IMPROVE THE CONSISTENCY OF COMPONENTS BY USING AN IMPROVED BONDED MIX

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1 IMPROVE THE CONSISTENCY OF COMPONENTS BY USING AN IMPROVED BONDED MIX Zhaoqiang Tan, Shun Li, Xin Xu, Ola Litström and Lifang Chen Höganäs China Co., Ltd Waiqingsong Road, Qingpu, Shanghai, China, Paul Skoglund Höganäs AB, Höganäs, Sweden Xuan Zhang Porite Yangzhou Technology & Industry Co., Ltd. 399 Hanjiang South Road, Yangzhou, Jiangsu, China, ABSTRACT One of the main advantages of PM technology is its ability to make near net shape components with a cost efficient process. However, variation in the dimensions is shown after compaction and sintering that often calls for sizing or machining to reach the desired tolerance class. Organically bonded mixes have been developed to improve the consistency by reducing the segregation of the elements in mix and improving the filling characteristics. An improved bonded mix combined with a recently developed lubricant, was evaluated by indutrial production of a complex shape component with tight tolerances. Many benefits of dimenisonal consistancy, weight precision, better lubrication as well as clean surfaces by using the improved bonded mix are presented in the paper. INTRODUCTION Powder metallurgy offers the ability to manufacture net-shape or near net-shape components to a variety of mechanical performance levels. As the PM market grows, new applications will not only require high mechanical performance, but also an increasing focus on the ability to improve component consistency of density and dimensions. The demand of close dimensional tolerance is and will be a key factor for successful PM production. In a time when parts with high precision requirement for the automotive industry gets more and more complex in geometry, resulting in deep and narrow filling gaps, the use of a powder mix and handling system, providing excellent filling performance and no or limited segregation are essential. 1 A number of factors have influences on the component consistency, including the material, mixing, handling, compaction and sintering, which increase the dimensional and weight scatter of the finished components. 2 Organic binders are widely used to bond the graphite as well as lubricant in order to minimize scatter in carbon content, and also dusting of the mixes. The Starmix TM system as effective solutions for fillability, weight variation and increased productivity during compaction has been demonstrated by comparing with un-bonded. 3

2 Besides good consistency during compaction, there are several more requirements on high performance mixes, such as good lubrication during ejection out of die and stain-free components after sintering. One recently developed lubricant exhibits many benefits, better lubrication compared with Amide wax and stain-free components after sintering. 4 In this paper, an improved bonded mix combined with the recently developed lubricant was investigated by an industrial production VVT stator with high precision requirement. One conventional bonded mix was selected as reference for consistency comparison. EXPERIMENTS Materials Three types of mixes were included in the investigation, see tableⅠfor composition of the mixes. Two types bonded mixes were tested for consistency comparison and one non-bonded was as reference for powder properties benchmarking. The three mixes were of the same nominal composition (Fe-2%Cu-0.8%C) according to MPIF FC-0208 based on atomized iron powder AHC However, different lubricants were used. In the recipe design, 0.8% Amide wax and 0.6%Kenolube were respectively used in the Starmix TM 500 and, in which is normal content for recipe design, and a recently developed lubricant, named Lube E, was added in Starmix TM 500i for comparison. In order to minimize the impact from copper powder on consistency of component, the Distaloy ACu was added as copper source. This is an atomized iron powder in which 10% of fine particulate copper is diffusion bonded. The natural graphite UF4 was mixed as carbon source. Mix type Starmix TM 500i Starmix TM 500 TableⅠ: Mixes recipe. Mix composition AHC %Distaloy ACu+0.8%C-UF4+0.6%Lube E AHC %Distaloy ACu+0.8%C-UF4+0.8%Amide wax AHC %Distaloy ACu+0.8%C-UF4+0.6%Kenolube Testing The Hall flow rate and apparent density was measured according to ISO Standards 4490:2008 and :2008. The cylindrical specimens with diameter 25 mm were compacted to measure the compressibility according ISO 3927:2011. The ejection properties were measured according to Höganäs AB s internal method. A ring with OD/ID 55/45 mm and the height of 15 mm was used for ejection properties measuring. The static peak ejection force is the max force where the component starts to move. The ejection energy is calculated based on the ejection force and displacement during ejection of the component. Both the peak static ejection force and the ejection energy are divided by the surface area in contact with the tool die during the ejection. This creates a normalized value, enabling a rough comparison between different components. A dust track aerosol monitor was used to measure the dusting of the powder mixes. Five grams of powder flow from a hall funnel into a sample container. Air from the container is extracted by a rate of 3L/min and fine particles in the extracted air are monitored. The bonding degree of graphite was measured with an air jet sieve (20μm). 2500Pa air pressure was set to separate the free graphite particles from the 100 grams mix and the carbon content was measured before and after sieving to calculate the bonding degree as Equation 1. Bonding degree (%) = C C After sieving Beforesieving 100% Equation 1 The filling index for the mixes was calculated using a die-filling simulator, shown in Figure 1 and more details are described in Reference 5. The equipment has rectangular dies with different widths between 1

