May 2005

HVC set to move on to multi-level PM applications

Large high-density single-level PM parts can be manufactured cost-effectively using high-velocity compaction. The next essential step is to master the art of multi-level HVC production to broaden the range of applications

In traditional compacting the main ram motion of a press is either force- or position-controlled through hydraulic cylinders, crank drives or knuckle drives. The top tooling is constantly connected with the ram and upon entering the die cavity exerts a continuously increasing pressure on the powder charge.

High velocity compaction (HVC) employs a ram with a discreet mass, which is accelerated to a predetermined speed and then impacts with the top tooling which has previously been brought into contact with the powder charge in the die cavity. This energy-controlled motion compacts powder by a shock wave. It reaches impact speeds of up to 10m/s.

Powders typically start to precompact - particles start to mechanically bond - at a density far below the desired green density. To avoid inhomogeneous density distribution and/or cracks between sections of a multi-level part independently movable punches for each level are required as soon as the desired height difference between levels represents a substantial percentile of the overall length of the part. The multiple punches are either force- or position-controlled or their motions relative to each other accommodate a uniform and simultaneous compaction of all sections of the part.

It appears to be extremely difficult to synchronise the auxiliary motions of multiple punches with a top tooling that is accelerated by an impact and decelerates while the impact energy is released at a speed that is two orders of magnitude higher than conventional, continuous compaction.

It has been suggested that the various sections of a multi-level part compact could be compacted by impact in sequence rather than simultaneously. This would not only require additional means to prevent radial powder transfer into not-yet-compacted sections, but it would certainly lead to cracks between the sequentially compacted sections.

Figure 1: Dual operating mode cycle. a) Under-fill position b) Position-controlled precompaction position c) Impact compaction position d) Ejection position.

A more successful approach is the introduction of a dual operating mode for the main ram. With the first operating mode a multi-level pre-form is compacted to a density that provides suitable green strength, utilising force- or preferably position-controlled drives for the main ram and multiple punch motions. In the second operating mode the main ram uses an energy-controlled drive to further compact the pre-form to high density, while no or few auxiliary punch motions are required. A dual operating mode was first seen in conjunction with the short-stroked electro-dynamic shock compaction.

The adaptation of the dual operating mode to high velocity compaction enables single-sided multi-level shapes to be made using multiple bottom punches.

The HVC dual-operating mode uses closed-loop, position-controlled hydraulic drives for all tooling members to compact the pre-form. Preferably the pre-form features length to density ratios of its various sections that allow further compaction to a uniform high density by impact with no auxiliary punch movements. Thus the bottom punches can rest on rigid mechanical stops to maximise impact energy discharge within the green part.

Figure 2: Pre-form stage alternatives prior to impact. a) Density gradient in pre-form b) Outer punch in float mode c) Retracted outer punch.

Alternatively, if a highly responsive pressure relief system that enables floating of pre-lifted punches is feasible, the pre-form can also be of uniform density. In this case a pre-lifted punch will be driven onto its mechanical stop by impact, see Figure 1b. In the absence of a pressure relief system and with a pre-form of uniform density, a pre-lifted punch would have to be retracted onto its mechanical stop prior to impact, leaving a gap between punch and preform.

To extend the principle to multiple top punches for two-sided multi-level shapes appears not feasible because the tooling with adaptation and drive systems represent a large dampening mass detrimental to shock wave transmission.

Levels with moderate drop heights compared to the overall length of a part however can be formed with a stepped upper punch. The limits are similar to those of conventional compaction, with shape distortion at high densities adding another factor to consider.

In the absence of a suitable multi-level HVC press, the concept of single-sided multi-level compaction can be suitably simulated by producing the pre-form in a conventional powder press and than inserting the pre-form into stepped tooling in a single-level HVC press.

To minimise impact energy losses, the whole HVC press system including tools has to be as rigid as possible. While the shape to be formed inherently limits the optimisation of tools, and to a lesser extent tool adaptation, the tool rig design is more open to improvements toward high rigidity. A new design concept has been developed, which is substantially more compact and therefore rigid compared to existing solutions [4].

Figure 3: Multi-level shapes requiring a bottom punch per level and a single (stepped) top punch.

The main characteristic of the new design is that a number of concentric cylinders operate from essentially the same elevation rather than stacked on top of each other. This is made possible by supplying the pressure medium for the internal pistons through the base plate of the cylinder block and the internal cylinder housing(s). Linear encoders for position control are connected from the base plate. The concentric cylinders drive punches and cores. An additional, stationary punch can be placed on the internal cylinder housing. Lateral cylinders located in the corners of the cylinder block actuate the die plate. The top plate, which drives the top punch in the first positional-controlled mode can be operated the same way. To avoid spacers for punch length adjustments, adjustable mechanical stops can be integrated into the base plate. It has yet to be determined, whether adjustable stops compromise the desired tool rig rigidity to an unacceptable degree, in which case they would be replaced by hard stops.

The new tool rig concept and the proven HVC press frame form a very compact and robust unit. The tooling stack up, with punch cross-sections and aspect ratios dictated by the part geometry, is now the limiting factor for rigidity of the whole system. This holds particularly true with each added level, increasing substantially the length of the innermost punch.

Figure 4: Tool rig with three concentric cylinders, adjustable mechanical stops and lateral cylinders to drive not shown die platen.