Study on Coating Mechanism and Properties of Die CrN+DLC Layer

This paper introduces the principle of PACVD coating technology, technical characteristics, equipment composition and material characteristics of CrN+DLC. Taking H13 steel as the research object, its surface was treated with CrN+DLC. The microstructure, bonding state and hardness of the interface were studied by means of metallography, SEM, hardness and component distribution of the surface layer. The anti-crack ability and grade of DLC layer were analyzed by studying the shape of crack distribution with Rockwell hardness indentation, and the high quality layer with crack grade of HF1 was obtained. With the dual properties of diamond and graphite of DLC, it can make the die surface have lower friction coefficient, higher hardness, higher impact toughness, better solid lubrication performance and higher corrosion resistance. Surface DLC coating technology provides a new solution to improve the performance of the die.

Performance Comparison of Zn-Based and Al–Si Based Coating on Boron Steel in Hot Stamping

The coatings of boron steels play an important role in affecting the quality of hot stamping parts, so it is important to evaluate the hot stamping performance of coatings before designing processes. Taking the U-type hot stamping part of boron steel as research objects, the surface quality, microstructure and temperature variation of samples with GA (galvannealed), GI (galvanized) and Al–Si coatings were observed and analyzed to evaluate the anti-oxidation, forming and quenching performances of different coatings. The results show that all the GA, GI and Al–Si coatings could provide good oxidation protection and also act as the lubricants for avoiding the friction damage of sample substrates and die-surface. But the different compositions of GA, GI and Al–Si coatings will contribute the different colors. Under the same deformation degree, the Al–Si coating can provide the best substrate protection and the GI coating will induce cracks in the substrate because of the liquid metal-induced embrittlement phenomenon. There is no significant difference between the quenching performances of GA, GI and Al–Si coatings, and the thermal conductivity of the GI coating is slightly better than Al–Si and GA coatings.

A Novel Sample Preparation Method for Frontside Inspection of GaN Devices after Backside Analysis

Frontside die inspection by Scanning Electron Microscopy (SEM) is critical to investigate failures that appear dispersed over the GaN die surface and that will be very difficult to localize by the typical Focus Ion Beam (FIB) or Transmission Electron Microscopy (TEM) analysis. Frontside sample preparation is; however, extremely challenging if the device was already subjected to sample preparation for backside Photo Emission Microscopy (PEM). In this paper, a novel sample preparation method is presented where all front side layers are removed and only the 5μm GaN die is left for inspection.

Springback prediction and compensation method for anisotropy sheet in multi-point forming

Multi-point forming (MPF) is widely used for three-dimensional (3D) curved surfaces, and the multi-point die (MPD) is reconstructed with the desired curvature and the compensated curvature which is caused by springback. Springback is affected by mechanical anisotropy of material in MPF. To predict the springback of anisotropy sheet in MPF process, the finite element models were established based on the three different yield criteria namely; von Mises, Hill's 48 and Yld2004-18p. To compensate the springback of anisotropy sheet, an algorithm with consideration of anisotropy for doubly curved sheet was proposed based on the results of the finite element simulation and the elastic perfect plastic biaxial bending model. The direct curvature adjustment (DCA) method and the Non-Uniform Rational B-Splines (NURBS) method are used to generate the die surface after springback compensation. The final shape of the workpiece is obtained by the execution of iterative compensation. The forming experiments were carried out and the results show that the anisotropic model is closer to the experiment than the isotropic model. Finally, the influences of blank thickness, part shape and forming radius on compensation accuracy were discussed to verify the applicability of the algorithm.

Weakening affinity of SUS304 asperities to die surface with TiN coating for preventing delayed cracks in deep-drawn cups

