LIFE PROGRAM

The project started on July 2017 and successfully concluded on August 2021.
Here we can summarize the project development ad the technical results obtained

PRESS PROTOTYPE INTEGRATING A DIE WALL LUBRICATION SYSTEM

The first project actions regarded the study and development of a ‘green’ lubricant and lubrication techniques, the design and construction of a prototype press coupled with a die wall lubrication automatic system.

The approach for the development of a “green and efficient” lubricant has been based on the following main considerations: the high pressure that the lubricant have to withstand, the cleanliness needed to avoid metal powder contamination and the easy way to deposition on mould surfaces. In order to obtain an effective layer of lubricant on the mould wall, many techniques have been considered but spraying has proved to be the most effective: an effective stable liquid suspension based on solid lubricant has been studied and produced.

The effectiveness of the different lubricants have been tested in term of extraction stress after warm die compaction, measured by a universal testing machine. Extraction tests have been demonstrated the effectiveness of the DWL compaction system as shown in Figure 01. The extraction stress can be reduced in the range from 55% up to more than 75% adopting the DWL compaction system respect to the standard internal lubrication.

Figure 01

Figure 02 shows the benefits of DWL respect to the standard practice and highlights the effectiveness of DWL in terms of improved green density: the target value of 7.30-7.40 g/cc foreseen in the project have been successfully reached and overcame for many materials normally adopted in the PM standard practice.

Figure 02

The press prototype integrating the automatic Die Wall Lubrication system has been studied, designed, and developed by the project partner SACMI. The construction of the press was successfully completed and tested. All the samples and the related data here presented have been obtained using the press prototype shown in figure 03.

Figure 03

ALLOY DESIGN

Another project task regarded the alloy design in order to define the chemical composition, copper and nickel free, of low alloy PM steel. Silicon and Manganese are the two element that have been choose and tested: they are environmentally friendly and they increase the hardenability of steels.

Figure 04 and figure 05 show the alloy design, based on Master Alloy (MA), benefits after sintering in term of improved dimensional stability and increased hardenability. Increasing the hardenability increase the hardness due to microstructure evolution to harder phase.

Figure 04

Figure 05

For this investigation Transverse Rupture Strength (TRS) samples have been adopted as shown in figure 06.

Figure 06

METERIAL CHARACTERIZATION

Another important project action regarded the production of complex structural steel samples by DWL and its microstructural and mechanical characterization.

In this phase also DWL gears prototypes have been successfully develop and tested.

GREEN STRENGTH – AN OUTSTANDING RESULT

TRS samples have been adopted to test mechanical and dimensional properties after compaction (i.e. green strength) and after sintering or heat treatment.

An outstanding results have been obtained in term of green strength: figure 07 shows the green strength of standard PM and DWL component. The orange line (i.e. 30-35 MPa) represents the resistance threshold that a green sample must have in order to sustain mechanical processing. This critical point can not be overcome by standard PM material whereas DWL one can easily do it. The green strength of the DWL components (from 40 up to 65 MPa) is more than double respect to the Standard PM (from 15 up to 25 MPa) and open the interesting possibility to perform some dry machining operation before sintering.

Figure 07

This aspect has been also successfully tested as shown in figure 08: a green DWL cylinder, external diameter 45 mm, machined and drilled. This results is really important because the dry machining operation on green components require less energy respect to the sintered components and therefore offers more environmental benefits.

Figure 08

SINTER DENSITY

Very high density levels have been obtained and the project target of 7.30 g/cc has been easily overcome end near full density conditions, more than 7.67 g/cc for all class of material investigated have been obtained. Under this conditions also DWL gears (De=45mm) have been successfully produced as shown in figure 09.

Figure 10 shows the sinter density evolution as function of the compaction pressure.

Figure 09

Figure 10

DIMENSIONAL STABILITY

Object of investigation was also the dimensional stability of the DWL components respect to the standard PM one.

