HYBRID LAYERED MANUFACTURING
Direct Rapid Metal Tool Making Process
K P Karunakaran, Sreenathbabu Akula and Vishal Pushpa
Mechanical Engineering Department, Indian Institute of Technology, Bombay, India
Abstract: Fabrication of direct metallic tools presented here combines synergic MIG-MAG welding
and CNC machining processes in order to exploit the benefits of both while avoiding their limitations.
A PC/PLC based low cost XYZ manipulator is fabricated with stepper motor diver in open loop
control. The tool head will hold the welding torch and milling cutter. At any time either milling or
welding will take place, for that the welding gun can be moved up and down with a pneumatic
operation. The welding parameters such as current, voltage, arc length etc are controlled externally.
The framework also consists of a software program which uses the zeroth order edge approximation
uniform slicing strategy to calculate each slice thickness to be deposited with the required metal as
successive layers from lower to topmost layer with the welding process. Further it generates the
required CNC code for machining from top to bottom layer direction of the deposited metallic layers to
attain the required contour profile shape with user specified accuracy. By attaining this cutting edge
technology the manufactures find metallic RP in the adventure of the growing years.
Keywords: Rapid Prototyping, STL File, CNC Machining, MIG-MAG Welding, Molds and dies.
1. INTRODUCTION successful RP process for making paper based
prototypes, have been used for making metallic
The demand for shorter development time, time-to- laminated tools and parts . The metallic
market and reduced product life cycle resulted for laminates in this method are bolted together rather
the emergence of a new paradigm called Rapid than gluing or brazing, thus it lacks the complete
Prototyping (RP) or Layered Manufacturing (LM) adhesion between layers. This method is not
process [1, 2]. The success of automotive, suitable for processes such as injection molding
electronics, telecommunications, biomedical, where liquid metal is involved. Furthermore, this
packaging appliances, leisure and sports industries LOM process suffers from high material wastage
in responding to rapidly varying customers demand and inherent inability to have adaptive layer
for higher quality standards with lower costs thickness .
depend on the ability to develop the product with Various droplet production processes thermal
the application of state-of-art technology. spraying, micro-casting and welding have been
Layered Manufacturing introduced in late 80’s developed for making metallic objects and tools to
as a design visualization tool, has revolutionized improve the deposition quality and to attain the
the way products are designed and manufactured desired contour profile shape. Comparatively three-
today. Although the prototypes could be made out dimensional welding has the ability to produce
of only soft material, very soon, innovative strong, fully homogeneous dense metal parts in
methods of making the short-run and production layers . Adaptation of a weld cladding technique
tools were developed, called as Direct Rapid has enabled the production of parts wider than
Tooling Processes . normally possible from single weld beads. But
Selective Laser Sintering (SLS) is the attaining the required surface finish and required
commercially successful metallic RP process so contour profile is difficult with the weld deposition
for. Direct Metal Sintering relies on laser induced . So to attain better dimensional accuracy and
melting of powder particles together. Since surface quality for building metallic dies and molds
significant thermal gradient exist using this route, it was proposed to develop a unique methodology
the powder bed temperatures are controlled to a with a hybrid approach of an additive and
value just below the powder melt temperature to subtractive processes comprising MIG-MAG
produce parts with minimum internal stresses. welding and CNC machining respectively.
Porosity is a problem that is normally reduced by
post sintering or infiltration [4, 5].
The principles of Laminated Object
Manufacturing (LOM) another commercially
2. HYBRID LAYERED MANUFACTURING the deposited metallic layers from top to bottom
layer direction to attain the required contour profile
The proposed Hybrid Layered Manufacturing shape with user specified accuracy.
(HLM) process has a numerical controlled system
that integrates the synergic MIG-MAG welding 2.1 Slicing Strategies
process , which provides the controlled heat
and mass transfer with precise depth of bead Fabrication of the prototype model through RP
penetration, and CNC milling process, which process requires the thickness of each layer to be
enhances both the surface quality and dimensional found before deposition of the material. To
accuracy with great manufacturing agility. calculate the Slice Thickness, i.e the distance
Synchronization of welding process and work- between the top profile contour and bottom profile
piece/substrate motion with milling operation offer contour of a specific layer [12, 13].
a new accelerator way of building desired metal
parts and tools . Based on the layer thickness, the slicing strategies
can be broadly grouped into two.
