An Experimental Investigation of Surface Quality in Single Point Incremental Forming of Cu-Al Composite Sheets

Single-point incremental forming (SPIF) has been developed as a customized substitute of traditional sheet metal forming techniques. SPIF is a great method for rapidly prototyping sheet metal objects, although it faces challenges in achieving surface quality and thickness distribution. In this investigation, sheets of Cu-Al bimetallic composite were formed utilizing a hemispherical-headed tool with a variety of step depths, spindle speeds, forming speeds, layer arrangements, and lubrication types. Surface roughness and minimum thickness were evaluated as response parameters in relation to the significance of process variables. The best surface quality was attained with 0.4 mm step depth, 750 mm/min forming speed, and 1500 rpm spindle speed with liquid lubrication support for both types of layer arrangements. Layer arrangement and lubricant type were the decisive factors for the minimum thickness response parameters.


Introduction:
The single point incremental forming (SPIF) technique is thought to be an inexpensive method for producing intricate and specialized components with a rapid turnaround time in low volume.This advantage has positioned SPIF apart from traditional sheet metal forming procedures.In SPIF, a hemispherical-headed tool deforms the metal sheet locally along a predetermined tool path to get the required shape.[1] [2].Surface quality and thickness variation were regarded as the key considerations for the acceptability of the formed component.There have been several studies done to improve surface quality by varying input process parameters and using lubrication.The mixture of white petroleum jelly and MoS 2 powder combined in the right proportion can be employed as a lubricant to improve the surface finish when forming CP Ti blanks incrementally [3].The surface finish has also been altered by the application method and the lubrication type.The ceramic coating of MoS2 or solid graphite, and the Nano-K2Ti4O9 whisker-improved solid lubricating layer were added during the formation of the AZ31 sheet.It was claimed that solid graphite or ceramic coating of MoS2 had an amazing greasing and self-lubrication effect.[4].
One of the forming flaws that restrict the process's widespread deployment has always been sheet thinning.According to a study on multiple passes of forming, an increase in the overall plastic deformation zone leads to improved thickness uniformity during the process [5].The incremental forming of DC04 sheet demonstrates that, when employed with a conventional tool trajectory, the minimum thickness is strongly correlated with tool diameter, whereas the region of the minimum thickness is substantially determined by step depth [6].The tool and sheet blank contact conditions have an impact on surface irregularity.Forming AA7075T0 sheet reveals that, in comparison to a nonspinning tool, a rotating forming tool provided a greater surface quality [7].The surface roughness of Al 5052 initially increases with increasing incremental depth up to a particular angle, and subsequently reduces during incremental forming [8].The arrangement of the layers is essential for describing surface quality in terms of surface roughness.The surface quality of the bimetal blank was impacted by friction conditions between its surface and the tool throughout the forming process.For low-carbon steel/CP-titanium bimetal blank, when a tool made contact with a layer of titanium in a bimetal sheet, there was sticky friction that caused the metal blank to adhere to the tool's tip and increase the surface irregularity.However, the St-Ti arrangement has produced improved surface quality [2].In comparison to SUS/Al, it has been found that Al/SUS arrangements exhibit early fracture incidence.However, after the occurrence of fracture, the SUS/Al layer arrangement experiences delamination.The forming wall angle and the surface irregularity are both positively impacted by the low values of the step size.The SUS/Al layer arrangement has been demonstrated to have a greater forming wall angle and excellent surface consistency [9] [10].It was clear that sheet thinning occurred in the pyramid's wall section during the formation of St/CP-Ti sheets.It was discovered that the corners of the pyramid form showed the greatest rate of thickness variation [11].Numerous investigations have been carried out to look into bimetallic composite forming and the process variables involved [12] [13].However, there is relatively little research available on incremental bimetallic sheet forming.The present experimental investigation concentrated on the influence of step depth, forming speed, and spindle speed for two lubrication conditions and layer arrangements.Surface roughness and minimum thickness were two important responses taken into account for the present experimental investigation.

Material:
The Cu-Al bimetallic composite sheet that was employed for the studies has layers of Copper 11000 and Aluminum 1060 that are each 0.20 mm thick and 0.80 mm thick, respectively.table 1 demonstrates the composition of both layers.The size of the test samples is 150 mm x 150 mm x 1 mm.The review of the literature demonstrates that lubrication has a greater overall influence on the surface irregularity of the manufactured component [4] [14].For the current investigation, two lubricants-SAE-10W-30 lubricating oil and Molybdenum disulphide lithium base grease were employed as lubricants.

Process parameters and response measurement
The continuous process variables used in the investigation are step depth, forming speed, and spindle speed.Layer arrangement and type of lubrication were considered categorical process parameters.In order to establish the range of the input variables prior to the core experimental work, a number of trial experiments were conducted.For the experiments, three levels of continuous parameters and two levels of categorical parameters were considered.The levels of the input variables taken into consideration are displayed in Table 2.

Input Variables
Level-1 Level-2 Level-3 Step depth (mm) 0. Formed samples were cut from the center to measure the minimum thickness and surface roughness.Figure 2 (a) depicts the formed sample that has been cut out of the center and the surface where roughness will be assessed.Readings close to the fracture location were acquired to determine the minimum thickness value.In order to examine the surface finish for the current investigation, the average roughness value (Ra) was assessed.Measurements of the roughness were made on the inside of the produced workpiece.Mitutoyo SJ-400 was used to utilize to gauge the roughness of the surface at the predetermined region.The setup for the measurement of the formed sample on the surface roughness tester is shown in Figure 2(b).In order to ensure precision, three measurements were obtained at the same location.The sample length for surface roughness measurement was 0.80 mm, while the evaluation length was 4.00 mm.

