in eliminating as much as possible of the unstable internal stress, without affecting the stable stress condition. The reduction in the residual stress is achieved by creating variable stress in the metal part. The part undergoes vibration at resonant frequency for a given period depending on certain mechanical characteristics of the part. Vibrating activates latent internal energy caused by internal stress, thus accelerating the rearrangement process of atomic planes (the systematic displacement) and the internal energy dissipation in the material. An accelerated or artificial ageing of the material can be said to occur [10], [11], [13]. Dimensional stabilization through vibration consists in using controlled vibration to reduce the residual stress in metal parts or structures [12]. This method is not a complete replacement of the method of stress relief by thermal cycle, but it is an efficient alternative to dimensional stabilization and stress relief of parts in any production stage, with no structural changes and with no influence on mechanical properties and fatigue strength in the material. The method consists in applying low frequency mechanical vibration, with controlled amplitude, in the part undergoing treatment.

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Application of vibration leads to a change in the positions of unstable atoms moving within very short distances in order to stabilize each crystal and the whole structure at the same time. The vibration stress relief results in achieving a more stable structure with low microscopic residual stress [14], [15], [16]. This treatment can be applied both to ferrous and non-ferrous parts; it can also be applied during the welding process, in order to avoid cracking. Practically this method does not require any limits regarding the weight of the part undergoing treatment. There are currently several types of vibration stress relief pieces of equipment [17], [18]. All these pieces of equipment are based on the material intended for stress relieving undergoing controlled mechanical vibration in terms of frequency and amplitude. The system carries out verification of the degree of vibratory stress relief in metal parts, and this process is achieved by primarily checking resonance peaks stability within the frequency range of the parts, both as amplitude and displacement [19], [20]. In other words, through vibration, the resonance peaks within the frequency range of the parts are moving and changing in amplitude. When these changes stop, we can say that the part is dimensionally stabilized. II. DESCRIPTION OF THE SYSTEM The system presented in this paper in Figure 1 is a monitoring and control system for the resonance peaks within the frequency range of the metallic parts, carried out by process computer. The vibratory stress relief process is computer achieved using the software application developed with LabVIEW programming [1] based on the algorithm shown in Figure 2. The hardware architecture of the stress relief system is shown in Figure 3. DAQ (Data Acquisition) is simply the process of bringing a real-world signal, such as voltage, into the computer, for processing, analysis, storage or other data control. A physical phenomenon represents the real world signal that we measure. In order to optimize the characteristics of a system in terms of performance, handling capacity and cost, the relevant subsystems can be combined together. Analog data is generally acquired and transformed into the digital form for the purpose of processing, transmission and display.

Rapid advances in Personal Computer (PC) hardware and software technologies have resulted in easy and efficient adoption of PCs in various precise measurement and complex control applications. A PC based measurement or control application requires conversion of real world analog signal into digital format and transfer of digitized data into the PC. A data acquisition system that performs conversion of analog signal to digital data and the digital data to analog signal is interfaced to a PC to implement the functions of measurement and control instrumentation applications [4], [5].

Fig. 3. The hardware architecture of the stress relief system.

The hardware components of the stress relief system are: • Process computer; • Control and monitoring module: DAQ NI USB-6009, signal conditioning system Model CA2 Honeywell, frequency inverter Model D700-SC Mitsubishi; • Acceleration transducer Model MAQ14 Honeywell; • Three-phase asynchronous motor; • Eccentric vibrator; • Rubber buffers. DAQ NI USB-6009 specifications: − −

Fig. 1. System for controlled vibration stress relief.

8 analog inputs (14-bit, 48 kS/s); 2 static analog outputs (12-bit); 12 digital I/O; 32-bit counter; − Bus-powered for high mobility; built-in signal connectivity − OEM version available; − Compatible with LabVIEW, LabWindows™/CVI, and Measurement Studio for Visual Studio .NET. Charge-mode piezoelectric transducers require charge amplifiers to convert their output to useful levels. Honeywell in-line charge amplifiers are versatile and convenient solutions for the use of charge-mode piezoelectric transducers. Charge Amplifier Honeywell Model CA2 specifications: − − −

Fig. 2. The algorithm underlying the developed software application.

