QUALITY ANALYSIS OF THE ULTRASONIC WELDING PROCESS OF AUTOMOTIVE ELECTRIC WIRES

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Nonconventional Technologies Review Roania, June, Roanian Association of Nonconventional Technologies QUALITY ANALYSIS OF THE ULTRASONIC WELDING PROCESS OF AUTOMOTIVE ELECTRIC WIRES Lăcriioara
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Nonconventional Technologies Review Roania, June, Roanian Association of Nonconventional Technologies QUALITY ANALYSIS OF THE ULTRASONIC WELDING PROCESS OF AUTOMOTIVE ELECTRIC WIRES Lăcriioara Nică 1, Dinu Gubencu 1 Politehnica University of Tiișoara, Politehnica University of Tiișoara, ABSTRACT: Technological processes involved in autootive industry ust provide products conforing to the specifications, in order to avoid failure of safety-related functions. This iplies eliinating or diinishing errors and therefore achieving, for all processes, the target of zero defects. As a consequence, it is very iportant to identify and apply different cost-effective quality anageent ethods to attain this obective. Thus, the paper presents the results obtained using the ran correlation ethod for hierarchy the causes that influence the quality of the autootive electric wires produced by ultrasonic welding. KEY WORDS: ultrasonic welding process, ran correlation ethod, quality anageent, root cause 1. INTRODUCTION In autootive industry, for achieving an electrically functional overall syste, the individual coponents such as power generator, battery, consuer and electrical and electronic display, control and regulatory systes ust be electrically connected to one another. These are created by the cable harness. In order to e.g. distribute power in the cable harness, several individual wires ust be connected to one another. This connection can be created, aong other ways, through ultrasonic welded nots. 48 The quality target for this process is zero defects because soe of the ost iportant quality characteristics are strictly related with wire welding and any errors that ay occur could generate fatal consequences. Moreover, these faults (Table 1) ay reain undetected depending on their type and could be passed on to the end custoer [4]. So, it is iportant to appeal to in-process approaches that help to ove the quality concept upstrea fro final product inspection [1], based on the experience of experts, without spending tie and oney. Table 1. Possible faults and consequences during ultrasonic welding of electric wires Characteristic Fault Type Causes Possible Consequences No connection at ties Welded oint Too wea, strands inserted Iportant functions not Welded not tears incorrectly available, vehicle not roadworthy Welded oint Insulation of welded nots Too wea, strands dirty before welding Single strands pierce the insulation During ultrasonic welding, the wire strands fro several wires are connected to each other, producing two types of welded nots: end or parallel splices. The wires are stripped iediately prior to the welding process and positioned anually in the welding ig, where: the wire strands are pressed and copacted onto one another by applying pressure after the oxidation coating is reoved fro the wire strands by applying friction, the adacent strands of the single wires rub against one another, aing possible the welding of the contact surfaces. On ultrasonic welding achines, an electrical vibration is converted into a echanical vibration Welded not tears or electrical conductance too low Short circuit No connection at ties Iportant functions not available, vehicle not roadworthy Sudden failure of iportant functions, ris of fire with a frequency of 0-40 Hz. The frequency of 0 Hz perits the best possible use of energy. The values of 5 or 40 Hz are suitable for welding processes which require saller aount of energy. Ultrasonic systes are generally coposed of the following aor eleents: a generator, converter with piezoelectric ceraics, booster, sonotrode, pneuatic press and holding fixture, as seen in Figure 1 [4], [5]. During ultrasonic welding, a coplex process is triggered, involving static forces, oscillating shearing forces and a oderate teperature increase in the welding area. The agnitude of these factors depends on the thicness of the worpieces, their surface structure, and their echanical properties. Piezoelectric aterials used, located in the converter, change their geoetry through application of an electrical field, which is used to convert the electrical vibration into echanical vibration. Then, this vibration is stabilized by the booster. Finally, the sonotrode achieves the desired values of the oveent aplitude and frequency.. ANALYSIS OF THE CAUSES THAT INFLUENCE THE QUALITY OF THE WIRES ULTRASONIC WELDING PROCESS The causes and the corresponding faults types that influence the quality of the ultrasonic welded oint are the following: X 1 : Confority of electrical wires The operators ade a istae when selecting the wires, regarding their colour, diensions and/or nuber; Wires to be welded are deteriorated (dirty, bended); Partial stripping caused by a too long storage period; Partial insulation was reoved too soon, not ust before delivering the wires to the ultrasonic welding achine; Incoplete or bad reoving of the insulation fro the part to be welded (Figure); a. unclean insulation cut Figure 1. Coponents of an ultrasonic welding achine Materials used for both etallic conductor and the insulation sleeve lying over it. X : Welding paraeters Wires diaeters; Total diaeter of wires per welded oint; Set tie for welding; Copression height; Joint height; Power/energy consued during the process. X. Weld insulation ode Welds ust be insulated before being placed in the wiring for preventing