3.1 Microstructure and tensile test
As seen in Fig. 1, Ultra high strength DP1200 steel contains a very high-volume fraction of the tough phase, martensite. This value contains about 75% martensite and 25% soft phase ferrite. Also, as seen in this figure, an explosive blasting joint with a wavy interface is provided. Since the Al sheet used is a well-known, unalloyed, single-phase structure, it has not been further characterized as microstructural. According to this figure and the EDS result, it can be said that the formation of intermetallics (Al-Fe) at the interface was limited [29], [30].
Intermetallic formation at explosive welding of dissimilar metals is a well-known phenomenon. However, this is observed in limited areas of an interface (Fig. 1). Another phenomenon in explosive and pulse welding, the joining interface offers three morphologies: wavy, flat, and fused layers. These morphologies are a very interesting and complex topic [4], [7], [30], [31]. The interfacial formation, for technical purposes, these morphologies depend on the speed and angle of impact [5], [31]. The interface developed is related to two important events during bonding: sparsity wave interaction and mechanical friction. The first factor that comes to mind is the material's spread of pressure and tension waves due to the impact and shock caused by the explosion and their interaction. Secondly, it is responsible for the acceleration of the flyer plate to the base plate and sliding due to jet formation and its interaction with both the wing and base plates [5]. These phenomena can cause several changes in metallurgical properties, such as deformation at the interface, especially melting of the low melting metal, and if possible, the formation of stoichiometric intermetallics (partial or general) between two metals [7], [32]
The tensile test results obtained in this study are given in Table 3. As can be seen from these results, unalloyed did not show a significant yield strength / tensile strength ratio, while DP 1200 sheet steel showed approximately 0.84 and DP1200-AA1100 plate composite showed about 0.675. In this way, while the yield strength/tensile strength ratio improved with composite production, there was no significant decrease in the elongation value. In addition, the strength of the composite decreased due to the field effect of Al. These results are approximately expected results according to composite rules about combining materials [7], [13]
3.2. Piercing process of the samples
3.2.1. The Piercing process of the base materials
Fig 4 shows cutting force vs time graphs of the DP1200 punching experiment. In addition, Fig 5 compares punching forces in punching operations using 5 different punch tips. As seen in these Fig., the shearing force was the maximum value of 45452.2 N when punch "0" was used. The shearing forces were obtained as 19949, 17118.9, 16535.5, and 11237.8 N when the punches R1, V16, R2, and 16 were used, respectively. Consequently, it is evident that the cutting force is very high in using the "0" punch type and sharply decreases when the other punches are used. On the other hand, as seen in Fig. 5., when experiments were conducted with theoretical studies using FEM analysis, it was found that the results were compatible. The shearing force obtained from FEA Program is 45725N, and the experiment result is 45452N for the punch "0". Similarly, the results were obtained as 20089N and 19949N for R1, 15959N, 16535N for R2, 11002N, and 11237N for punch 16, 17245N, 17118N for punch V16, respectively. This situation was associated with the higher cutting and stripping forces when using the flat punch because of the more significant blanking area.
In addition, Firstly, Fig. 4. shows that the force vs time graph of the piercing process using a P0 straight punch is interpreted as follows;
- Elastic stage: Elastic deformation is when the force rises when the punch encounters the sheet and continues until the plastic deformation begins.
- Elastoplastic stage: The stage of plastic deformation. In this stage, the force increases until the crack occurs
- Punch penetration: When the crack occurs, the force decreases sharply, and at this stage, a slight force appears, which is the force that the punch penetrates the sheet
Secondly, Fig. 4. shows also that the piercing operation performed using the P2-16 punch is interpreted to define the cutting process in the force vs time graph, and the following result emerges. Unlike piercing with a P-0 punch, it has been observed that there are four different stages in piercing with this angled punch:
- In this elastic deformation stage, the force rises when the punch encounters the sheet and continues until the plastic deformation begins.
- In this plastic deformation stage, the force increases until the first crack occur at point (A).
- After the occurrence of the first crack, a decrease in the force can be observed in this area because it has become a shear force and continues with the same value until it approaches point (B), so the force rises again until the occurrence of the second crack.
- When the second crack occurs, the force decreases sharply, and at this stage, a slight force appears, which is the force that the punch penetrates the sheet.
In related literature, similar works were performed in this study manner. For example, Hambali and Guerin [32] reported that during the punching-blanking process, the part is exposed to complex conditions such as deformation, work hardening, and crack initiation and propagation. It is possible to understand the different stages of the cutting process, which ends with the complete separation of the sheet metal of such operations, by defining the force-time graphs. Still, they can be challenging to model. They also show force vs time graphics as described in this study, but there are some differences. This way, accurate knowledge of the failure process is essential for selecting an appropriate damage model. A precise understanding of the failure process is critical to choosing a proper damage model. In the case of sheet shearing by shearing operations, numerous authors have studied the different physical mechanisms that lead to eventual breakage and proposed their models. Dos Santos and Organ [33] performed a viscoplastic study of the rectangular bar blanking operation. They analyzed the deformation of a pattern engraved on the surface of the cut area. As a result, he studied the different physical mechanisms that lead to many eventual ruptures in the case of cutting sheet metal by shearing operations and proposed his models.