The aim and scope of the proposed process is Uniform Slicing where the layer thickness is
To develop a RP machine for making injection same for all layers of the object
molds and dies. Adaptive Slicing where the thickness of each
To develop the know-how to retrofit any layer is dependant on the local geometric
exiting CNC machining center with the above properties and the user specified cusp height.
To develop the software required for this HLM Slicing schemes can be classified based on the edge
process that does approximation, that is how each bottom and top
- Slicing process contour is connected with respect to the build
- Path generation to control welding torch and direction .
milling cutters Zeroth Order Edge Approximation: The edges
- Generation of codes to control process of the layers are along the build direction. The
parameters such as spark initiation, boundary of the final part will be a stepped
- Control the speed and feed of the milling approximation of the CAD model.
cutter First Order Edge Approximation: The slice
- Post-processing to make the code suitable for thickness has been formed by a curve of first
the controller of any CNC m/c order i.e. a straight line. This is also referred to
as ruled slicing.
The HLM process consists of a software program Higher Order edge approximation.
which uses the zeroth order edge approximation
uniform slicing strategy of the RP paradigm to The stereolithography (STL) file of the body is
calculate each slice thickness that to be deposited sliced at regular intervals with the consideration of
with the required filler metal as successive layers Flat Surfaces, Peak Points and Branching
from lower to topmost layer with the welding constraints to form the Coarse Slices and then
process. Further in accordance to the first order further to have Fine Slices as shown in Figure 1
edge approximation slicing strategy of RP it and Figure 2.
generates the required CNC code for machining of
Tool to be built Coarse Slicing Uniform Slicing Adaptive Slicing
Fig.1 Slicing Strategy
Coarse Slicing Coarse Slice 1 Coarse Slice 2 Coarse Slice 3
Fig.2 Coarse Slicing Process
Layer to be deposited Weld Path Deposited Layer
Fig.3 Weld Path
In a Coarse Slicing, the number of loops in the motion and the inner part is filled with raster
top z_level and that in the bottom z_level will be approach. These area-fill paths are then fed to the
the same i.e, for each loop in the bottom z_level drive controller that integrates the welding machine
there will be a corresponding loop at the top level. to initiate the welding arc and the XY table of the
In this slicing process it is essential to establish the HLM machine for deposition of the model in a
mapping between the top and bottom contours of a layer-by-layer fashion as shown in Figure 3.
coarse slice. The weld path is optimized to transfer heat
In Fine Slicing, each individual coarse slice is uniformly over the layer as the heat build-up due to
further sub-divided in accordance to the user the welding process may result in part
defined/required uniform slice or adaptive slice malformation and collapse of the structure. With
thickness. the consideration of the limitations of the welding
Though the above slicing procedure for the process the path for the welding gun has to be
STL file takes more computational time, the results generated to deposit the filler metal for attaining
will be more accurate with the simplification of the the required slice profile shape and thickness.
geometry of the complex CAD model. The first Control of parameters and trajectory is added to the
step i.e, Coarse Slicing is helpful in establishing start and the end portions of the weld in order to
contour mapping and in eliminating branching make their thickness and width similar to that of
constraint. The second step, Fine Slicing is an the central portion of the weld. The number of
iterative process to calculate slice thickness based discrete joints of weld passes will decrease
on the local curvature of the body. The data precision of the work piece. To improve the
attained through the Fine Slices is utilized for the deposition speed and precision it is essential to
path generation of the MIG-MAG welding gun, minimize the number of weld passes and joints
and face milling operation. between them.
