Process optimization normally involves the combination of mathematical and statistical techniques which can be approached by distinct ways. Despite the fact that different methods can be found in the literature, the response surface methodology raised as one of the most effective ways for performing process optimization, by combining design and analysis of experiments, modeling techniques, and optimization methods. However, practical guidelines for response surface methodology and critical analysis of its applications are quite scarce. Thus, this paper aims to present the theoretical principles and practical guidelines for carrying response surface methodology as well as to provide empirical evidence on its critical aspects for manufacturing optimization. In order to accomplish with this objective, 49 papers published in the International Journal of Advanced Manufacturing Technology (IJAMT) from 2014 to 2017 were investigated and reproduced, allowing the analysis of 123 response surfaces. Surprisingly, more than 75.29% of the models have presented a saddle shape. The practical meaning of this finding is that the stationary point is not a suitable solution for the optimization of those surfaces. Besides, multiple response surfaces are more commonly found in the literature than individual ones. From this amount of papers, 71.88% of the works investigated have presented significant correlation with their peers and 87.61% have convexity incompatible with the optimization direction. Most of the optimization solutions have found outside of experimental region which reveals a preponderant neglect of the nonlinear constraints involving the definition of the experimental region. It was also verified that the proportion of functions in saddle format corresponds to 96.86% of the models estimated in flat regions. Moreover, it was found that the number of center points is commonly changed and all the manufacturing processes investigated are driven by at most five control parameters. Finally, considering the theoretical principles, the practical guidelines, and the obtained results, a follow-along example involving the optimization of AISI H13-hardened steel turning with PCBN wiper, previously published by the authors in IJAMT, was revisited by using response surface methodology. The results corroborated the proposed framework suitability.

Response surface methodology for advanced manufacturing technology optimization: theoretical fundamentals, practical guidelines, and survey literature review

\n A foldable smartphone is an exciting new approach for future mobile systems. With some early models being introduced to the market, research and development is being performed to increase the reliability of future systems. Mechanical failure of the display is the major source of foldable smartphone's reliability issues. The display is a laminate structure including an organic light emitting diode (OLED) layer (emission layer and pixel electrode layer), a touch panel, a polarizer, and multiple protective layers in addition to multiple functional coatings and supporting structures. The display is exposed to repeated folding, finger touch, and the impact of external sharp objects. In addition, mechanical behavior changes through the thickness of the display; which adds extra challenges in preventing damage to vital components of the display. This review covers more than 100 U.S. patents about the mechanical failures in foldable smartphones published or filed after 2015. Different inventions reviewed are to design a foldable configuration to prevent mechanical failures resulting from the following four mechanisms: (1) plastic deformation (yielding, creep, and fatigue); (2) fracture; (3) delamination; and (4) buckling (warping, buckling, wrinkling, and creasing).

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A Review on United States Patents to Prevent Mechanical Failures in Foldable Smartphones

Tube hydro-forging − a method to manufacture hollow component with varied cross-section perimeters

An analytical springback model for bending of welded tube considering the weld characteristics

Unequal-thickness billet optimization in transitional region during isothermal local loading forming of Ti-alloy rib-web component using response surface method

A hydro-pressing method was proposed to solve the problems in tube hydroforming process, such as too high pressure, nonuniform thickness distribution, and difficult forming of section corners. The process of hydro-pressing a tube seems like pressing of a solid bar. Theoretical analysis was performed, and the calculation formulas of hydro-pressing process parameters were given. Experimental research was conducted on hydro-pressing process of rectangular cross section with curved sides and bowtie cross-sectional components. The effects of supporting pressure, displacement of the pressing on section shape, and thickness distribution were investigated. The difference was compared between the hydro-pressing forming and the conventional mechanical pressing. It is demonstrated that the supporting pressure and pressing displacement are the essential parameters that influence cross-sectional shape and thickness. The dent defect disappears gradually as the supporting pressure increases and the thickness varies unobvious during hydro-pressing process of a rectangular cross section with curved sides. When the supporting pressure is 15 MPa, the maximum thinning of the rectangular cross section with curved sides is only 1.92 %. For a bowtie cross section, the required pressure is far less than that of conventional hydroforming for the same corner radius. Thickness distribution of section corner zone is more uniform than that formed by conventional hydroforming. Compared with conventional hydroforming, the hydro-pressing is a valid method to form the closed section tubular parts with the same perimeter and different cross-sectional shapes, which remarkably improves thickness distribution and reduces the forming pressure.