SUS304 stainless steel has strong affinity to carbon tool steel surfaces. Therefore, the flow of material in the flange portion during the deep drawing process is retarded, leading to increase in amount of wall thicknening along the cup edge and rising risk for delayed cracks. In this paper, TiN coating is applied to the drawing die surface to weaken the affinity. Under elevated blank holding forces (BHF), the experimental results showed that the crack-free BHF range for the TiN coated and the uncoated dies are 5~10 kN and 12 kN, respectively. The entire BHF range for successful drawn cups formed with the coated die are crack-free. The crack-free BHF magnitude is successfully lowered and the range is enlarged with the coating. Crack-free cups having large elongated height and low amount of wall thickening along the cup edge are formed. The weak affinity is evidenced by the low estimated coefficient of friction (COF) obtained from a FE simulation model based on the Coulomb’s law of friction. In contrast, the estimated COF of the uncoated die is high even at the low BHF due to the strong affinity. Therefore, delayed cracks are observed under BHF range of 7~11 kN. At BHF of 12 kN, wear fragments are formed in the boundary layer as a result of the continuous polishing of the SUS304 asperities by the uncoated die asperities. The COF is sharpyly decreased due to the smooth relative movement of contacting surfaces facilitated by the particles and the formation of cracks is prevented. However, the segments tend to penetrate into the SUS304 surface under excessive BHF of 13kN and above. The relative motions of the segments are prohibited, resulting in the reformation of the cracks.

Precision Polishing Techniques for Metal Molding Dies and Glass Forming Technology “Slumping Method”

Precision manufacturing techniques are required for the fabrication of small and large optical components in various fields. To prepare molding dies with highly precise geometric shapes and surface roughness that are used in certain molding processes, polishing techniques have been investigated for many materials. In this research, the polishing techniques used for a SUS310S stainless steel molding die for the glass forming technology “slumping method” were investigated. The surface roughness of the polished SUS310S molding die surface was below Rz = 120 nm (P–V), Ra = 20 nm after 35 h of polishing with 0.5% alumina polishing liquid under a pressure of 1.7 kPa. In addition, the centerless polishing machine was designed and manufactured to polish cylindrical molding die surfaces with same polishing conditions. As the result of using cylindrical molding dies that made by this centerless polishing machine, the surface roughness of the glass plate formed using the slumping method with the polished molding die was below Ra = 20 nm. These results indicate that the surface roughness of the molding die had a small effect on the glass plate surface formed using the slumping method.

The Interaction between the Sheet/Tool Surface Texture and the Friction/Galling Behaviour on Aluminium Deep Drawing Operations

The increasing demands for lightweight design in the transport industry have led to an extensive use of lightweight materials such as aluminium alloys. The forming of aluminium sheets however presents significant challenges due to the low formability and the increased susceptibility to galling. The use of tailored workpieces and controlled die roughness surfaces are common strategies to improve the tribological behaviour, whilst galling is still not well understood. This work is aimed at analysing the interplay between the sheet and tool surface roughness on the friction and galling performance. Different degrees of Electro Discharge Texturing (EDT) textures were generated in AA1050 material strips, and tooling presenting different polishing degrees were prepared. Strip drawing tests were carried out to model the tribological condition and results were corroborated through cup drawing tests. A new galling severity index (GSI) is presented for a quick and quantitative determination of both galling occurrence and severity. The present study underlines the key role of die topography and the potential of die surface functionalization for galling prevention.

Eliminating Delayed Cracks in Deep-Drawn SUS304 Cups Under Wider Range and Lower Magnitude of Blank Holding Forces Using Tin Coated Die

The effect of TiN coated die on eliminating delayed cracks in deep drawing processes of stainless steel SUS304 cylindrical cups under elevated blank holding forces (BHF) using a commercial lubricant at room temperature is investigated in the experiment. For comparison, the experiment is repeated using an uncoated and finely polished die under the same conditions. The results shows that the crack-free BHF range for the coated and the uncoated dies are 5~10 kN and 12 kN, respectively. Both the magnitude and range of the crack-free BHF are successfully lowered and enlarged by applying TiN coating to the die surface. Lower magnitude and wider range for BHF are preferred in the industries as it is difficult to maintain a high, constant and precise BHF during the deep drawing process using coil springs or die cushions. The elimination of the cracks is mainly due to the decrease in amount of strain-induced martensite resulting from the lower amount of wall thickening, particularly in the valley points along the cup earring profiles. The improved tribological performance by the coating enhances the radial flow of the materials into the die cavity resulting in lower amount of wall thickening. The chance for delayed cracks is reduced with decreasing amount of wall thcikening. Overall, the amount of tensile residual stresses along the outer surface of the cup, particularly in the upper portion is reduced with the coated die due to its low BHF. Therefore, the risk for the cracks is reduced.