3 mm to 20mm, having a fixed length and depth of 30 mm. The dies were filled automatically at different speeds and the powder was subsequently collected on a scale for calculation of filling densities from the weight and measured volumes of the cavities. From the results, a filling index was calculated as shown in Equation 2 as the relative difference in powder density for two cavities of different widths (2 mm and 13 mm). AD13mm - AD Filling Index( %) = AD 13mm 2mm 100% Equation 2 Figure 1. Die Filling Simulator. Production Trial The two bonded mixes were investigated by production trial. A hydraulic CNC press TPA 250/3 HP was used in this investigation. For this trial, a mass production component (VVT stator) was utilized to evaluate the performance of the improved bonded system. A picture of the stator and the dimensions are shown in Figure 2. The wall thickness of the component was only 4.5 mm and the required variation in density was within 0.1g/cm 3. The nominal density of the component was 6.95 g/cm 3 and the weight was around 300g. The parts were continuously compacted at two speeds, 4.7 and 7 strokes/ min and 100 parts for each running were sampled from the 201 st for investigation. There components were sintered at 1120 for 30 minutes in 90/10 N 2 /H 2 atmosphere. D3 D2 D1 Figure 2. Pictures and dimensions of the VVT stator. All sampled parts were weighed. Two inner diameters marked D1 and D2, and the outer diameter marked D3, 10 points height were measured by the coordinate measuring machine Gage2000 before and after sintering, shown in Figure 2 and Figure 3. Groups of 20 parts were also sampled from every fifth in these 100 parts for diameter and height measuring. After sintering, three parts from each trial were randomly chosen for density distribution investigation. Each component was cut into 10 segments and the segment without hole was marked A and the segment with hole was marked B, shown in the Figure 3. The sintered density of each segment was measured by weighing in air and water according to ISO 2738:1999. The dimensional change of height was calculated on the average of 10 points from green to as-sintered and that of outer diameter was also calculated from green to as-sintered component. B Filling direction A Figure 3. Measuring points of height and as-sintered density.

4 RESULTS AND DISCUSSIONS Powder Properties Flow rate and apparent density of the mixes are presented in TableⅡ. Similar flow rate was obtained for these three mixes. Due to higher adding content of wax and its character, the apparent density of Starmix TM 500 is much lower than the Starmix TM 500i and. Highest AD was obtained from premix, which is the result of Zn-containing Lube. TableⅡ: Hall flow rate and apparent density. Starmix TM 500i Starmix TM 500 Hall flow rate, s/50g AD, g/cm The compressibility of Starmix TM 500i was as good as the and better than the Starmix TM 500, more pronounced in higher compaction pressure, shown in Figure 4. However, even higher density was obtained from i when compacted in warm die. The ejection properties were measured at different compaction pressures, shown in Figure 5. Slightly lower ejection energy was shown in low compaction pressure by Starmix TM 500, due to higher lubricant content. But when comes to higher compaction pressure, lowest ejection energy was presented by Starmix TM 500i. It is also shown that 15% lower in ejection energy and static ejection force for Starmix TM 500i when applied 70 (158 )warm die compaction compared to room temperature compaction. As the results, Starmix TM 500i showed higher compressibility by warm die compaction. On the other hand, its excellent lubrication allows lower the lubricant content needed of ejection. Lower lubricant amount can also contribute to higher compressibility. Therefore the way of using lower lubricant content combined warm die compaction is a solution for the component with higher density. 7.3 Compaction pressure, tsi Compacted density, g/cm i i-70 (158 ) Compaction pressure, MPa Figure 4. Compressibility

5 Ejection energy, J/cm Compaction pressure, tsi i i-70 (158 ) Compaction pressure, MPa Static ejection force, N/mm i 35 i-70 (158 ) Compaction pressure, MPa Figure 5. Ejection properties: Ejection energy, Static ejection force Compaction pressure, tsi Bonding degree The results of dusting and graphite bonding degree are presented in Figure 6. Less dust of fine particles can be collected for bonded mixes, compared to and even less dust was obtained from Starmix TM 500i, compared to Starmix TM 500. The Figure 6.b presented that 13% improvement of graphite bonding degree can be obtained from Starmix TM 500i compared to the Starmix TM 500. Starmix TM 500i as improved bonded version can further minimize the powder segregation during packing, transportation and compaction and while improve the component tolerance, due to higher bonding degree, and meanwhile reduce dust formation of fine particles and improve the working environment in production plant. Dust, mg/m i Time, second Peak Aver SM500i mg/m 3 SM mg/m mg/m 3 Graphite bonding degree 100% 90% 80% 70% 60% 50% 40% 30% i Figure 6. Bonding degree: Dusting, Graphite bonding degree. Filling ability The results of the filling index, which were obtained from studies of the powder mixes in the filling simulator, are shown in Figure 7. Lower filling index means smaller filling density variation between the wide and narrow cavity according to the definition. Similar filling index at low speed of filling shoe were shown for these three types of mixes. When the filling shoe speed increased from 115 mm/s to 162 mm/s, the filling index of all mixes is increasing, but the increasing rate for Starmix TM 500i is much slower, which illustrates that Starmix TM 500i can keep the good filling ability even at higher filling shoe speed, compared with other two types of mixes, thus Starmix TM 500i has the capability to decrease the density distribution for complicate geometry components.

