The friction at tools-sheet metal contacts in DDP is a complex phenomenon. Combined with tool geometries and blank holder pressure, it is considered as a key parameter responsible for the occurrence of necking or tearing, wrinkling and surface defects in the final product [1–4]. Generally, Coulomb’s figure lubrication contact, - relative sliding velocities, - applied load, - and lubricant viscosity. In addition to the mentioned parameters, several research works affirm that the friction in the DDP can also be influenced by other parameters: contact area, lubricant, temperature, surface topography and oil film thickness. These parameters have been the subject of several experimental and numerical studies [4, 7–18] which state the need to take a variable COF or at least to evaluate it in different zones of the forming process.
Plastic strain-induced and anisotropy are among the most important forming characteristics in DDP. These two parameters have a significant influence on the COF and the surface roughness [19–23]. Trzepieciński and Fejkiel  used a strip drawing test to evaluate the COF for different specimens taken from steel sheets at 0° and 90° to the rolling direction (RD). Both contact conditions, dry and lubricated, are considered in the experiments. The results show that the roughness increases with plastic strain level, however the COF decreases. This is explained by the increase in the sheet hardness as a function of the plastic strain that results from the work hardening phenomenon. In the work of Wu et al. , the frictional behavior was also evaluated on pre-strained specimens in strip drawing tests. The measured COF and surface roughness increase with plastic strain level. The sheet materials used are: coated/uncoated DC06 and HX340LAD. Azushima and Sakuramoto  used bending under tension test to investigate the effects of plastic strain on COF and surface roughness. Tests were carried out on an aluminum sheet (A1100). The results show that at lower pressure, the surface roughness increases and the COF remains constant as the average elongation (plastic strain) increases. At a higher pressure, the surface roughness decreases due to the flattening of surface asperities and the COF decreases with the average elongation. Bending under tension tests were also performed on the DC04 sheet metal in the work of Trzepiecinski . The findings indicate that the COF increases with the average elongation in both dry and lubricated conditions following the two RD (0° and 90°). This is explained by the increase in surface roughness due to plastic strain. Masters et al.  investigated the frictional behavior of three commercially automotive aluminum grades using strip drawing tests. Strips were pre-stretched following three RD (0°, 45° and 90°). In this work, the found COF can increase or remain constant depending on the plastic strain. According to the authors, this is due to changes in surface roughness. From the results of the above research works on the influence of plastic strain on the COF, all authors agree that the roughness of different surfaces increases with the plastic strain level for most of materials, but the obtained COF may increase, decrease or remain constant. Therefore, the COF has not a clear tendency according to surface roughness. Masters et al.  indicated that the lack of a generally accepted model for friction resulted in the use of constant COF values, and this is adopted in numerical modeling of DDP for certain specific combinations: forming operating conditions, material, roughness and lubricant. However, they recommend to incorporate a friction law into a numerical model by relating it to the plastic strain-induced during sheet metal forming.
From an overview, tribological tests for DDP were performed at macroscopic scale such as: strip drawing tests, draw bead tests, pin-on-plate tests, draw bend friction tests [4, 24]. These tests have a surface-to-surface contact for which average COF values are obtained and therefore do not allow a suitable analysis of the effect of grain orientation for anisotropic sheet metal, especially when this latter is influenced by plastic strain. At microscopic scale and depending on the sheet material and its fabrication process, the grains vary in both size and orientation. At this scale, the influence of grain orientation on COF can be faithfully analyzed using microscratch tests, because the friction response could potentially be used to reproduce a tribological event such as contact and deformation of asperities [25, 26].
It is well known that microscratch test with a conical indenter is originating from asperity micro-mechanisms. This test was used to study broad quantitative aspects of the effects of material properties, especially work hardening characteristics, on the friction of different pairs of materials [25, 27]. Shugurov et al.  used also this test to investigate the effect of crystallographic grain orientation of polycrystalline Titanium on plastic ploughing. Peng et al.  recommend single-cone scratch test in future works to investigate the effect of grain size on the adhesive and ploughing frictional behaviors of polycrystalline metals in forming process. In the literature, very little information is found on the principles behind the influence of inhomogeneous plastic strain-induced and anisotropy on the frictional behavior in DDP. In order to contribute to this study, this experimental work attempts to evaluate the COF via microscratch tests under conditions of pre-strained specimens following different RD (0°, 45° and 90°) similar to the condition of the plastic strain-induced in DDP.