Result and Discussion:
Several tests have been carried out to evaluate the surface irregularity and thickness of the formed components in order to examine the influence of input variables.The following process variables are taken into account for establishing the quality of the formed workpiece in terms of surface roughness and minimum thickness value: step depth, spindle speed, forming speed, lubricants, and sheet layer arrangements.Figure 1 (b) illustrates the generated component employing the truncated cone-type shape from the Cu and Al sides.Surface roughness (Ra) and minimum thickness values were measured on samples cut from the center.The graphs are constructed using data points that represent the average of five measurements, and the error bar represents the standard deviation among these measurements.It was evident that minimum thickness and surface roughness were crucial factors for successfully manufactured components.The results were discussed in this section while taking into consideration the effects of the lubrication condition and layer arrangement.To study the impact of step-depth on surface roughness and minimum thickness, spindle speed and forming speed were maintained constant at mid-range values.Figure 3(a), illustrates that the value of surface roughness increases as the step depth increases.Lower roughness values have been achieved for both the Al-Cu and Cu-Al layer arrangements using liquid lubricants.To achieve good surface quality, a Cu-Al layer with liquid lubrication assistance was used, as depicted in figure 3(a).Figure 3(b) demonstrates that for both types of lubrication conditions, lower minimum thickness values were obtained for the Cu-Al layer arrangement.The minimum thicknesses have also increased as the step depth increased.However, for the Al-Cu arrangement, the minimum thickness has somewhat lowered with an increase in step depth with solid lubrication.The error bar plot depicted on the graphs reveals a significant decrease in measurement error, indicating improved accuracy.Step depth and forming speed were held constant at mid-range levels to evaluate the effects of spindle speed on surface roughness and minimum thickness.As seen in Figure 4(a), under both lubrication conditions, the value of surface roughness rises with spindle speed for the Al-Cu layer configuration.As the spindle speed rises, the surface roughness for the Cu-Al configuration slightly reduces.Liquid lubrication enhances the surface quality of both types of layer arrangements.To assess the impacts of forming speed on surface roughness and minimum thickness, spindle speed and step depth were maintained at mid-range levels.Figure 5(a) illustrates that surface roughness appears to be somewhat unaffected by forming speed.However, the Al-Cu layer arrangement shows significant variations with various lubrication conditions.Cu-Al configuration with solid and liquid lubrication support has demonstrated greater surface quality.The minimum thickness values have grown up to the middle level with an increase in forming speed before decreasing, as illustrated in figure 5. (b).In comparison to Cu-Al arrangement, Al-Cu arrangement delivers superior minimum thickness values for both lubrication conditions.The error bars overlapping each other for surface roughness and minimum thickness indicate that the impact of lubrication on the Al-Cu arrangement was not found to be significant.The outcomes of the experiment highlight the significance of layer arrangement and lubrication during the forming of Cu-Al bimetallic sheets.As compared to the lubrication type, layer arrangement is crucial, with response factors including minimum thickness and surface roughness being taken into account.

Conclusion:
On a Cu-Al bimetallic blank, the SPIF is performed, with the layer arrangement and lubrication type acting as categorical process parameters whereas step depth, forming speed, and spindle speed are considered continuous process parameters.Surface roughness and minimum thickness were studied on samples that had been formed while taking into consideration a combination of several process factors.The experimental investigation yielded the following findings.
 The minimum surface roughness values of 0.52 µm for Cu-Al arrangement and 1.05 µm for Al-Cu arrangement have been achieved using 0.4 mm step depth, 750 mm/min forming speed, and 1500 rpm spindle speed with liquid lubrication support. The average surface quality in terms of surface roughness is better in the Cu-Al arrangement for liquid lubrication assistance than in the Al-Cu arrangement.The average minimum thickness value of an Al-Cu arrangement with solid lubricant is higher than that of a Cu-Al arrangement. Layer configuration, step depth, lubricant type, and spindle speed have been identified as the process variables that have a significant effect on surface roughness.However, the impact of the forming speed was very minimal. The most important factors that influenced the minimum thickness response parameter included layer arrangement, lubricant type, forming speed, spindle speed, and step depth, respectively.
Figure 1.(a) Set-up arrangement on the machine; (b) Formed component.For the SPIF experiments on the Cu-Al bimetallic sheets, a test setup was constructed.The clamping plate, support column, and hemispherical-headed shaping tool comprised the setup for the experiments.A CNC milling center was equipped with the developed setup, and a hemisphericalheaded tool was fixed on the spindle.The hemispherical-headed tool is made of tungsten carbide.

Figure 1 (
a) depicts the arrangement of the setup and the holding of a Cu-Al bimetallic blank on the setup.Figure 2 (b) depicts the formed component for the Cu-Al and Al-Cu layer arrangements using truncated geometry.The Cu-Al layer arrangement refers to the forming tool coming into contact with the Cu layer as it is being formed, and vice versa.

Figure 2 .
Figure 2. (a) Formed samples cut from the center; (b) Measurement on roughness tester.

Figure 3 .
(a) Effect of step depth on surface roughness; (b) Effect of step depth on minimum thickness.

Figure 4 (
b) illustrates how the minimum thickness value rises as spindle speed rises.In comparison to a Cu-Al layer arrangement, there is a significant variation in the minimum thickness value for an Al-Cu layer arrangement.The analysis of the error bars suggests that there is no significant variation in the minimum thickness for the Cu-Al arrangement at 1000 rpm.The liquid lubrication has obtained greater minimum thickness values for both types of layer arrangements.(a) (b) Figure 5. (a) Effect of forming speed on surface roughness; (b) Effect of forming speed on minimum thickness.