Applying TRIZ for systematic manufacturing process innovatio(3)

An experimental program run on this setup allowed to demonstrate the following process performance improvements.

5.1. Process force reduction

Using a force sensor mounted on the robot wrist, the reduction of the process forces for SPIF with dynamic heating support, compared to forming at ambient temperature, could be demonstrated (Fig. 10). From this figure it can be observed that, by choosing an appropriate heating temperature, the dominant force component parallel to the tool axis can be reduced to approximately 50%. It is obvious that the resulting stress levels in the vicinity of the tool contact zone are reduced accordingly, effectively reducing the areas in which the yield strength of the material is exceeded. This can be expected to lead to systematic accuracy improvement, as illustrated in the next section.

Joost R. Du ou and Joris D’hondt / Procedia Engineering 9 (2011) 528–537535

16001400

120010000

Temperature [°C]

Force [N]

5.2. Accuracy improvement

Figure 10: Forces measured along the tool z-axis for Al5184 in function of the local heating temperature in the tool contact zone.

Fig. 11 shows the effect of localised heating on the overall accuracy of the workpiece. It should be noted that in the underlying experiment constant heating was applied. On one hand this leads to an improved approximation of the intended CAD geometry, e.g. in Zone A. However, the local heating also results in groove formation in Zone B (the bottom surface of the part not being processed). Timely switching off the heating, in accordance with separation principle 4, could allow to minimise this effect.

m

5.3. Formability improvement

Figure 11: Accuracy improvement for a conical workpiece with indication of the envisaged CAD geometry used for toolpath generation.

Some materials are known for their limited formability under ambient temperature conditions. TiAl6V4 sheets of 0.6mm thickness were used to compare the formability behaviour under dynamically heated conditions with test results obtained at ambient temperature.

Conical shapes with maximum outer diameter 140mm and increasing wall angle were formed using a backing plate with a 180mm aperture. A spiralling toolpath with a continuous incremental step size (pitch) of 0.5mm was used. The initial feed rate of the robot was set to 1000mm/min. This feed rate was varied for some of the dynamically heated tests to assure a constant energy input density for different laser spot diameters (tests with diameter 14 and 15mm in Table 1).

536 Joost R. Du ou and Joris D’hondt / Procedia Engineering 9 (2011) 528–537

Non-heated wall angle

[°] 30 35 32 34 33

obtained result OK failed OK failed failed

wall angle spot size

[°] [mm] 45 12.0 50 12.0 55 12.0 53 12.0 55 12.0 57 12.0 56 12.0 56 14.0 57 15.0

Heated energy input

[J/mm²] 0.875 0.875 0.875 0.740 0.740 0.740 0.740 0.740 0.740

obtained

result OK

OK failed OK OK failed failed OK failed

Table 1: Formability test results for 0.6mm TiAl6V4.

At room temperature (20°C), it was possible to form cones with slope angles up to 32°, whereas tests with a higher wall angle resulted in cracks appearing before a 30mm depth was reached (see Table 1).

With an effective power input of less than 200W the maximum forming angle could be substantially shifted. Varying the spot diameter and the energy input level successfully allowed to form cones with slope angles up to 56°. Optimising the temperature field by adjusting the cooling conditions may allow to further increase this limit.

Similar tests demonstrated an increase in formability for materials with a higher formability at room temperature as well. 65Cr2 sheets with a 0.5mm thickness could, for example, be formed into cones with slope angles up to 57° at room temperature without any cracks appearing before a 40mm depth was reached.

The same material, tool and process parameters were used to produce samples under dynamic heating conditions. Assuring a process temperature in the forming zone of approximately 350°C, parts were made with a wall angle of 64° without any part failure occurring. According to the sine law approximation [2], and in terms of principal strain, the wall angle increase of 7° corresponds to a strain increase of 44.5%. 5.4. Residual stress reduction

Warm forming of sheet material is known to result in lower elastic springback [8]. Question was whether dynamic, localised heating would yield similar results, or might, on the contrary, lead to residual stresses that could cause additional elastic deformations upon unclamping or trimming of the workpiece.

A B

Figure 12: Obtained workpiece geometry for non-heated (A) and heated process variants (B) after unclamping.

Figure 12 depicts the results of tests conducted with TiAl6V4 after unclamping of the test specimen. While for the left part (A) a 30° cone was created at ambient temperature conditions, for the part at the right a 50° cone was generated using dynamic local heating support. The residual elastic stress levels, that visibly deform the workpiece upon unclamping when cold formed, are significantly reduced in the heat supported process variant.

Joost R. Du ou and Joris D’hondt / Procedia Engineering 9 (2011) 528–537537

6. Conclusions

In this paper the applicability of TRIZ analysis and problem solving techniques for the systematic enlargement of

the manufacturing process window of a newly emerging forming technique was illustrated. The physical conflict resolving separation principles lead to the specification of a process variant that proved to offer superior performance results compared to the original process capabilities. Taking the favourable results …… 此处隐藏：5731字，全部文档内容请下载后查看。喜欢就下载吧 ……