loss or interruption of the signal. In order to attain this purpose the following insulation aterials are used: PVC and cloth tape; Insulation heads ade of plastic in various sizes and colours, having on the inside several striations. X 4 : Wires insertion in the welding tool In order to achieve the required quality of a welded not, it is essential that the conductor strands ust be bunched according to specifications and positioned in the welding tool. In doing this, the operators ust tae into account the: Position of the wire strands to each other because of the required not geoetry and the not strength (Figures and 4) b. untidy cut c. single strands cut-off short Figure. Bad wire end Figure. Correct position of end splice 49 Figure 4. Correct position of parallel splice Position of the wire strands to the welding tool because of the not strength The arrangeent of the wire strands on the sonotrode whilst taing into account the wire and strand cross sections due to the energy transfer to the welded not and the not strength X5: Type of welded nots End splices (Figure 5) Parallel splices (Figure 6) b) Visual, attributive characteristics operator self inspection of each weld: Knot geoetry and version (e.g. no protruding single strands); Knot discolouration and structure of the surface; Condition of wire insulation; Strain relief of not. Also, Quality Departent perfors other inspections of the variable characteristics by taing rando saples (Figure 8): Pull-off strength for parallel splices; The peel strength on end splices; Knot diensional characteristics (height and width) Figure 5. End splice Figure 6. Parallel splice X 6 : Joints inspections The operator fulfils two categories of inspections: a) On-process 100% easureent of variable characteristics through ultrasonic welding achine (Figure 7): Copressed size and weld size of the not; Welding tie and energy applied during the welding process; Figure 7. Integrated inspection equipent Figure 8. Pull-off strengths gauge X 7 : Incorrect aintenance of the equipent X 8 : Teaching of the equipent For welding achine onitor to show a difference between bad and good welds, welding reference ust be saved in the syste for each weld. This process is nown as Teaching and is updated by the aintenance departent. Only the best nots ust be saved as reference.. STEPS FOR APPLYING RANK CORRELATION METHOD It was appealed to a nuber of = 15 specialists, woring in autootive branch, to ran the = 8 causes identified that influence the quality of ultrasonic welded nots. Each expert had to coplete a survey for, where all the potential causes of process error. The priary table, containing a i rans assigned by each expert i to causes analyzed, is presented in Table. The steps for statistical data processing are []: a) The priary results of the investigation were processed by suing the rans assigned to each cause (colun): A ai 1,..., i 1 (1) Based on this calculus, the overall rans are assigned θ (1) in the priary table (Table ). 50 Table. Priary inquiry table Causes Expert i X 1 X X X 4 X 5 X 6 X 7 X A i (1) θ (1) b) Because there were specialists who have given identical rans to causes, the priary table data were corrected by coupling the rans. This has resulted to obtaining a secondary table. Table. Secondary table c) The secondary table (Table ) data were processed by applying relation (1) and assigning global rans θ (). Expert i Causes Ti X 1 X X X 4 X 5 X 6 X 7 X , A i (1) θ () Δ ² d) The adequacy of the data fro the priary table to the secondary table was checed by calculating the correlation coefficient: 6 rs 1 ( 1) ( ) 1 () As the correlation coefficient has the value: r 0,98 1, s the secondary table data can be used for the next steps of the procedure. e) Consistency between the views expressed by experts has to be ensure by calculating the coefficient of consensus: w 1 where: 1 i1 T i, () T i i1 a i 1 1 i1 t t i 1,..., a i (4) 1 (5) t representing the nuber of identical rans assigned by the i expert. Since 7, the criterion suited that ust be used for verifying is: calc 1 w (6) Seeing that: χ² calc =46.06 χ² tab;α;-1 = χ² tab;0.05;7 = (7) it can be concluded that there is a significant correlation between the views of professionals. The ordering results are represented, choosing the axis values 1000/A i (Figure 9). 4. CONCLUSIONS Following the hierarchy of causes, based on statistical processing of the views of 15 experts involved in autootive field, the root cause that influence the occurrence of nonconforing welds, according to the specifications prescribed in the docuentation was identified. The research revealed that the ost possible reason for obtaining defects after welding process is X 1 the confority of electric wires. So, both operators and quality engineers have to focus for finding corrective easures related to this cause. Even if they were also raned, order obtained for other causes has less relevance, because the calculated values are relatively close. In other words, the results regarding these causes should be analyzed with caution, the ethod applied being subective. For a higher confidence of the results, ethod can be resued, with the weighting of expert responses, Figure 9. Coponents of an ultrasonic welding achine depending on their copetence and interest in research. 5. REFERENCES 1. Borror, C.M., The Certified Quality Engineer Handboo, Third Edition, ASQ Quality Press, Milwauee, Wisconsin, (004);. Mnerie, A., Nanu, A., Mnerie, D., Slavici, T., (009), SOME Aspects of autoated electrical erosion achine with wire electrode, ISSN , Volue XIII, nr.1/009. Nichici, Al., Cicală, E., Mee, R., Prelucrarea datelor experientale, UPT, (1996). 4. accesed on accesed on
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