Розмір шрифту: 

UDC 621.7.044

Knyazyev M. K., PhD, Associate Professor

 

Samantha Joseph, Student

 

National Aerospace University “Kharkiv Aviation Institute” – KhAI, Kharkiv, Ukraine

EXPERIMENTAL RESEARCH OF ELECTROHYDRAULIC IMPULSE FORMING
OF LARGE SHEET FITTING HALF-PIPES

Electrohydraulic impulse forming (EHF) method proved its high efficiency for manufacture of sheet parts under individual and small-batch production conditions [1]. EHF Laboratory in KHAI University possesses experimental and serial electrohydraulic presses. Fairly often the Laboratory is contracted to produce batches of sheet parts of sophisticated shapes inconvenient for manufacture by traditional “punch-and-die” method, because of big cost of “hard” tools.

Fitting sheet half-pipes are widely used in various branches of industry including aircraft engines manufacture. The half-pipe limit ratio h/d ≤ 0.55 (where d is diameter of half-pipe, is h depth of a formed final shape) is applied for single-step electrohydraulic deep-drawing process. According to the recommendations [2] form and sizes of initial sheet blank for EHF depend on angle and diameter of half-pipe. For 90° turn angle α deep-drawing ratio K = D_max/d = 3.5, where D_max is maximum width of initial flat blank. Thus, for three needed d diameters 200, 250 and 300 mm the calculated parameter D_max is 700, 875 and 1050 mm, respectively.

Electrohydraulic deep-drawing process is greatly influenced by widths of external a and internal b flanges of initial blank. It is recommended to calculate their values from the formula

D_max = a + d + b.                                                              (1)

Here the condition a/b = 0.42 with numerical value determined from the graph [2] for the 90° turn angle is used. The calculation results are: a = 148, 185, 221 mm and b = 352, 440, 528 mm, respectively.

Experimental investigations were planned with aim to check feasibility of recommendations [2]. Tests were carried out in semi-industrial installation UEHF-2 equipped with multi-electrode discharge unit. It is worth to note that space between columns of technological unit for accommodation of tooling equals 645 mm and this value is smaller than the maximum width of initial blank even for the smallest diameter part (700 mm).

Maximum width of initial flat blank D_exp for tests was calculated with respect to the length of generating line of half-torus geometric body D_exp = πd/2 + a_f + b_f, where a_f and b_f are widths of external and internal flanges of the formed final shape, respectively. For three diameters selected for analysis D_exp = 485, 580, 625 mm, respectively, which are smaller than the recommended values calculated above and gives the opportunity to conduct tests in UEHF-2 installation. The configuration and exact dimensions of sheet blanks including a_f and b_f were determined during tests performance. For example, for half-pipe of diameter 200 mm a_f = 50–55 mm and b_f = 105–110 mm. Tests were conducted with plain carbon steel 08kp of sheet thickness 1.2 mm.

Tests results showed that position of initial blank relative to the die cavity greatly influences success of deep-drawing process and final shape quality. Position of sheet blank before the forming is mainly determined by a and b values. Thus, for 200-mm half-pipe a = 110 mm, b = 175 mm and ratio a/b ≈ 0.63, the latter being much larger (1.5 times) than the recommended value. Similar values were obtained for half-pipes of diameters 250 and 300 mm with ratio a/b ≈ 0.5 for both cases. For these dimensions small folders were observed along the half-torus quarter adjoined to internal flange in several formed parts.

Excessive width of internal flange b (with ratios a/b ≤ 0.5) can result in folding along the internal quarter of half-torus and pulling out the external flange from under hold-down plate. Therefore the authors would recommend to use the larger ratio a/b = (0.55–0.60).

Also smaller blank widths D_exp were tested in comparison with the recommended values D_max. Tests results give reason to recommend deep-drawing ratio K = (2.1–2.4) for the turn angle α = 90° with smaller values K for larger diameter d.

The materials [2] contain recommended design for the end elements of half-pipes needed for proper realisation of deep-drawing process. The authors of this paper have proposed another design with shorter length (Fig.) in order to save material of sheet blanks and material for the tooling. Thus, the recommended length [2] is determined by diameter d, and the proposed design length is (0.25–0.3)d.

Results of conducted experimental investigations allows to propose improvements for the recommended method for determination of sizes of initial sheet blank for EHF of half-pipe shapes with turn angle α = 90°. First of all, maximum width of blank can be reduced 1.44–1.68 times with respective decrease of deep-drawing ratio K to (2.1–2.4) to be included into the calculation method.  Here also the ratio of flanges widths a/b = (0.55–0.60) is recommended for better quality. The length of initial blank may reduced by value of (1.4–1.5)d along middle line taking into account the proposed design of the end elements. All these improvements give opportunity for significant reduction of material consumption for producing tooling and sheet blanks, saving the production costs and shortening preproduction period.

Further investigations for larger turn angles α are planned with model tooling of scaled sizes.

 

References

1. Чачин В.Н. Электрогидравлическая обработка машиностроительных материалов. – Минск: Наука и техника, 1978. – 184 с.

2. РТМ-1.4.449–78. Электрогидроимпульсная штамповка листовых деталей сложных форм. – Москва: НИАТ, 1978. – 109 с.

electrohydraulic; sheet; forming; half-pipe