2.2 Weld Path 2.3 Face Milling
For the proposed HLM process shrinkage and The width and height of the weld bead depends on
machining allowances are to be added to the applied voltage, diameter of the filler wire and
compensate the shrinkage of the deposited filler weld speed. The deposited weld bead will be
metal during the solidification and to eliminate the generally in convex shape. Due to the heat sink, the
weld defects of undercut and blowhole defects. For penetration is lower at the initial position. Thus the
that reason contours of the slice are provided with weld bead thickness is higher than the normal
offset. The outer contours will have a positive portion of the weld. These uneven portions of the
offset while the inner ones with negative offset layer will influence succeeding layer metal
value. So during the zeroth order edge slicing deposition. And at the end portions of the weld
strategy the top and bottom z_level contours of a pass, because of the viscous nature of the molten
layer are merged to obtain the union of the outer metal, an inclined shape is created. After a few
contour and intersection of inner contours. To the layer depositions, a significant deviation from the
Fine Slice contours thus obtained after offsetting originally expected shape will be accumulated.
and merging is passed for mesh generation in order Face Milling of the slice as shown in Figure 4
to achieve the zigzag tool path segments for the ensures the correct vertical height accuracy of the
weld deposition . To minimize the surface error built layer.
the outline of the contour is filled with vector
Face Milling End Milling Scan Milling
Fig 4 CNC Milling Operation
On the newly deposited layer a thin oxide layer halting the welding process and activating the face
will be formed due to the oxidation phenomenon. milling process and vice versa. For this operation a
Subsequently if the next layer is deposited on the pneumatic system is used to swivel between
top surface of previous layer, a significant bond is welding gun and the milling cutter. So that at any
not achieved. Initially small in size, the defect is time either welding or milling operation will only
amplified with succeeding multi-layer deposition take place. The necessary functions for the
until the deposition is not feasible. To minimize operation of synergic MIG-MAG welding machine
and correct this deviation, a face milling operation with shielding gas were monitored through the
is performed on the top surface of the deposited numerical controller. While the majority of the
metallic layer. welding process parameters such as current,
voltage, arc gap, shielding gas composition etc are
2.4 End Milling controlled externally.
With the welding process, attaining the accurate 3.2 Rough Machining the near-net shape of the
contour profile shape of the coarse slice is difficult. tool to final dimensions
For that end milling operation is performed to
attain the shape and accuracy. The tool path of the Face milling operation is performed on the top
end mill is generated in accordance to first order surface of each deposited metal layer to get the
edge approximation. Slicing with first order edge required thickness of the layer. The instability of
approximation will be made use of ruled surfaces arc welding process may cause a
. As the coarse slice passes the information of malfunction/defect in the middle of the weld bead.
loops between the top and bottom layers a ruled To minimize the deviation and correct the vertical
surface has to be find within the upper limit height accuracy of the built layer in successive
deviation. In order to calculate the deviation, a line multiple layer deposition, face milling operation is
is fitted to connect a point in the bottom loop and performed on all the horizontal surfaces of the
the corresponding point in the top loop without deposited layer. Repeating the above process of
violating the tolerance constraint ‘cusp height’. The layer deposition and face milling operations to
corresponding point in the top loop is the one generate near net layers deposited one over the
having the same surface normal vector and nearest other from bottom most to top layer till a casting
to the bottom point. The tool path generated like rough shape is obtained.
contains a list of vertices that contains the co- As the size of the deposited layer is made to be
ordinates of the points. These paths are then fed to little bit larger than the actual required profile
the HLM machine for the generation of the final shape considering into account the shrinkage and
shape of the model by scan milling using a flat end weld defects, the edges of the layers are still rough.
mill cutter as shown in Figure 4. The tool path of the end mill is generated for
machining the final casting like rough shape from
3. METHODOLOGY top to bottom layer direction of the deposited
metallic layers to attain the required contour profile
The proposed HLM process consists of the shape with user specified accuracy in accordance to
following stages: first order edge approximation strategy slicing
Building a near-net shape of the tool. principle of RP paradigm.
Rough Machining the near-net shape of the
tool to final dimensions 3.3 Heat treatment for stress relieving and
Heat treatment for stress relieving and strengthening
Finish Milling to get the required surface finish The temperature gradients within the deposited
and quality. layer with the severity of cooling, influences on the
generation of internal stresses and on the resulting
3.1 Building the near-net shape of the tool microstructure of the final deposited layer. The
stress distribution consists of nearly uniform
The substrate plate is rigidly placed with proper tension in the newly deposited layer, with tension
fixtures on the XY table, and its motion is guided at the top of the substrate and nearly uniform
by motor drives. The required weld paths are compression at the bottom half of the substrate. To
generated in relation to the Fine Slice contours for relieve these undesirable residual stresses, a
deposition of the layer thickness is used to deposit suitable heat treatment is performed using
the bottom most layer with a simple zigzag pattern. normalizing and annealing processes. As these
As the metal deposition in the bottom most layer is residual stresses are unchecked, they may induce
completed the shielding gas nozzle is turned off warping, loss of edge tolerance and delaminating
and the switching functions are invoked to change thereby reducing the strength and influencing on
over between welding and face milling process by the tool life.