During the blanking process, shearing and stripping forces occur between the die, punch, and sheet material. When the movement of the punch is at right angles to the die, the force required in a blanking operation can be determined as follows;
F = Lt . Ss (1)
Where F is the force required for blanking, L is the total cutting length (perimeter), t is the sheet thickness, and Ss is the material's shear strength.
The compressive stress in the punch is calculated as follows;
Sp = F / Ap (2)
Where Ap is the cross-sectional area of the punch.
Stripping force is needed to free the blank from the die or the strip from the punch and increase the total blanking force. Stripping force is calculated as follows;
Lst = k. As (3)
Lst is the stripping force, k is a stripping constant, and as is the cut surface area [34].
When examining the forces changes that occurred during the punching operations (Fig.4.), it can be said that while using the punch "0", blanking/piercing operations co-occur in one shot along the shearing line. Therefore, cutting and stripping forces are also high. Using other punches (R1, V16, R2, and 16) is not instantaneous and takes time. The cutting and stripping forces also decrease depending on the decreasing cutting area. The significance of these results is that the cutting/deformation relationship can be seen. The change in the cutting area also affects the thickness changes and the quality of the cutting surface.
All experiments and FEM analyses conducted using five different punch shapes and AA1100 sheet material have shown that the shearing force is significantly affected by the punch form, as given in Fig. 6. As experimental results, the shearing force was the maximum value when the punch "0" was used. The shearing force was obtained as 7879.5 N for the Al-1100 sheet material. However, it was found that punch V16 comes in second place instead of punch R1, and the shearing force was 4741.8 N. The shearing forces were measured as 3781.3 N, 2954.8 N and 2546.4 N for the punches R1, R2, and 16, respectively.
It was found that when comparing the shearing force of the punch "0" with the rest of the punches, the shearing force decreases markedly. Changing the punch shapes reduced the cutting forces by 39%, 53%, 63%, and 68% using V16, R1, R2, and 16 punches, respectively, compared to flat-ended (0) one (Fig. 6). When these results are compared with the results of the DP1200 shown in Fig. 4 and 5, it is seen that the rate of decrease in the force is lower than expected. This situation can be explained by the low strength of the Al sheet and the reduction in the piercing resistance.
When the experimental results and analysis results are compared, it is seen that the required shearing forces for cutting the Al-1100 sheet material are very close. The shearing force obtained from Deform Program is 7.4 KN, and the experiment result is 7.8 KN for the punch "0". Both results are 3.7 KN for punch R1. The other results were obtained as 2.9 KN and 2.7 KN for the punch R2, 2.6 KN and 2.5 KN for punch 16, and 4.69 KN and 4.7 KN for the punch V16.
3.3. Piercing/Punching of the DP1200 – AA1100explosive welded plate composite by Experimental and FEM
Piercing/punching tests and analyzes were carried out on DP1200 – AA1100explosive welded sheet materials experimentally and theoretically. The maximum shearing force was 47541 N with the punch "0". While the shearing forces were obtained at 20485 N, 21644 N, 18088 N, and 13374 N for punches R1, V16, R2, and 16, respectively, as seen in Fig. 7. and 8. Like base materials, plate composite punching force is reduced by changing the punch shapes. Reducing portions in the cutting forces were 54.5%, 57%, 62% and 72% by using V16, R1, R2, and 16 punches, respectively, compared to flat-ended (0) one (Fig. 8). It is also seen that the required shearing forces obtained from experimental studies and analyses for cutting off the DP1200-AA1100 explosive welded sheet material are very close. The shearing force obtained from Deform Program is 48435N, while the practical result is 47541N for punch 0. The other results were 21274 N and 20485N for the punch R1, 18088N and 18706N for the punch R2, 22204N and 21644N for the punch V16, 13673 N and 13374N for punch 16.
3.2. Experimental and FEM studies of Piercing of the DP1200 – AA1100pairs (welded by explosive and unwelded)
DP1200-AA1100 sheet specimens with and without welding were also tried to shear by piercing to see the welding process's effect on the shear force. It is known that both components' hardness increases after the explosive welding process. Hardness is increased with increasing explosive ratio, and the highest hardness values are obtained near the bonding interface [1, 8, 9]. Therefore, the cutting force of the materials combined with explosive welding was somewhat higher than the not welded (Fig. 9). However, the force increase was minimal. That may be interpreted as reducing the thickness of the shearing area due to the deformation mentioned above.
Besides, it was determined that there was slippage on the contact surfaces when cutting unwelded sheet materials together. No slippage was observed when cutting explosively welded sheet materials (Fig. 10).
On the other hand, in the cutting with punch 16, the cutting surfaces of the falling apart and the blank are not smooth. It was observed from the measurements that these cutting surfaces are angular. Deformation of the falling part and blank when using punch – 16 are shown in Fig. 10. When the interface of explosively welded DP1200-AA1100 bimetal was also investigated as macrostructure, it was observed that the composite components did not separate from each other during punching in which shear forces reached up to 47 kN. Therefore, it can be said that the punch process can be used for layered composite produced by explosive welding.