− − − −

Input voltage: ±15 V dc or 24 V dc to 32 V dc; Input current: 20 mA; Sensitivity: Programmable (0.05 mV/pc to 6.4 mV/pc); Input range: 780 pc to 100000 pc; Output: ±5 V RMS maximum; Frequency response: 3 Hz to 30 kHz (-3 db); Time constant, Model CA2: 50 ms.

The MAQ14 is a self-generating piezoelectric transducer which has no internal electronics and requires no external power for operation. Accelerometer Model MAQ14 specifications: − − − − − − −

Dynamic range: 1000 G; Sensitivity: 50 pC/G; Transverse sensitivity: Less than 5 %; Temperature sensitivity: 0.145/°F; Frequency range: 1 Hz to 10 kHz; Amplitude linearity: Better than 1 % linearity Mounted base resonance: 18 kHz.

The three phase asynchronous electric motors are simple, reliable and have a low cost, for this reason the choice for these motors for industrial application will be highly prevalent. Since the power supply frequency is usually constant at 50Hz, the motor speed is constant and can be modified for various applications by changing the structure of the winding. After choosing the speed, the motor always operates at constant speed, for example approximately 3000 rpm or 1500 rpm. The solution for this problem consists in using a frequency converter, which is a device converting the constant voltage and the power supply frequency into variable voltage with variable frequency. This converter is installed between the power supply and the motor [6], and allows the continuous adjustment of the speed, turning a standard single winding motor into a flexible actuator mechanism, with adjustable speed. The speed of the connected motor can be modified continuously by changing the converter frequency and output voltage.

software also prevents a programmer from having to carry out register-level programming or complicated commands to access the hardware functions. The application software is based on state machines. A state machine is programming architecture that can be used to implement any algorithm that can be explicitly described by a state diagram or flowchart. It introduces a way to create a program such that it responds to user events (such as key strokes and mouse events) or in-state calculation (such as comparing system variables). This architecture provides a distinct advantage to using a sequential architecture that uses data dependency to force execution in exactly the same way every time. Our application software generates a report which is an essential part of a professional application. Reports must contain all the information that the user wants. In addition, reports must look professional and be carefully formatted. The LabVIEW Report Generation Toolkit for Microsoft Office is a library of flexible, easy-to-use VIs for programmatically creating and editing Microsoft Word and Excel reports from LabVIEW. The application interface software is developed in a menubased system for easy and efficient management of vibratory stress relief phases and is presented in Figures 4-7.

The eccentric vibrator is produced by ICMET Craiova and has a welded, robust frame, with a fastening lug, providing increased operating safety. The fastening of the vibrator on the part undergoing treatment is achieved using bulky catches. The rubber buffers isolate the part mechanically, allowing its vibration at maximum amplitude. III. DESCRIPTION OF THE SOFTWARE APPLICATION

Fig. 4. The interface for scanning part.

The application’s development software is LabVIEW, which is a powerful programming graphical environment, highly used for signal acquisition, measurement analysis and data presentation, providing flexibility to traditional programming languages and at the same time a user-friendly interface. LabVIEW works on a data flow model in which information within a LabVIEW program, called a virtual instrument (VI), flows from data sources to data sinks connected by wires [8]. The software transforms the PC and the data acquisition hardware into a complete data acquisition, analysis, and presentation tool. Without the software to control or drive the hardware, the data acquisition device does not work properly. Driver software is the layer of software for easy communication with the hardware. It forms the middle layer between the application software and the hardware. The driver

Fig. 5. The interface for stress-relief part.

• It applies to a wide range of internal voltage parts. REFERENCES [1] [2] [3] [4] [5] [6] [7] Fig. 6. The interface for the acceleration signal analysis before the stressrelief of the part.

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[12] [13]

Fig. 7. The interface for the report generation with comparative acceleration waveform graph.

IV. CONCLUSIONS The controlled vibration stress-relief equipment is portable, and can be used for parts in any size, shape or weight achieved through welding or casting. Small parts can be treated by fastening to a vibration plate. This method is ideal for the applications requiring critical size parts, which would require very large ovens for thermal stress-relief. The advantages of the equipment are:

[14]

[15]

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[17]

[18]

• It reduces the distortions during and after mechanical processing and applies to a wide range of metals; • It lowers the risk of cracking during welding;

[19]

• It does not affect the mechanical properties; • Several parts can be treated at a time; • The treatment can be applied in the point where the mechanical processing is carried out; • It consumes a fraction of the energy required by the thermal stress-relief process;

[20]

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