The material homogeneity of the prototype to
be obtained in the proposed method is between
those of cast and machined parts. Thus, this process
is not suitable for making forging dies where very
high impact forces are encountered. The die used in
injection molding, die casting and sheet metal
forming undergo considerably less fatigue loading
during the operation, so these tools can serve the
purpose even without any homogenization
operation such as Hot Isostatic Pressing (HIPping)
process. Further these die halves are free from
overhanging features, as they need to open and
close in operation. Building such dies and mold
with free from re-entrant profiles by the proposed
methodology will not require support structure,
thus making the process less complicated.
Fig 5a. XYZ Manipulator
3.4 Finish machining
Finish mill all the contour profiles of the tool to the
required surface finish.
Machined and fastened structure of XYZ
manipulator is fabricated as shown in Figure 5 for
the proposed process with low investment cost. The
horizontal X, Y motions are attained by the
movement of substrate table on ball lead screw
arrangement and vertical Z motion is achieved by
the movement of tool head. Each axis has its
motion control by an individual stepper motor in
open loop drive with rapid and linear
Fig 5b. XYZ Manipulator
interpolations, such that all 3 axes can be moved
simultaneously. The maximum transverse in X, Y
& Z axis is 400, 300 & 300mm respectively with
an accuracy of 0.05 mm and maximum attainable
speed of 2500mm/min. The load on the XY table
can bear up to 100kg.
The arrangement of tool head as shown in
Figure 6 is made to hold the MIG-MAG welding
torch and the face-milling cutter. The welding torch
is connected to the wire feeder, shielding gas
nozzle and power source of the weld machine. For
the face-milling cutter a motor is fitted on a fixed
frame from which it is driven through a belt and
spline shaft. The welding gun is made to move up
and down with a pneumatic controller after Fig 6a Tool Head
completing the metal deposition for the desired
layer thickness. This arrangement makes sure that
at any time either milling or welding will only take
place. The NC codes M08 and M09 are used to
invoke the switching functions of the pneumatic
piston to move up and down respectively. Further
the NC codes M03 and M04 are made to control
the on/off of the welding torch during the metal
deposition as per the requirement. Adequate
isolation to protect the motion control electronics
from welding spikes is implemented. Process
parameters such as welding current, voltage, arc Fig 6b Tool Head
length, weld speed and shielding gas of welding 5.4 Weld Path
process are controlled externally and the tool The area-filled weld paths along with the face mill
change, spindle speed, coolant, etc of the milling paths are generated and can be viewed as in Fig 7.4
process are governed through the numerical control
of the software.
5. CASE STUDY
A step-by-step processing of a sample model is
described in this below section. All the subsequent
sequence of operations is carried out on it to
generate the weld deposition paths and the coarse
slice machining paths. Fig 7.4 Weld path for a uniform slice
5.1 Input Model 5.5 Milling Paths
Using the Pro/E CAD package, a solid model The .STL model is fed for the machining path
bracket without any overhang features is drawn as generation. The paths thus generated along with the
shown in Figure 7.1 and its STL output file is tool withdrawal and reentry points can be viewed
generated. This .STL file acts as an input for the in Figure 7.5.
software. Then the user defines the process
parameters required for the further subsequent
sequence of operations to be carried out on it to
generate the weld deposition paths and the coarse
slice machining paths.
5.2 Coarse Slice Model
Next the .STL model is coarse sliced to generate
the coarse slice levels as shown in Figure 7.2. This Fig 7.5 Machining paths
is followed by generation of the solid coarse slices
and their display 5.6 Output Files
The output files generated from the software i.e,
the weld deposition path, face mill cutter path and
the coarse slice-machining path are of standard NC
format as in Figure 7.6. These output files are fed
to the NC drive controller to generate the milling
and deposition paths.