Figure 11 summaries the results in term of dimensional analysis about the low alloyed chromium steel; it is possible to note the very high benefits of DWL:

• Springback is a critical expansion that occurs during the extraction of the green components from the die after compaction. The rising of the springback increase the possibility to cracks formation (i.e. structural defects) that can negatively affect the mechanical properties. HDPM by DWL can reduce the springback more than 38% defining a very good result.
• Shrinkage occurs during sintering when the residual porosity into the material reorganize itself. Because of the metal powder morphology the shrinkage is not homogeneous and strongly affect the dimensional and geometrical stability of the component. Samples obtained by DWL can reduce the shrinkage of 89% defining an outstanding result.

Figure 11

MECHANICAL PROPERTIES

About the target values in term of mechanical properties for DWL components after sintering and heat treatment, the Yield Strength (YS), Ultimate Tensile Strength (UTS) and elongation at fracture (Ef) have been overcome.
The mechanical strength of the material was measured by uni-axial tensile test. Dog-bone tensile bar were produced following the ASTM B925 standard. The tensile tests were carried out using a universal machine model MTS Criterion C45. The tests were conducted in accordance with the ASTM E8M standard by adopting a “gage length” strain gauge equal to 12mm.
For the DWL material Astaloy Mo 0.5%C after heat treatment of quench and tempering the following values have been reached.
In brackets the target project values YS=1309MPa (1000); UTS=1700MPa (1200); Ef=2.7% (2)
Figure 12 summarize these results and emphasize the DWL benefits on mechanical properties respect to the best practice of the standard PM. The elongation at fracture is normally a weakness point for the sintered components where value above 1.5-1.7% can not be reached by standard PM after quench and tempering. For this reason the DWL value of 2.7% can be considered a very good result.
The Yield Strength and the Ultimate Tensile Strength are improved around 20% and the elongation at fracture around the 70%.

Figure 12

Figure 13 shows the final microstructure of the DWL material. The LOM analysis after chemical etching shows a fine high density, high resistance martensitic microstructure.

Figure 13

About the gear DWL prototype also fatigue performances have been investigated. The material 20MnCr5 used for power gear applications (i.e. forged gears from machining) have been adopted like benchmark.

Figure 14 shows the fatigue strength after thermochemical treatment of DWL gears respect to the forged/machined material. Standard PM steels normally show a fatigue limit in the range of 50% of the forged one.

The DWL gear having, high density, a well calibrated case hardened profile, a fine microstructure, allow to obtain performance in the range of 70% respect to the forged materials.

Figure 14

LOW PRESSURE CARBURIZING OF DWL SINTERED MATERIALS

Another important project action regarded the design and development of the Low Pressure Carburizing (LPC) thermo-chemical treatment for high density sintered steel components having complex geometry: achieve a demonstrator (i.e. spur gear) featuring an effective case hardened profile useful to meet high mechanical performance requirements.

The LPC process has been studied and a numerical model, that allows to define the process parameters to conduct the thermochemical process LPC at industrial level, has been developed.

The machined 20MnCr5 is a Gas Carburized gear adopted for a commercial planetary gearboxes that the DWL Gear developed in the project can be the substitute.

Figure 15 shows the comparison of the macro hardness HRC measured after case hardening by means of LPC process. The DWL LPC gear shows a surface hardness of 58.4 HRC that exceed the 55 HRC minimum requested for power gear applications (threshold orange line). This result is very good having in mind that the best macro hardness for the best practice of case hardened standard PM does not exceed 52-53 HRC.

Figure 15

Figure 16 shows a detail of the carbon penetration on cross section of the gear teeth after LPC. The samples have been observed at LOM after chemical etching in order to highlight the microstructure. It is possible to see the well defined carbon profile, also measured from the microhardness investigation, and both confirm:

• the goodness of the numerical model to define the process parameters
• the goodness of the high density DWL component to thermochemical treatments.

The high level process control allows also to obtain a very good, defects free, martensitic
microstructure as shown in figure 17.

Figure 16

Figure 17

In conclusion we can assert that all the technical objectives of the project have been reached ad in some cases overcome.