Foundation item: Project(2012ZX04010-081) supported by the National Science and Technology Major Project of the Ministry of Science and Technology of China

Received date: 2014−09−01; Accepted date: 2014−12−30

Corresponding author: ZHOU Jie, Professor, PhD; Tel: +86−15215014161; E-mail:cqzf0223@163。com

optimize the related parameters。 Centering on the stamping process of head, lots of researches have been done。 The reasons were presented in Refs。 [8−12] that the defects, such as fracture and wrinkle, occur。 ZHANG and WANG [13] took wall thinning into consideration to obtain optimal parameters。 The optimization of forming parameters was got for sheet metal forming  process based on multi-objective algorithm such as surrogate and artificial bee colony algorithm, response surface method, Pareto-based genetic algorithm [14−16]。 However, the defect prediction model and multi-object optimization of head forming have never been raised。

In this work, in order to reduce the defects, such as fracture, wrinkle and serious thinning, of stamping forming of the head and keep multi-objective balance, we established the forming defects function with the parameters of head forming process, based on experimental design and collaborative optimization response surface method。

2Analysis of deep drawing process of electric-head

2。1Defects of head stamping forming

The drawing process of head is similar to stamping drawing process of ordinary sheet metal。 Under the effect of punch force (FP) and blank holder force (Fy), there are four important deformation regions in the forming process of electro-thermal head, such as flange region A, die fillet region B, hanging region C and ellipsoidal region D, as shown in Fig。 1。 Meanwhile, the deformation of stress and strain of each region has also been shown in Fig。 1。 It has been shown that the absolute value of tangential strain ε2 of flange region A and hanging region C is the maximum, which easily leads to wrinkling, such as Fig。 2(a)。 There is an excessive deformation in the die fillet region B because blank in this region bears a large radial tensile stress σ1, which easily lead tos fracture if the thickness is slight, as shown

Fig。 1 Diagrammatic sketch of head

Fig。 2 Defects of electric heads: (a) Wrinkle; (b) Fracture

in Fig。 2(b)。 Since the material of ellipsoidal  zone D bears large radial tensile stress σ1 and tangential stress σ2, forming a bulging deformation zone of head, the material of this region becomes thinner [3]。 Because the  stress and strain are complex in stamping forming of the head, it leads to several difficulties in the forming process of the head。 One is to ensure no fracture in the die fillet region; one is to avoid drum kits in the hanging region, and another is to avoid serious thickness。

2。2Determination of process parameters

Through the analysis of defects of drawing forming, it is predictable to optimize and control the forming defect, using multi-objective optimization based on certain material properties and the shape of parts。 Certainly, in order to minimize the defects of stamping forming, we should select the right parameters which need to be optimized and create a proxy model to seek the optimal process parameters。 In this work, we select four optimization variables, namely。 the fillet radius, the position of draw-bead (draw-bead center line diameter), the blank size and blank holder force to diminish objective value of fracture, wrinkle and thickness, so  as to improve the forming quality of head。

If the matrix radius is small, it will be difficult for the material of flange region to flow into the die。 Hence, fracture occurs easily。 If the magnitude of the matrix radius is too large, the area that the blank contacts with the die is too small, so, the vertical compressive stress σ3 tends to 0。 Hence, wrinkle occurs easily。 Meanwhile, since there exists serious defect of bulge in the hanging region, it is necessary to control the flow of initial blank material strictly by changing the blank-holding force, the fillet radius, the position of draw-bead and the blank size, which can control the size and distribution of the flow of