6 Filling index % 45% 40% 35% 30% 25% 20% 15% 10% 5% 0% i Filling shoe speed, mm/s Figure 7.Filling index Production Trial The consistency of component including compaction consistency between components, filling consistency within one component, and consistency of dimensional change, was investigated. The compaction consistency was evaluated by the weight and diameter scatter of sampled components. The standard deviation of weight is presented in Figure 8. The standard deviation of the average of 10 measuring points of diameters is presented in Figure 9. A little bit better weight and diameter scatter was shown by Starmix TM 500i at compaction speed compared to Starmix TM 500, and more pronounced in 7 strokes/min. Therefore, the better control of weight and diameter can indicate smaller variation of powder amount in each filling into die for Starmix TM 500i. Higher compaction speed gives a little bit higher scatter, as expected. Stdev of weight, g strokes/min i Figure 8. Weight scatter. Stdev of Diameter, mm Inner D1 Inner D2 Outer D3 i Stdev of diamter, mm Inner D1 Inner D2 Outer D3 i Figure 9. Diameter scatter:, 7 strokes/min.

7 Variation in the filling of different cavity can cause the variation of density distribution of component. To verify the filling properties in this component, the density distribution of as-sintered component according to filling direction was analysed. The density range was calculated by five pieces for both segment A and B, presented in Figure 10. As can be seen the density variation of Starmix TM 500i was smaller in the both of segment A and segment B, which was also proved by the filling index. Therefore, the testing result of the filling index is in accord with the production trial, lower filling index can contribute more even density distribution in production trial. As the results Starmix TM 500i can fill the different sections homogenously, thus better density distribution was obtained Density distribution range, g/cm strokes/min Density distribution range, g/cm strokes/min i i Figure10. Density distribution: segment A, segment B. The close control of carbon content is a pre-requisite for small dimensional change scatter. Bonding fine particles is a method to minimize the segregation and serves for carbon control in sintered components. Therefore higher graphite bonding degree can give more consistent dimensional change from green to as-sintered component. Figure 11 presented the dimensional change consistency of outer diameter (D3) and height of the component. It is pronounced that slightly reduced scatter was obtained for Starmix TM 500i, compared to Starmix TM 500, probably due to higher bonding degree and better fillability Stdev of DC, % strokes/min Stdev of DC, % strokes/min i i Figure 11. Dimensional change consistency: Outer diameter, D3, Height The final consistency of component is the sum of consistency of weight and dimension between components, density distribution and dimensional change, which are influenced by many factors, like powder properties, filling performance as well as the bonding degree of fine particles. Base on the results tested by industrial production component, the Starmix TM 500i showed better control of weight and diameter at higher compaction speed, smaller density variation in both segment A and B, and also offers better control of dimensional change, due to the improvement in filling ability and bonding degree.

8 CONCLUSION The improved bonded mix combined with a recently developed lubricant, can deliver many benefits. As the results tested by industrial production component with high precision, better consistency between components, smaller density distribution, and better control of dimensional change were achieved by Starmix TM 500i. It also gives excellent lubrication and can be used with warm die compaction (60-80 ) to obtain higher density. The improved bonded mix can also reduce dust formation of fine particles and improve the working environment in production plant and offers stain-free components after sintering. ACKNOWLEDGEMENT The authors would like to thank colleagues from Asia Technical and Education Centre of Höganäs, and Ping Guo from Porite Yangzhou Technology & Industry Co., Ltd. for assistance with the experimental testing in this study. REFERENCES 1. D.Edman and P.Hofecker, Improvement in Dimensional Consistency using Starmix TM Bonded Products, 2004 International conference on of Powder Metallurgy & Particulate Materials June 13-17, Chicago, USA. 2. M.Larsson and J.Rassmus, Higher Tolerances by Improved Materials and Processes, 1998 Powder Metallurgy World Congress, Granada, Spain. 3. D.Edman, H.Vidarsson, B. Johansson, Press capacity improvements utilizing Starmx TM powder, Proceedings of EURO PM2003 in Valencia, Spain on October 20, Å. Ahlin, A.Ahlqvist and O.Litström, Newly developed lubricants for high performance metal powder mixes, PM2008 in Mannheim, Germany, September 30, H.Vidarsson and J.Arvidsson, Die Filling Capability of Powder Mixes, Proceedings of Powder Metallurgy World Congress 2000, Kyoto, Japan, November 2000.