Research on hydro-pressing process of closed section tubular parts

A combination method of beneficial wrinkle and preform shape is proposed to solve the problem of non-uniform thickness distribution during hydroforming of a variable diameter tubular component The effect of loading path for the hydroform part and the preform shape are simulated by LS-DYNA At the same time, the wall thickness deviation of the maximum and minimum of the hydroformed part is considered as the optimal target and the loading path for the preform shape is optimized to obtain the beneficial wrinkles by response surface methodology Simultaneously, hydroforming experiments were conducted to verify the simulation results Study shows that the loading path plays an important role in the forming of the beneficial wrinkles, which influence the cracking and the wrinkling defects Only the loading path is appropriate can the beneficial wrinkles be formed and the wrinkles are flattened in the subsequent calibrating process Response surface methodology is an effective way to obtain the optimal loading path, and the desired loading path can be achieved by controlling the relationship of the axial feeding and the internal pressure

Thickness improvement in hydroforming of a variable diameter tubular component by using wrinkles and preforms

Bending springback plays an important role in hydroforming of curved hollow components, and the effect will be more apparent with the increase of the strength of the material. In order to predict and eliminate the effect of springback on hydroforming process, springback of DP590 welded tube was investigated in computer numerical control (CNC) bending process using theoretical analysis, numerical simulation, and experimental methods. The effects of the bending angles, the diameter-thickness ratio d/t, and the relative bending radius R/d on springback are studied, and the influence factor of the weld position is considered. The theoretical and experimental results show that the springback of the DP590 welded tube with 65 mm in diameter and 2.6 mm in thickness varies within the range from 4.0 to 4.9 % and the amount of springback is influenced by the tube diameter and thickness. The springback reduces from 7.0 to 3.5 % when the diameter-thickness ratio is increasing from 10 to 40. On the contrary, the springback increases from 3.7 to 7.2 % when the relative bending radius is increasing from 1.2 to 4.0. The effect of weld position on springback is very little, but it has a negative effect on defects of hydroforming and wall thickness distribution. It is liable to crack when hydroforming if the weld locates on the outside or neutral layer of the bend. Finally, springback rules and weld effect are applied to form a control arm. A sound part had been successfully manufactured to avoid flash and crack defects considering the appropriate springback compensation and weld position.

Springback and compensation of bending for hydroforming of advanced high-strength steel welded tubes

With the increasing use of advanced high strength steel for automotive components, hydro-formed hollow components have the advantage of both high strength and high stiffness. In order to manufacture a 780 MPa torsion beam satisfied with both improved safety and lightweight, hydro-forming process of 780 MPa torsion beam was studied by experimental research and numerical simulation. Pre-form design according to central composite method was carried out to avoid forming flash and bursting during hydro-forming. Response surface model of the effect of pre-form structure on thickness distribution was developed, and significance of structural parameters on the response value was discussed. According to the genetic algorithm, an optimum pre-form shape has been obtained. An experiment was carried out on the basis of the optimum pre-form shape, and the effect of loading paths was deeply discussed on hydro-forming process. It is shown that the axial feeding within acceptable range can significantly improve the thickness distribution and forming accuracy. Finally, a well hydro-formed torsion beam is manufactured.

Optimization of pre-form shapes by response surface methodology for hydro-forming of 780 MPa torsion beam

Large diameter thin-walled tubes are prone to wrinkle due to a low stability of anti-wrinkles, non-uniform material flow, and contact sequence to the die cavity during hydroforming process. Although wrinkling defects can be controlled by reducing the diameter of the initial tube to some extent, the severe thinning will occur. A method of using local constraints was proposed to eliminate wrinkling defects that occurred on thin-walled hydroformed tubes. The effect of axial constraint, hoop constraint, and axial-and-hoop constraint on wrinkling behavior of large diameter thin-walled tubes has been investigated by using numerical simulation and experiment methods. In addition, the trajectory of principal stress of the wrinkling zones with different local constraints has been discussed. The results show that axial compressive stress of the two wrinkling zones can be reduced by 6.8 and 23.0% by using axial constraint, respectively. Hoop constraint reduces axial compressive stress by 11.7% at the two wrinkling zones. And the axial compressive stress remarkably decreased by 22.3 and 31.8% using axial-and-hoop constraint. The results reveal that the application of local constraints can effectively restrict the occurrence of wrinkle by decreasing axial compressive stress during hydroforming of large diameter thin-walled tubes. Finally, a desired part without defects has been manufactured successfully by axial-and-hoop constraint.

C. Han

Papers

Research on hydro-pressing process of closed section tubular parts

Thickness improvement in hydroforming of a variable diameter tubular component by using wrinkles and preforms

Springback and compensation of bending for hydroforming of advanced high-strength steel welded tubes

Optimization of pre-form shapes by response surface methodology for hydro-forming of 780 MPa torsion beam

Study on wrinkling behavior in hydroforming of large diameter thin-walled tube through local constraints