N0001 G28 Z0
Fig 7.1 Fig 7.2 N0002 G28 X0 Y0
Model dimensions Coarse slices display N0003 M08 // To move pneumatic piston down
N0004 G10 L2 P1 X99.0 Y97.0 Z-160.0
5.3 Uniform Slice Model N0005 G55
N0006 G90 G01 G21 F800
The model is uniformly sliced in accordance to the N0007 G00 X0 Y0
user-defined settings and the uniform layers are N0012 M03 // To initiate the welding process
generated to display as shown in Figure 7.3. : :
N0200 M04 // To turn off the welding process
N1000 M09 // To lift the pneumatic piston up
Fig 7.3 A uniform slice/layer Fig 7.6 NC file format generated by software
1 2 3 4 5 6 7 8 9
Fig. 8 Test Tool Build with HLM Fig 9 SEM Image Microstructure of Fig 10 Hardness variation with Layer
process Layer number
6. DISCUSSION strength can be improved by subjecting the
produced tool to HIPping process.
Traditionally dies and molds for injection molding, Experimental trails are performed with MIG-
forging, and die-casting are manufactured using MAG machine on a clean Mild Steel substrate plate
CNC machining process. Initially the metal blocks of size 100X 100X 12 mm3 that is free of initial
of annealed condition are rough machined with residual stresses. A DIN 8559 weld wire with
HSS and Carbide tool cutters. Further smooth diameter size 1.2 mm was used to deposit the metal
finish milling is operated on it at a low feed rates in short circuit mode with proper weld parameters.
and to achieve the desired accuracy and shape an The composition of shielding gas used is of 80%
EDM process is performed. The time needed for Ar + 20% CO2. Total ten number layers are
these process to machine or configure the die shape deposited on the substrate plate, with the weld
will increase significantly for harder materials, as speed of 12 mm/sec as show in the Figure 8. A
the feed rate and spindle speeds have to be reduced. simple zigzag weld path was generated to built the
So time and cost incurred in the manufacture of the test trail tool. The average layer thickness of metal
tools cannot be justified for small and medium deposition is 2.1 mm with a bead width 2.4–2.8
batch production. But during the last two decades mm. For initial test trails no external cooling
the steep acceleration in product and market procedure was implemented to monitor the
variations demanded for higher quality standards microstructure and induced residual stresses. As
with lower costs.Direct production of the metal part shown in Figure 9 the SEM images showed a fine
is unique among current RP techniques. Although microstructure formation with a hardness of 28
MIG-MAG welding cannot generate the required HRC. But the hardness varied for each layer as
accuracy, it is economical, safe, portable and easy shown in Figure. 10.
to maintain. Adequate isolation to protect the Complex shapes and sizes are possible with
motion control electronics from welding spike is least number of jigs and fixtures. The process does
implemented. The number of joints of weld passes not pose any restriction or loss of accuracy on the
decrease the precision, so the reduced number of prototype as its size grows. Since the part size is
weld passes and joints in the deposited layer to limited only by the transverse available on the
enhances the speed and precision of weld CNC machine, larger CNC machine can be used to
deposition. With welding process, all the desired produce larger tools. The size of the prototype thus
material properties cannot be attained. Strength of attained is independent of its accuracy and is
the weld depends mainly on the availability of limited only by the table transverse. The tools
suitable filler wires. The material homogeneity and produced using this process may be inferior to their
Characteristic SLS Proposed HLM
Principle Powder Metallurgy (PM). GMAWelding and CNC machining.
Slicing type Uniform slicing of 0th order edge. Adaptive slicing with 1st order edge
approximation is possible.
Accuracy Limited by particle and layer sizes. Same as CNC machining
Density Porous part. Density close to 100% can be achieved.
Post-processing Impregnation with copper in a furnace is No post processing for density
required which takes several hours. improvement.
Strength Since it is not totally steel, not very strong. Strength depends only on the availability
of suitable filler wires
Overall cycle time Slower than HLM process. Much faster than SLS process.
Available as A complete and expensive machine. A retrofit or complete machine.
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