The experimental results of shearing processes of the DP1200, AA1100, and DP1200-AA1100 explosively welded specimens. It is convenient to use the punch "0" in blanking and piercing processes. The R1, R2, and V16 punches can be used just for piercing processes. Using these punches for the blanking processes leads to uncontrolled part sizes because the side form of the falling part is not flat. It was also determined that punch 16 is unsuitable for blanking and piercing processes since it allows lateral travel of the cut part along the cutting edge. Therefore, the cutting surfaces of both sides are obtained as angular edges. For this reason, it is unsuitable to be used in manufacturing processes, not only the blank but also falling apart.
3. 3. Pierced-punched-Sheared surface and areas examination
The cutting-punch-piercing surface images of DP1200 obtained in this study are given in Fig. 11. It has been achieved to define the rollover, shear, fracture, and burr regions generally expressed in the literature in these photographs [33], [35]. In addition, in this study, SEM images of the cutting cross surface of the DP1200-AA1100plate composite samples are given in Fig. 12. The measurement analysis of the micro images taken from the mid of the samples is shown in Fig.13, with the measured state of the cutting surface regions. It has been achieved to define the rollover, shear, fracture, and burr regions, generally expressed in the literature in these photographs [19], [23], [36]. Correctly, in the case of two different materials combined with this explosion in these images, it is observed that these regions almost formed in both parts.
Punch 16 was used, and the falling part and blank cutting surfaces were not smooth. It was observed from the measurements that these cutting surfaces are angular. Deformation of the falling part and blank when using punch 16 are shown in Fig. 10. When the interface of explosively welded DP1200-AA1100 bimetal was also investigated as macrostructure (Fig. 11), it was observed that the composite components did not separate from each other during punching in which shear forces reached up to 47 kN Therefore, it can be said that the punch process can be used for layered composite produced by explosive welding.
share surface quality is one of the most important factors that provide flexibility and stretchability during expansion and flanging in the punching process. Therefore, it is an essential criterion in the quality approach of the cutting process [37]–[39]. This study determined that four different regions can be analyzed according to the literature's surface texture and cutting surface. As mentioned before, these areas are; burr, cutting, tipping and breaking areas are primarily expressed. It is desirable to have a large cutting area and smaller burrs for a good quality cutting surface. In addition, in the surface roughness criteria, the fracture and tipping areas are desired to be minor, while the cutting area is more significant. As shown in Fig.s 11, 12 and 13, the punch type has a distinct effect on the cutting surface. In this study, the cutting speed was kept constant. It is accepted that the cutting speed also affects the region and areas here.
In some studies, it has been mentioned that the cutting surface properties are affected by the clearance. When the clearance is too low, the second finishing area is formed due to the contact of the fractured surfaces [15], [32], [33]. In 2D numerical studies, it has been observed that the broken surfaces touch each other. Surface cutting is slower in punches with inclined geometry because the shear angle and crack propagation move perpendicular to the punch surface according to the inclination angle[42]. In this case, the deformation increases and the rollover size increases due to slow sliding. That can be understood from the top view of the rollover regions formed due to stapling with different staples [43].
Another important subject where cutting processes are combined is punching and hole boring in automotive body production applications. The quality and undamaged performance of these processes are very important indicators in terms of the quality of the subsequent parts and service performance. Hole boring test is one of the most important processes to understand the cutting surface quality and production and service conditions in sheet materials. In this context, the ductility and formability of plate and sheet metals must also meet the requirements of automotive and other applications. Typically, the hole expansion ratio (HER) is a good indicator of the sheared edge ductility of sheet materials, emphasizing the importance of surface quality and pain caused by hole formation used in hole formation and downstream processing. In this context, room temperature punch-cutting is the most cost-effective way to create profile cuts compared to other methods. Therefore, understanding the punching process and its effect on subsequent cutting behavior is beneficial for the validity of punch cutting [44], [45]. It is performed as a cutting process that divides the samples into two parts by cutting the profile hole and forms the first hole surfaces with the areas affected by the cutting edge shearing (SAZ) in the remaining space. (SAZ) in the remaining space. Wu et al. [[46] It is performed as a cutting process that divides the samples into two parts by cutting the profile hole and forms the first hole surfaces with the areas affected by the cutting edge shearing (SAZ) in the remaining space. Wu et al. [[46] showed that plastic deformation and effects caused by punch cutting have a high degree of strain localization in the first hole-edge region. This results in a deformation gradient where the stress decreases as you move away from the cutting edge. The effect of this shear stop for subsequent deformation processes is very obvious. In this study, the development and final conditions of the surface quality as susceptibility to the next production processes were evaluated in detail and it was shown that these results could be considered as an important criterion when starting the next processes. In this context, the cutting edge properties of the plate composite flat product produced by both dual-phase steel and burst welding and the cutting forces resulting from the process are given in great detail.